Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B17/00—Systems with reflecting surfaces, with or without refracting elements
  • G02B17/02—Catoptric systems, e.g. image erecting and reversing system
  • G02B17/06—Catoptric systems, e.g. image erecting and reversing system using mirrors only, i.e. having only one curved mirror
  • G02B17/0647—Catoptric systems, e.g. image erecting and reversing system using mirrors only, i.e. having only one curved mirror using more than three curved mirrors
  • G02B17/0663—Catoptric systems, e.g. image erecting and reversing system using mirrors only, i.e. having only one curved mirror using more than three curved mirrors off-axis or unobscured systems in which not all of the mirrors share a common axis of rotational symmetry, e.g. at least one of the mirrors is warped, tilted or decentered with respect to the other elements
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B64—AIRCRAFT; AVIATION; COSMONAUTICS
  • B64G—COSMONAUTICS; VEHICLES OR EQUIPMENT THEREFOR
  • B64G1/00—Cosmonautic vehicles
  • B64G1/10—Artificial satellites; Systems of such satellites; Interplanetary vehicles
  • B64G1/1021—Earth observation satellites
  • B64G1/1028—Earth observation satellites using optical means for mapping, surveying or detection, e.g. of intelligence
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/28—Investigating the spectrum
  • G01J3/2823—Imaging spectrometer
  • G01J2003/2826—Multispectral imaging, e.g. filter imaging
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/02—Details
  • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
  • G01J3/0208—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using focussing or collimating elements, e.g. lenses or mirrors; performing aberration correction
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/02—Details
  • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
  • G01J3/021—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using plane or convex mirrors, parallel phase plates, or particular reflectors
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/28—Investigating the spectrum
  • G01J3/2823—Imaging spectrometer

Definitions

• Optical imaging systems are useful in many applications such as imaging planets or stars.
• Known optical system designs for satellite imaging include a traditional Three Mirror Anastigmat (TMA) design and a Korsch design.
• TMA Three Mirror Anastigmat
• Korsch Korsch design.
• Existing solutions to optical imaging have drawbacks with regard to size and corresponding resolution capability. Improvements in optical imaging are therefore desirable.
• SUMMARY In one aspect, an all-reflective optical system is disclosed.
• the all- reflective optical system comprises a concave primary mirror having a central aperture and a radius, the primary mirror having one of a parabolic, non-parabolic conical, or aspherical surface; a convex secondary mirror facing the primary mirror, the secondary mirror having an aspherical surface, where an optical axis extends from a vertex of the primary mirror to a vertex of the secondary mirror; a concave tertiary mirror arranged behind the primary mirror, the tertiary mirror having one of a parabolic, non-parabolic conical or aspherical surface; a concave quaternary mirror arranged in the central aperture of the primary mirror or behind the primary mirror, the quaternary mirror having one of a spherical, parabolic, non-parabolic conical or aspherical surface; and at least one image plane having one or more aggregated sensors.
• the optical system may additionally comprise an entrance pupil positioned near the primary mirror or the secondary mirror, and an exit pupil or Lyot stop positioned at one of 1) near the quaternary mirror, 2) between the tertiary mirror and the quaternary mirror, and 3) between the quaternary mirror and the image plane.
• the optical system may additionally comprise one or more folding mirrors arranged to deflect rays from the quaternary mirror to the image plane, wherein the one or more folding mirrors may be configured to fold a ray path.
• the exit pupil may be positioned between the tertiary and the quaternary mirror, or between the quaternary mirror and the first folding mirror.
• One of the folding mirrors may be tilted at a specific angle to an optical axis of the system.
• One of the folding mirrors positioned at the front of the image plane may widen the field of view with reflective and transmissive sections over a same spectral range, wherein each section may correspond to a specific sensor of the one or more sensors.
• One of the folding mirrors positioned at the front of the image plane may enable simultaneous multi-color imaging, wherein the one of the folding mirrors may be reflective over a first spectral range and transmissive over other spectral ranges, and may be reflective over a second spectral range and transmissive over other spectral ranges, wherein one of the aggregated sensors may be dedicated to the first spectral range and a different one of the aggregated sensors may be dedicated to the second spectral range.
• a form factor defined as a ratio of a distance between the secondary mirror and the tertiary mirror to an effective focal length of the optical system, may be less than 0.09.
• Vertices of the primary mirror and the secondary mirror may form an optical axis, which may be a geometric reference line extending from the vertex of the primary mirror to the vertex of the secondary mirror.
• the primary mirror and the secondary mirror may be symmetric or periodic about the optical axis.
• a diagonal of a periodic mirror may have an angle of zero degrees or 45 degrees from a diagonal of the image plane.
• the optical axis of the tertiary mirror may not coincide with a mechanical axis.
• a radius of the secondary mirror may be in a range of 1% to 3% of an effective focal length, and a radius of the tertiary mirror may be in a range of 2% to 3% of the effective focal length.
• a radius of the quaternary mirror may be in a range of 6% to 22% of an effective focal length.
• the folding mirrors may enable simultaneous multi- color imaging, wherein each of the folding mirrors may be reflective over a particular spectral range and transmissive over other spectral ranges, and wherein each added folding mirror and a corresponding one of the aggregated sensors may be associated with a different spectral range.
• a distance from the tertiary mirror to the image plane along the optical axis may be in a range of 3% to 9 % of an effective focal length and the distance from the secondary mirror to the tertiary mirror along the optical axis may be in a range of 4% to 9% of the effective focal length.
• the system may have an imaging resolution better than 1 m at a 500 km altitude.
• the system may be adapted to support simultaneous multi-color imaging, including 1) panchromatic and RGB and near-infrared, 2) visible and infrared (near-infrared, shortwave infrared, mid-wave infrared, or longwave infrared), 3) visible and visible, 4) infrared and infrared, 5) UV and visible, or 6) UV and infrared imaging.
• a diameter of the primary mirror may range from 3% to 8% of an effective focal length.
• a focal point distance from the primary mirrors may be in a range of 1 % to 6 % of an effective focal length.
• An effective focal length may be in a range of 300 mm to 20,000 mm.
• the optical system may further comprise a supporting structure for one or more of the mirrors.
• the supporting structure may be additively manufactured.
• the image plane may comprise a charge coupled device (CCD)-in CMOS time delay integration (TDI) sensor.
• the CCD-in-CMOS TDI sensor may be a multispectral TDI, backside illumination imager.
• the CCD-in-CMOS TDI sensor may comprise seven CCD arrays of 4096 x 256 pixels each.
• the CCD-in-CMOS TDI sensor may comprise four panchromatic CCD arrays of 16384 x 96 pixels each and eight multispectral CCD arrays of 8192 x 48 pixels.
• the primary mirror may have a circular or a non- circular shape
• the tertiary mirror may have a segmented non-circular shape
• the quaternary mirror has a circular or non-circular shape.
• the non-circular shape of the primary mirror may enhance a modulation transfer function (MTF) and a signal to noise ratio (SNR).
• MTF modulation transfer function
• SNR signal to noise ratio
• the quaternary mirror may face the tertiary mirror and may be positioned to avoid interference with rays from the secondary mirror to the tertiary mirror.
• the optical system may additionally comprise a supporting structure of the mirrors including a cylindrical tube or a conical baffle of the primary mirror.
• the four mirrors may be constructed of zero-CTE materials, low-CTE materials, or mild-CTE materials, wherein the four mirrors and a supporting structure may be made of one material.
• the system may be adapted to provide imaging in the modes of starring, scanning or pushbroom, video, stereo, BRDF (Bidirectional Reflectance Distribution Function), HDR (High Dynamic Range), polarimetric and low-light.
• the system may be adapted to be installed onboard satellites purposed for a non-imaging mission including communication satellites, or installed on imaging satellites, quasi-imaging satellites, or scientific mission satellites.
• the system may be adapted to be installed onboard airplanes, drones, unmanned aerial vehicles, and balloons.
• an all-reflective optical system comprising a concave primary mirror having a central aperture and a radius, the primary mirror having one of a parabolic, non-parabolic, conical, or aspherical surface; a convex secondary mirror facing the primary mirror, the secondary mirror having a hyperbolic surface, where an optical axis extends from a vertex of the primary mirror to a vertex of the secondary mirror; a concave tertiary mirror arranged behind the primary mirror, the tertiary mirror having one of a parabolic, non-parabolic, conical and aspherical surface; a concave quaternary mirror arranged in front of the central aperture of the primary mirror, the quaternary mirror having one of a spherical, parabolic, non-parabolic, conical or asp
• an all-reflective optical system comprising a concave primary mirror having a central aperture and a radius, the primary mirror having one of a parabolic, non-parabolic conical, or aspherical surface; a convex secondary mirror facing the primary mirror, the secondary mirror having an aspherical surface, wherein an optical axis extends from a vertex of the primary mirror to a vertex of the secondary mirror; a concave tertiary mirror arranged behind the primary mirror, the tertiary mirror having one of a parabolic, non-parabolic conical, or aspherical surface; a concave quaternary mirror arranged in the central aperture of the primary mirror or behind the primary mirror, the quaternary mirror having one of a spherical, parabolic, non-parabolic conical, or aspherical surface; at least one image plane having one or more aggregated sensors, wherein the image plane is positioned at a radial distance from
• the second image plane and the third image planes may each be positioned close to the primary mirror at a radial distance from the optical axis that is greater than the radius of the primary mirror.
• the first beam splitter and the second beam splitter may be each positioned between the primary mirror and the secondary mirror.
• the first beam splitter and the second beam splitter may each have a tilt angle ranging from 65 to 115 degrees with respect to the optical axis.
• the optical system may additionally comprise an exit pupil or Lyot stop positioned at one of: 1) near the second and third quaternary mirrors, 2) between the second and third tertiary mirrors and the second and third quaternary mirrors, and 3) between the second and third quaternary mirrors and the second and third image planes, and wherein intermediate focuses may be formed near the first folding mirror and the second folding mirror.
• a reflective and cata-dioptric optical system comprising a concave primary mirror having a central aperture and a radius, the primary mirror having one of a parabolic, non-parabolic conical, or aspherical surface; a convex secondary mirror facing the primary mirror, the secondary mirror having an aspherical surface, wherein an optical axis extends from a vertex of the primary mirror to a vertex of the secondary mirror; a concave tertiary mirror arranged behind the primary mirror, the tertiary mirror having one of a parabolic, non-parabolic conical, or aspherical surface; a concave quaternary mirror arranged in the central aperture of the primary mirror or behind the primary mirror, the quaternary mirror having one of a spherical, parabolic, non-parabolic conical, or aspherical surface; a main folding mirror or beam splitter configured to receive light rays from the quaternary mirror;
• the reflective and cata-dioptric optical system may further comprise a first beam splitter and a second beam splitter configured to separate specific spectral ranges of light rays, wherein the first beam splitter and the second beam splitter may have opposing tilt angles with respect to each other, and wherein the first beam splitter and the second beam splitter may each receive light rays reflected by the secondary mirror; a first folding mirror receiving light rays from the first beam splitter and a second folding mirror receiving light rays from the second beam splitter; a first group of lenses receiving light rays from the first folding mirror and a second group of lenses receiving light rays from the second folding mirror; and a third image plane receiving light rays from the first group of lenses and a fourth image plane receiving light rays from the second group of lenses, the third and fourth image planes may each have one or more aggregated sensors that convert light into electrical signals.
• the third image plane and the fourth image plane may each comprise a commercially available sensor, and wherein a focal length of the first group of lenses and the second group of lenses may each be adjusted independently to match an optical resolution of each of the first group of lenses and the second group of lenses to a pixel size of each of the commercially available sensor.
• the first group of lenses and the second group of lenses may each comprise lenses having spherical or aspherical surfaces.
• the first beam splitter and the second beam splitter may each be positioned between the primary mirror and the secondary mirror.
• the first beam splitter and the second beam splitter may each have a tilt angle ranging from 65 to 115 degrees with respect to the optical axis.
• the reflective and cata-dioptric optical system may additionally comprise an exit pupil or Lyot stop positioned in the first group of lenses and the second group of lenses and before the third and fourth image planes, and wherein an intermediate focus may be formed near the first folding mirror and the second folding mirror.
• the lenses of the first group of lenses and the second group of lenses may be radiation hardened or resistant.
• the folding mirrors may perform scanning to cover a field of view of the optical system with a smaller number of sensors than when the folding mirrors do not perform the scanning.
• the reflective and cata-dioptric optical system may further comprise an inertial measurement unit connected to the first and second folding mirrors to compensate for unwanted motion of the system by stabilizing the line of sight of the system or the instantaneous field of view of image sensors positioned at the third and fourth image planes.
• the reflective and cata-dioptric optical system may further comprising a first beam splitter, a second beam splitter and a third beam splitter configured to separate light rays of a specific spectral range, and wherein the first, second and third beam splitters may each receive light rays reflected by the secondary mirror; a first folding mirror receiving light rays from the first beam splitter, a second folding mirror receiving light rays from the second beam splitter, and a third folding mirror receiving light rays from the third beam splitter; a first group of lenses receiving light rays from the first folding mirror, a second group of lenses receiving light rays from the second folding mirror, and a third group of lenses receiving light rays from the third folding mirror; and a third image plane receiving light rays from the first group of lenses, a fourth image plane receiving light rays from the second group of lenses, and a fifth image plane receiving light rays from the third group of lenses, the third, fourth and fifth image planes may each
• the reflective and cata-dioptric optical system may further comprise a first beam splitter configured to separate light rays of a specific spectral range, wherein the first beam splitter may receive light rays reflected by the secondary mirror; a first folding mirror receiving light rays from the first beam splitter; a first group of lenses receiving light rays from the first folding mirror, wherein the first group of lenses may include a second beam splitter or a reflective polarizer; and a third image plane receiving light rays from a first path in the first group of lenses and a fourth image plane receiving light rays from a second path in the first group of lenses, the first and second paths being based on the second beam splitter or the reflective polarizer, the third and fourth image planes may each have one or more aggregated sensors that convert light into electrical signals.
• the reflective and cata-dioptric optical system may further comprise a first beam splitter configured to separate light rays of a specific spectral range, wherein the first beam splitter may receive light rays reflected by the secondary mirror; a first folding mirror receiving light rays from the first beam splitter; a first group of lenses receiving light rays from the first folding mirror, wherein the first group of lenses may include filter wheels or sliders comprising narrow-band spectral filters within a spectral range defined by the first beam splitter; and a third image plane receiving light rays from the first group of lenses, the third image plane may have one or more aggregated sensors that convert light into electrical signals.
• the reflective and cata-dioptric optical system may further comprise a first beam splitter configured to separate light rays of a specific spectral range, wherein the first beam splitter may receive light rays reflected by the secondary mirror; a first folding mirror receiving light rays from the first beam splitter or first filter wheels or sliders arranged between the first beam splitter and the folding mirror; a first group of lenses receiving light rays from the first folding mirror, wherein the first group of lenses may include second filter wheels or sliders when the first filter wheels or sliders may not be utilized, the first filter wheels or sliders and the second filter wheels or sliders may comprise polarizers at zero, 45, 90 and 135 degrees for polarimetric imaging; and a third image plane receiving light rays from the first group of lenses, the third image plane may have one or more aggregated sensors that convert light into electrical signals.
• the reflective and cata-dioptric optical system may further comprise eight beam splitters configured to separate light rays of a specific spectral range, and wherein each of the eight beam splitters may respectively receive light rays reflected by the secondary mirror; eight folding mirrors receiving light rays from respective beam splitters; eight groups of lenses receiving light rays from respective folding mirrors; and eight further image planes receiving light rays from respective groups of lenses, the eight further image planes may each have one or more aggregated sensors that convert light into electrical signals, wherein the eight further image planes may each be positioned close to the primary mirror at a radial distance from the optical axis that is greater than the radius of the primary mirror.
• FIGS. 1A and 1B are schematics of an embodiment of an optical system that may be used for imaging.
• Figures 1C and 1D are schematics of another embodiment of an optical system that may be used for imaging.
• Figures 1E and 1F are schematics showing diagonals for respectively a periodic mirror and an image plane.
• Figures 1G and 1H show example embodiments of optical systems having a periodic primary mirror.
• Figure 2A is a block diagram showing a schematic of an embodiment of a payload system for a satellite that may include the various optical systems described herein.
• Figure 2B is block diagram showing a schematic of an embodiment of an image plane circuit that may be used with the various optical systems described herein.
• Figures 3-5 are schematics showing various embodiments of configuration layouts for mirrors and an imaging plane that may be used with the various optical systems described herein.
• Figures 6-9 are schematics showing various embodiments of configuration layouts for mirrors, including one or more folding mirrors, and an imaging plane, that may be used with the various optical systems described herein.
• Figures 10-13 are schematics showing various embodiments of configuration layouts for mirrors, including one or more folding mirrors, and two imaging planes, that may be used with the various optical systems described herein.
• Figures 14A-14D are various views of an embodiment of a camera system that includes the optical system of Figure 1.
• Figures 15A-17B are graphical plots showing various embodiments of performance characteristics for the optical system of Figure 1A.
• Figures 18A-20B are graphical plots showing various embodiments of performance characteristics for the optical system of Figure 1C.
• Figures 21A and 21B are graphical plots showing distortion performance for, respectively, the optical systems Figures 1A and 1C.
• Figure 22 is a schematic showing an embodiment of a configuration layout for mirrors, including two beam splitters, two folding mirrors, and three imaging planes, that may be used with the various optical systems described herein.
• Figures 23-28A are schematics showing various embodiments of configuration layouts for mirrors and lenses, including one or more beam splitters, one or more folding mirrors, and two or more imaging planes, that may be used with the various optical systems described herein.
• Figure 28B is a diagram showing a projection of an embodiment of an image plane circuit that may be used with the configuration layout of Figure 28A.
• Figures 29A and 29B are schematics of another embodiment of an optical system that may be used for imaging.
• An optical system 100 is shown in Figure 1A and is one embodiment that may be used for providing high resolution imaging performance in a “micro” or small form factor (volumetric envelope).
• the optical systems may “piggyback” on other missions with existing high bandwidth capabilities.
• a constellation of satellites in orbit may operate in collaboration with each other for coordinated ground coverage.
• the orbits of the satellites in the constellation may be synchronized.
• the orbits may be geostationary, where the satellites may have orbital periods equal to the average rotational period of Earth and in the same direction of rotation as Earth.
• the orbits may be sun-synchronous, such as a nearly polar orbit around Earth, in which the satellite passes over any given point of the Earth’s surface at the same local mean solar time or the orbit precesses through one complete revolution each year so it always maintains the same relationship with the Sun.
• Synchronous systems introduce complexity by requiring dedicated platforms and sensors, launchers, and operation stations.
• typical examples of such synchronized constellations include the programs of PLANETSCOPE (a.k.a. DOVE), SKYSAT, BLACKSKY, and CARBONITE.
• PLANETSCOPE a.k.a. DOVE
• SKYSAT SKYSAT
• BLACKSKY BLACKSKY
• CARBONITE CARBONITE
• any of the optical systems or features thereof described herein may include any of the features of the micro optical and camera systems and other aspects described in “Study on the feasibility of micro camera systems for asynchronous, gigantic satellite constellation”, by Youngwan Choi, Proc. SPIE 11127, Earth Observing Systems XXIV, 111270Z (9 September 2019, available at https://doi.org/10.1117/12.2529090), the entire contents of which are incorporated by reference herein in their entirety.
• An asynchronous constellation may include camera systems onboard any available platforms, which have planned missions but can host additional payloads. It may be different from the nominal constellation in the sense that it will not be operated synchronously and not provide coordinated ground coverage with the sole purpose of only providing a stream of images.
• the most significant benefit of the asynchronous constellation is to avoid or minimize cost, time, and effort to develop a platform, require a specific launch system, and operate a dedicated ground control system, which can be a large fixed cost.
• An advantage of leveraging LEO broadband data relay satellites for asynchronous constellation imaging is its broad data bandwidth. CUBESATs or other platforms with dedicated imaging or other missions may suffer from decreased data bandwidth.
• asynchronous constellation with the LEO data relay satellites can stream image data in dedicated channels as the satellites stream movies or other content so that users can selectively receive, record, and process image data.
• much smaller or micro camera systems that have dimensional advantages and can accommodate themselves to any available space are needed.
• Recent developments of smaller cameras focuses on dimensional advantages only so that such development relies on optical designs that are easier to design, simpler to develop, or cheaper to build.
• optical system 100 and the other embodiments of imaging systems described herein may be used for constellation operations and be micro in physical dimension as well as be advanced in performance.
• the embodiments described herein may be onboard satellite platforms that are already planned, as a secondary payload or an additional system.
• the imaging system may have a size on the scale of a star sensor or tracker.
• the imaging system may be lightweight.
• the imaging system may minimize power consumption.
• the imaging system and its interface to a platform may be simple so that it can be installed and operated easily.
• the imaging system may be capable of proper imaging, which may be described by its specification.
• the imaging system may have proper MTF values.
• the imaging system may be designed to operate over a wide spectral range and equipped with a number of channels over the spectral range, with panchromatic, red, green, blue, and near infrared as a baseline set.
• the imaging system may be capable of a large field of view.
• an important requirement is distortion property.
• TDI time-delay-integration
• SNR signal-to-noise ratio
• the optical imaging systems described herein are based on a reflective or mirror system, which may be unusual for a small form, affordable system.
• Usual cameras for CAN- or NANO-SAT are based on a cata-dioptric design for its design simplicity and cost reduction.
• the examples are PLANETSCOPE (a.k.a. DOVE), SKYSAT, BLACKSKY, and CARBONITE.
• the design of the SKYSAT camera is based on a Ritchey-Cassegrain telescope, which has two mirrors (primary and secondary) and a small number of lenses.
• the CARBONITE camera is an example of commercially available, off-the-shelf astronomical telescope, which is modified to be accommodated to space environment, and equipped with a commercial CMOS sensor for color video imaging. Utilizing a commercial telescope seemed to be a smart move in a sense that development or manufacturing effort can be reduced, cost can be cut seriously, and operation management can be efficient. Whole processes were developed suitable for implementing constellation of Earth observation satellites. [0063] Different from those approaches, the optical system embodiments described herein for cameras are based on a reflective design that is a four-mirror system. The optical system described herein may have no limit of spectral range to be covered.
• Figure 1A is a perspective schematic view of an optical layout of a first optical system 100 showing optical path lines.
• Figure 1B is a perspective schematic view of the optical system 100 without the optical path lines shown for clarity. The optical lines may be indicative of multiple spectral bands.
• Figure 1C a perspective view of an optical layout of a second optical system 150 showing optical lines is illustrated.
• Figure 1D shows the optical system 150 without the optical lines for clarity.
• the first two mirrors of the optical systems 100, 150, a primary mirror 104 and a secondary mirror 105 in Figures 1A and 1B, and a primary mirror 154 and a secondary mirror 155 in Figures 1C and 1D, are responsible for power of the systems so that it can determine its effective focal length or resolution.
• Effective focal length as used herein has its usual and customary meaning, and includes without limitation the distance from a principal plane of an optical mirror to an imaging plane 118, 168.
• the entrance pupil may be the optical image of the physical aperture stop, as seen through the front (the object side) of the optical system.
• the corresponding image of the aperture as seen through the back of the optical system is called the exit pupil.
• the primary mirrors 104, 154 may be supported by a structural support 102 having radially extending beam 103 to support the mirror structure.
• the structure 102 and beams 103 may minimize the distortion on the primary mirror surface that may be induced by bonding and thermal environmental change. Also, it may protect the primary mirror from random vibration and shock that the camera may experience during launch.
• the various mirrors and supporting structures for any of the optical systems described herein may be formed of aluminum, ceramics, designed composite materials, other suitable materials, or combinations thereof.
• the one or more structures and/or the one or more mirrors can be manufactured by 3D printing technology also known as additive manufacturing technology.
• the mirrors and the supporting structure may all be additively manufactured as one monolithic piece.
• a tertiary mirror 113 in Figures 1A and 1B, and a tertiary mirror 163 in Figures 1C and 1D contribute to widening a field of view (FOV) and corrects corresponding residual optical aberrations.
• the tertiary mirrors 113, 163 may not include an optical axis, for example for simpler manufacturability, and two or more tertiary mirrors may be manufactured from one base piece.
• a quaternary mirror 114 in Figures 1A and 1B, and a quaternary mirror 164 in Figures 1C and 1D, may minimize distortion and control a back focal length.
• “Back focal length” as used herein has it usual and customary meaning, and incudes without limitation the distance between the last surface of an optical mirror to its image plane.
• the fields of view of the optical systems 100, 150 are designed so that the rays do not interfere with the respective quaternary mirror and the central aperture of the respective primary mirror.
• the quaternary mirrors 114, 164 reflect the respective light along the optical path to the imaging plane 118, 168.
• Figure 1D shows the second optical system 150 but without showing the optical path lines for clarity.
• the diameter of the aperture or central hole 110 in Figure 1B and 160 in Figure 1D is minimized to maximize the use area of the primary mirror and in some embodiments is not larger than the corresponding secondary mirror 105, 155.
• the diameter of the central hole 110 in Figure 1B and hole 160 in Figure 1D may be designed large enough to not interfere with the light rays travelling through the central holes 110 and 160.
• the primary mirrors 104, 154 and/or the secondary mirrors 105, 155 may be symmetric or periodic about the respective optical axis.
• Figures 1E and 1F are schematics showing diagonals for respectively a periodic mirror and an image plane. The diagonal of a periodic mirror may have an angle of zero degrees or 45 degrees from a diagonal of the image plane.
• FIGS 1G and 1H show example embodiments of optical systems 170, 190 respectively having a periodic primary mirror 174, 194.
• the optical systems 170, 190 further include, respectively, a secondary mirror 175, 195, a tertiary mirror 173, 193, a quaternary mirror 184, 198 and an imaging plane 189, 199.
• the optical systems 170, 190 may have the same or similar features and/or functions as the optical systems 100 or 150.
• the optical systems 100, 150 may include any of the same or similar features and/or functions as the other embodiments of optical systems described herein, and vice versa.
• the optical systems 100, 150 may include any of the same or similar features and/or functions as optical systems 210, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1430, 1460, and vice versa.
• the primary mirror may be concave and have a central aperture.
• the primary mirror may have a parabolic surface, a non-parabolic conical surface, or an aspherical surface.
• a “parabolic surface” as used herein has its usual and customary meaning, and includes, without limitation, a reflective surface used to collect the light energy and may have a shape that is part of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis.
• a “non-parabolic conical surface” as used herein has its usual and customary meaning, and includes, without limitation, a curve rotated about its axis where the curve is obtained as the intersection of the surface of a cone with a plane other than a parabola.
• the “non-parabolic conical surface” may be hyperbolic, elliptical, or circular.
• the secondary mirror may be convex and face the primary mirror.
• the secondary mirror may have an aspherical surface.
• the tertiary mirror may be concave and arranged behind the primary mirror. “Behind” may refer to a side of the primary mirror that is opposite the side of the primary mirror that reflects incoming light to the secondary mirror.
• the tertiary mirror may have a parabolic surface, a non-parabolic conical surface, or an aspherical surface.
• the quaternary mirror may be concave and arranged in the central aperture of the primary mirror, before the primary mirror or behind the primary mirror, for example as shown in Figure 3.
• the quaternary mirror may have one of a spherical surface, a parabolic surface, a non-parabolic conical surface, or an aspherical surface.
• the optical axis may be defined as a geometric reference line extending between the vertices of the primary and secondary mirrors.
• the vertex for a given mirror may be a point on the mirror's surface where the principal axis meets the mirror.
• the optical system 100 may have a larger primary mirror 104 and thus higher resolution relative to the primary mirror 154 of the optical system 150.
• the resolution of the optical system 100 may be better than 1 m at 500 km altitude.
• the optical system 150 may have a resolution of better than 2 m at 500 km altitude.
• the optical system 150 may have a larger field of view (FOV) than the optical system 100.
• the optical system 100 may have a narrower field of view (FOV) relative to the optical system 150.
• the optical system 100 may have volumetric dimensions of 200 mm (W) x 200 mm (H) x 250 mm (L).
• the optical system 150 may have volumetric dimensions of 100 mm (W) x 100 mm (H) x 150 mm (L).
• the optical system 150 may be lighter in weight than the optical system 100.
• the optical systems 100, 150 may both have a proper MTF for higher resolution imaging.
• Both the optical systems 100, 150 may have similar mirror types and optical paths. But their respective purposes and missions may be different.
• the purpose of the optical system 100 may be to map the surface of the Earth and acquire geospatial data.
• the purpose of the optical system 150 may be for remote sensing and environmental monitoring.
• the optical systems 100, 150 may achieve various parameters for orbital systems and/or imaging systems. Example parameters achievable with the optical systems 100, 150 are described in Table 1. For example, the design orbit may be set to 500 km, the spectral bands may be designed to be compatible with big satellites and scientific satellite imaging except the panchromatic band, etc.
• FIG. 1 is a block diagram of an example payload system 200 configuration for an optical system 210 in a satellite.
• the optical system 210 is shown in schematic form.
• the optical system 210 includes a concave primary mirror 204 having a central aperture 212.
• the primary mirror may have one of parabolic, non-parabolic conical or aspherical surface.
• a smaller convex secondary mirror 205 faces the primary mirror 204 and has an aspherical surface.
• the secondary mirror may have an aspherical surface..
• a concave tertiary mirror 213 is arranged behind the primary mirror 204.
• the tertiary mirror may have one of parabolic, non-parabolic conical or aspherical surface.
• a concave quaternary mirror 214 is arranged slightly behind the central aperture 212 of the primary mirror 204, where the quaternary mirror can have one of a spherical, parabolic, non-parabolic conical or aspherical surface.
• the primary mirror 204, the tertiary mirror 213 and the quaternary mirror 214 each have positive power or focal length, and the secondary mirror 205 has negative power. “Behind” may be defined as described above.
• An image sensor 216 having up to ‘n’ aggregated sensors that convert light into electrical signals is positioned behind the primary mirror 204.
• the image sensor 216 may deliver an output format of thirty-two sub-LVDS (low-voltage differential signaling) channels of digital data across an interface 218 to a control and processing electronics portion 220 of the satellite. In other embodiments, other output formats are used.
• the sensor 216 includes a readout integrated circuit (ROIC) used for infrared, visible, and other arrayed sensors.
• ROIC readout integrated circuit
• the functions supported by the ROIC include processing and shaping of an image signal and may include unit cell preamplifiers.
• Interface 218 also includes control signals from the control and processing electronics 220, where the control signals may include a serial peripheral interface (SPI) and a clock signal in some embodiments.
• SPI serial peripheral interface
• a data formatting and distribution subsystem 224 receives the data across the interface 218 and then further sends the data to a data processing with machine learning subsystem 226 and to a data storage and archiving subsystem 230 to be stored.
• the stored data from the data storage and archiving subsystem 230 can be sent directly to the data processing subsystem 226 for various types of processing.
• the output of processed data from the data processing subsystem 226 can be sent directly to the data storage and archiving subsystem 230 for storage.
• the output of processed data from the data processing subsystem 226 and data from the data storage and archiving subsystem 230 can be sent to a data formatting, encryption and transmission subsystem 228.
• the output of the data formatting, encryption and transmission subsystem 228, such as image data, is then sent to a satellite Bus for further distribution, which may include transmission to an earth station, relay satellite, or other entity that receives the data.
• the data processing subsystem 226 may include one or processors and one or more memories, such as a memory for program instructions and a memory and/or a cache for data.
• a payload control electronics subsystem 222 receives telecommands from the satellite Bus and provides housekeeping data to the satellite Bus.
• the payload control electronics subsystem 222 provides commands to portions of the payload system 200, including to the image sensor 216 and/or to a thermal control, temperature data and optical focusing subsystem 234.
• the thermal control, temperature data and optical focusing subsystem 234 provides control signals, such as thermal control and optical focusing, via an interface 232 to the optical system 210 and receives temperature data back from the optical system 210.
• a power conversion, distribution and telemetry subsystem 236 receives telecommands from the satellite Bus and provides telemetry data to the satellite Bus.
• the power conversion, distribution and telemetry subsystem 236 may also receive power, such as from solar panels or batteries of the satellite.
• the sensors for the optical systems described herein such as the aggregate sensor of the sensor 216 in optical system 210, may be developed and tailored to space applications and their consistency and stability may be validated.
• the optical system 210 and all the optical systems described herein may be calibrated according to the standard processes and with respect to each other so that all the image data from the systems is compatible with each other and also with reference systems.
• FIG. 2B an example embodiment of a sensor circuit for the image sensor 216 is shown.
• the image sensor 216 may include a readout integrated circuit (ROIC) 272 and a charged coupling device (CCD) array 270.
• ROIC readout integrated circuit
• CCD charged coupling device
• Photons incident on the surface of the CCD array 270 generate a charge that can be read by electronics and turned into a digital copy of the light patterns falling on the device.
• a charge coupled device in complementary metal–oxide– semiconductor (CCD-in-CMOS) time delay integration (TDI) sensor from IMEC International may be used for the optical system 210, even though a pixel size of 5 micrometers ( ⁇ P) is preferred.
• a backside illumination sensor uses a format of 4096 columns and 256 stages per multiband CCD array 270, combines a TDI CCD array with CMOS drivers and readout pixels on a pitch of 5.4 ⁇ m.
• An on-chip control and sequencer circuit may be included.
• a 130 MHz clock 262 may be an input to the image sensor along with a serial peripheral interface (SPI) for control.
• the imager may interface through the SPI and may integrate on-chip PLLs to deliver an output format of 32 sub-LVDS (low-voltage differential signaling) channels as part of the ROIC 272.
• a seven-band version of the circuit can contain seven CCD arrays of 4096 x 256 pixels each.
• other sensor circuits may be used for the image sensor 216, which may have differing sizes of the array 270 and a different ROIC 272 for the output of data.
• the image sensor 216 may include four panchromatic CCD arrays of 16384 x 96 pixels each and eight multispectral CCD arrays of 8192 x 48 pixels.
• backside illumination technology can be used. This consists of bonding the sensor wafer to a carrier wafer and thinning it from the backside. This directly exposes the CCD gates to the light, without obstruction of metal lines. An effective fill factor thus reaches 100% percent.
• Backside illuminated CMOS imagers feature a very high intrinsic light sensitivity and are very efficient in detecting (near) ultraviolet and blue light.
• ARCs antireflective coatings
• image quality is sensitive to platform motion, which can be represented by image smear MTF.
• the image smear MTF of the optical system 210 may be 0.974 with smearing of 0.2 pixels, the number of TDI steps at 128, and a clocking phase of 4. This may impose requirements of attitude stability of the platform, which may be twenty-two micro-radians per second ( ⁇ rad/sec) or 4.54 arcseconds per second (arcsec/sec). When the attitude stability requirement is relaxed to smearing of one pixel, then the smear MTF becomes 0.75 and the stability may be 23 arcsec/sec.
• the optical design of the optical system 300 may be different from a traditional Three Mirror Anastigmat TMA or three mirror Korsch design.
• the Korsch design may have an ellipsoid surface for the primary mirror, a hyperbola surface for the secondary mirror, and an ellipsoid surface for the tertiary mirror.
• the optical system 300 includes a concave primary mirror 304 having a central aperture 310, where the primary mirror can have one of a parabolic, non-parabolic conical, or aspherical surface.
• a smaller convex secondary mirror 305 faces the primary mirror 304 and has an aspherical surface.
• a concave tertiary mirror 313 is arranged behind the primary mirror 304, where the tertiary mirror can have one of a parabolic, non-parabolic conical or aspherical surface.
• a concave quaternary mirror 314 is arranged in front of the central aperture 310 of the primary mirror 304, where the quaternary mirror can have one of a spherical, parabolic, non-parabolic conical or aspherical surface.
• the primary mirror 304, the tertiary mirror 313 and the quaternary mirror 314 each have positive power or focal length, and the secondary mirror 305 has negative power.
• An image plane 316 having one or more aggregated sensors that convert light into electrical signals is positioned behind the primary mirror 304.
• the image plane 316 is positioned at a specific distance from an optical axis that is defined by a mechanical symmetry around a line through the vertices of the primary and the secondary mirrors, which may define the “optical axis.”
• the specific distance is within the physical radius (from the optical axis) of the primary mirror.
• the image plane will not exceed a cylindrical envelope that is defined by the radius from the optical axis of the primary mirror.
• the radius of the primary mirror may extend perpendicularly from the principal axis of the mirror to an outermost edge of the mirror.
• the principal axis may be a geometric reference line going through the center of the mirror that is exactly perpendicular to the surface of the mirror.
• the optical system 300 uses the secondary mirror 305 that is symmetric around the optical axis.
• the tertiary mirror 313 can have a segmented non-circular shape.
• the quaternary mirror 314 can have a circular or non-circular shape.
• the primary mirror 304 can have a circular or a non-circular shape, where the latter is to enhance modulation transfer function (MTF) and signal to noise ratio (SNR).
• MTF modulation transfer function
• SNR signal to noise ratio
• a circular shape is inscribed to a non- circular shape, which may be periodic about the optical axis.
• the incircle may be the shape of a primary mirror for traditional optical system design. If the radius of the incircle is “r,” then the area of the square will be larger by 4/pi. This is not usually an issue for a larger camera for which a large volume is allocated. However, for a small satellite, which is usually a cuboid, a primary mirror in a square shape can have a larger area by 4/pi and boost MTF and SNR. [0095] Both Korsch and other four-mirror optical designs do not use a parabolic surface for the primary and/or the tertiary mirrors.
• the optical system 300 can provide a unique, affordable solution to a mission with budget constraints.
• a parabolic surface a general test setup can be used for manufacturing, or a stitching measurement is possible.
• a commercial product line can be used for parabolic mirror manufacturing, especially when mirrors are smaller than 300 mm.
• non-parabolic conic or aspherical surface may require a dedicated test tool, including computer generated hologram (CGH) or nulling optics.
• CGH computer generated hologram
• the primary and the secondary mirrors 304, 305 forming the optical axis are symmetric or periodic about this axis.
• the primary and secondary mirrors face each other.
• the tertiary mirror 313 faces the back of the primary mirror 304 and may be a segmented mirror.
• “segmented mirror” includes its usual and customary meaning and includes, without limitation, an array of smaller mirrors designed to act as segments of a single, larger curved mirror.
• the optical axis of the tertiary mirror 313 may not coincide with a mechanical axis.
• the “mechanical axis” has its usual and customary meaning, and may includes, without limitation, a normal vector at the center or at the edge of the mirror.
• the tertiary mirror 313 is a segment of a larger mirror.
• the optical axis for the tertiary mirror 313 may refer to the optical axis of the larger mirror and the mechanical axis may refer to the axis of the segmented mirror.
• the quaternary mirror 314 faces the tertiary mirror 313 and is positioned to avoid interference with rays from the secondary mirror 305 to the tertiary mirror 313.
• the metering and supporting structure of the mirrors can be a cylindrical tube or a conical baffle of the primary mirror 304, such as those shown and described with respect to FIGS. 14A-14D.
• the cylindrical envelope may be coextensive with a cylindrical structure to limit the specific distance at which the imaging plane is located relative to the optical axis between the primary and secondary mirrors. For example, the location of the imaging plane may be radially limited by the radius of the cylindrical structure.
• the image plane 316 includes one or more sensors, which may be aggregated in an orderly manner.
• An entrance pupil of the optical system 300 can be positioned near the primary or the secondary mirrors 304, 305.
• An intermediate focus is formed around a vertex of the primary mirror 304, positioned between the primary and the secondary mirrors 304, 305, or between the primary mirror 304 and the tertiary mirror 313.
• An exit pupil or Lyot stop can be positioned near the quaternary mirror 314, between the tertiary and the quaternary mirrors 313, 314, or between the quaternary mirror 314 and the image plane 316.
• a “Lyot stop” has its usual and customary meaning and includes, without limitation, an optical stop that reduces the amount of flare which may be caused by diffraction of other stops and baffles in the optical system.
• the Lyot stops may be located at images of the system's entrance pupil and have a diameter slightly smaller than the pupil's image.
• the optical system 300 has a small form factor.
• the form factor is defined as the ratio of 1) a distance between the secondary and tertiary mirrors 305, 313 to 2) an effective focal length of the optical system 300.
• the optical system 300 has a form factor of less than 0.2 and of 0.09 in some embodiments.
• the form factor may be from about 0.09 to 0.2, from about 0.04 to less than 0.25.
• the form factor may be less than 0.25.
• the form factor may have the following values or about the following values: 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, .010, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24 or 0.25.
• the form factor may be less than 0.04, less than 0.05, less than 0.06, less than 0.07, less than 0.08, less than 0.09, less than .010, less than 0.11, less than 0.12, less than 0.13, less than 0.14, less than 0.15, less than 0.16, less than 0.17, less than 0.18, less than 0.19, less than 0.20, less than 0.21, less than 0.22, less than 0.23, less than 0.24, or less than 0.25.
• the optical system 300 has a benefit over the prior art in having a much shorter physical distance from the tertiary mirror 313 to the image plane 316.
• the prior art has quite a long distance between the tertiary mirror and image plane and mandates one or more folding mirrors to fit into a limited dimension. This configuration may lead to difficulty in optical alignment, thermal instability during operation, which may end with performance degradation.
• the optical system 300 because of the small form factor and the short distance between the tertiary mirror 313 and the image plane 316, eliminates unnecessary folding mirrors and simplifies the alignment and assembly and stability of operation.
• the optical system can be designed to have mirrors of zero coefficient of thermal expansion (CTE) materials (such as Zerodur, Fused Silica, Suprasil, Astrostiall, etc.), low-CTE materials (such as Borosilicate glass, like BOROFLOAT, Pyrex, etc.), and mild- CTE materials (such as Crown glass, like NBK7).
• CTE zero coefficient of thermal expansion
• low-CTE materials such as Borosilicate glass, like BOROFLOAT, Pyrex, etc.
• mild- CTE materials such as Crown glass, like NBK7.
• CTE matching a specific combination of mirror and structure materials are used for the optical system.
• Super-invar, invar, or designed composite material can be used for zero-CTE mirror materials.
• Invar, Kovar, ceramics, or designed composite material can be used for low-CTE mirror materials. Titanium, ceramics, or design composite materials can be used for mild-CTE mirror materials.
• a monolithic structure can be used for the optical system as an ultimate solution.
• Mirrors and structures can be made of one material, including aluminum, ceramics, designed composite materials, and is not limited to this list.
• the optical system 400 includes a primary mirror 404, a secondary mirror 405, a tertiary mirror 413, a quaternary mirror 414, and image plane 416.
• the primary mirror 404, secondary mirror 405, tertiary mirror 413, the quaternary mirror 414 and image plane 416 may have the same or similar features and/or functions as, respectively, the primary mirror 304, the secondary mirror 305, the tertiary mirror 313, the quaternary mirror 314 and image plane 316 of the optical system 300, and vice versa.
• the quaternary mirror 414 is located behind the primary mirror 404, but close to an aperture 410 in the primary mirror 404.
• the tertiary mirror 413 is positioned further behind the primary mirror 404 in the optical system 400 than in the optical system 300.
• the tertiary mirror 413 may be positioned a distance behind the primary mirror 404 that is in a range of 20% to 60%, of 30% to 50%, or of 35% to 45%, of the diameter of the primary mirror 404.
• the primary mirror 404 of the optical system 300 in Figure 3 can be manufactured in one body with the tertiary mirror 313 so that the center of mass is closer to the primary mirror 304 and the crosstalk moment of inertia (MOI) of the system can be reduced.
• the optical system 400 of Figure 4 is different from the optical system 300 with regard to effective focal length and field-of- view.
• An advantage of the configuration of the optical system 400 is that placing the quaternary mirror 414 closer to the primary mirror 404 may make it easier to set a Lyot stop on the quaternary mirror 414 and the central hole size or aperture 410 of the primary mirror can be minimized.
• the optical system 500 includes a primary mirror 504, a secondary mirror 505, a tertiary mirror 513, a quaternary mirror 514, and an image plane 516.
• the primary mirror 504, secondary mirror 505, tertiary mirror 513, quaternary mirror 514 and image plane 516 may have the same or similar features and/or functions as, respectively, the primary mirror 304, the secondary mirror 305, the tertiary mirror 313, the quaternary mirror 314 and the image plane 316 of the optical system 300, and vice versa.
• the quaternary mirror 514 is located a distance behind the primary mirror 504 that is greater than a distance between the quaternary mirror 414 and the primary mirror 404 of the optical system 400 (see Figure 4).
• the tertiary mirror 513 is positioned a distance behind the primary mirror 504 that is greater than a distance of the respective corresponding mirrors of the optical system 400. In certain embodiments, the tertiary mirror 513 may be positioned a distance behind the primary mirror 504 that is in a range of 45% to 55% of the diameter of the primary mirror 504.
• the optical system 500 may be designed for much smaller pixel sensors, such as having a pixel size of less than 4 micrometers in certain embodiments.
• the optical system 500 may be different from the optical system 300 with respect to effective focal length and field-of-view.
• the optical system 500 may have have a shorter effective focal length and a wider field-of-view relative to the optical system 300, which may allow the system 500 to include sensors with smaller pixel size. ut it may be relatively closer to an aperture 610 in the primary mirror 604.
• the tertiary mirror 613 is similarly positioned behind the primary mirror 604 as in the optical system 400.
• An added folding mirror 615 receives rays from the quaternary mirror 614 and reflects them to the image plane 616, which is positioned above the folding mirror 615.
• the image plane 616 is positioned to be above and parallel to the optical axis.
• Some embodiments of the optical systems may have a longer system optical path length between the quaternary mirror 614 and the image plane 616 using the folding mirror 615. If the image plane 616 is behind the tertiary mirror 613, the system optical path length is the distance between the secondary mirror 605 and the image plane 616. With using the folding mirror 615, the system optical path length is the distance between the secondary mirror 605 and the tertiary mirror 613.
• the image plane 616 may be positioned to satisfy the requirement of focal length and field-of-view.
• the configuration of the optical system 600 may provide a compact design. Another advantage is that the system 600 may allow for easier installation of a sensor cooler and a radiating plate for the cooler.
• a sensor for the image plane can be positioned closer to the primary mirror supporting structure and the sensor can be held in a more stable way.
• FIG. 7 another embodiment of an all-reflective optical system 700 having a folding mirror 715 is shown.
• the optical system 700 may have the same or similar features and/or functions as the optical system 600, and vice versa.
• the optical system 600 includes a primary mirror 704, a secondary mirror 705, a tertiary mirror 713, a quaternary mirror 714, and an image plane 716, which may have the same or similar features and/or functions as, respectively, the primary mirror 604, the secondary mirror 605, the tertiary mirror 613, the quaternary mirror 614, and the image plane 616 of the optical system 600, and vice versa.
• the quaternary mirror 714 is behind the primary mirror 704, as in the optical system 600, but is close to an aperture 710 in the primary mirror 704.
• the tertiary mirror 713 is similarly positioned behind the primary mirror 704 as in the optical system 600.
• the folding mirror 715 receives rays from the quaternary mirror 714 and reflects them to the image plane 716, which is positioned below the folding mirror 715.
• the image plane 716 is positioned to be below and parallel to the optical axis.
• the optical system 800 may have the same or similar features and/or functions as the optical system 700, and vice versa.
• the optical system 800 includes a primary mirror 804, a secondary mirror 805, a tertiary mirror 813, a quaternary mirror 814, and an image plane 816, which may have the same or similar features and/or functions as, respectively, the primary mirror 704, the secondary mirror 705, the tertiary mirror 713, the quaternary mirror 714, and the image plane 716 of the optical system 700, and vice versa.
• the image plane 816 is closer to the optical axis than the image plane 716 is close to its respective optical axis.
• the quaternary mirror 814 is behind the primary mirror 804 but is further behind it than in the respective corresponding components of the optical system 700.
• the tertiary mirror 813 is positioned further behind the primary mirror 804 than in the optical system 700.
• the folding mirror 815 receives rays from the quaternary mirror 814 and reflects them to the image plane 816, which is positioned below the folding mirror 815.
• the image plane 816 is positioned to be below and parallel to the optical axis.
• the optical system 800 is designed for smaller pixel sensors, which are usually commercial or MIL-STD.
• optical system 800 can utilize up-to-date sensors, including commercial or MIL-STD sensors.
• the optical system 900 may have the same or similar features and/or functions as the optical system 800.
• the optical system 900 includes a primary mirror 904, a secondary mirror 905, a tertiary mirror 913, a quaternary mirror 914, and an image plane 916, which may have the same or similar features and/or functions as, respectively, the primary mirror 804, the secondary mirror 805, the tertiary mirror 813, the quaternary mirror 814, and the image plane 816 of the optical system 800, and vice versa.
• the image plane 916 is closer to the optical axis than the image plane 816 is close to its respective optical axis.
• the quaternary mirror 914 is behind the primary mirror 904, in a similar distance to that of the optical system 800.
• the tertiary mirror 913 is positioned behind the primary mirror 904 in a similar distance to that of the optical system 800.
• the folding mirror 915 receives rays from the quaternary mirror 914 and reflects them to the image plane 916, which is positioned above the folding mirror 915.
• the image plane 916 is positioned to be above and parallel to the optical axis.
• optical system 900 Advantages of the optical system 900 are that a sensor of the image plane can be more stable against vibration and that a cooler with a radiator can be installed in an easier way than in other optical system configurations.
• an embodiment of an all-reflective optical system 1000 having multiple image planes and a folding mirror 1015 is shown.
• the optical system 1000 may have the same or similar features and/or functions as the optical system 600.
• the optical system 1000 includes a primary mirror 1004, a secondary mirror 1005, a tertiary mirror 1013, a quaternary mirror 1014, and a first image plane 1016, which may have the same or similar features and/or functions as, respectively, the primary mirror 604, the secondary mirror 605, the tertiary mirror 613, the quaternary mirror 614 and the image plane 616 of the optical system 600, and vice-versa.
• the first image plane 1016 is a similar distance to the optical axis as the image plane 616 is to its respective optical axis.
• optical system 1000 has a second image plane 1016’ similar to the first image plane 1016.
• the first image plane 1016 can be dedicated to a first spectral range and the second image plane 1016’ can be dedicated to a second spectral range.
• the quaternary mirror 1014 is behind the primary mirror 1004 and is close to an aperture 1010 in the primary mirror 1004 at a distance similar to that in the respective corresponding components of the optical system 600.
• the tertiary mirror 1013 is positioned behind the primary mirror 1004 in a similar distance to that in the respective corresponding components of the optical system 600.
• the folding mirror 1015 receives rays from the quaternary mirror 1014 and reflects some of the rays within a certain spectral range to the first image plane 1016, which is positioned above the folding mirror 1015.
• the folding mirror 1015 may be transmissive to rays within a second different range from that which is reflected.
• the optical system 1000 enables simultaneous multi-color imaging by having the folding mirror 1015 be reflective over the first spectral range and transmissive over the second spectral range.
• the first image plane 1016 is positioned to be above and parallel to the optical axis
• the second image plane 1016’ is positioned to be below and perpendicular to the optical axis on an opposite side of the optical axis as the first image plane 1016.
• the optical system 1100 may have the same or similar features and/or functions as the optical system 1000.
• the optical system 1100 includes a primary mirror 1104, a secondary mirror 1105, a tertiary mirror 1113, a quaternary mirror 1114, a folding mirror 1115 and a first image plane 1116, which may have the same or similar features and/or functions as, respectively, the primary mirror 1004, the secondary mirror 1005, the tertiary mirror 1013, the quaternary mirror 1014, the folding mirror 1015 and the first image plane 1016 of the optical system 1000, and vice versa.
• the first image plane 1116 is positioned at a greater distance to the optical axis than the first image plane 1016 is to its respective optical axis.
• the optical system 1100 has a second image plane 1116’ similar to the first image plane 1116.
• the first image plane 1116 can be dedicated to a first spectral range and the second image plane 1116’ can be dedicated to a second spectral range.
• the quaternary mirror 1114 is behind the primary mirror 1104 and is close to an aperture 1110 in the primary mirror 1104 at a distance similar to that in the respective corresponding components of the optical system 1000.
• the tertiary mirror 1113 is positioned behind the primary mirror 1104 in a similar distance to that in the respective corresponding components of the optical system 1000.
• the folding mirror 1115 receives rays from the quaternary mirror 1114 and reflects some of them to the first image plane 1116, which is positioned below the folding mirror 1115.
• the optical system 1100 enables simultaneous multi-color imaging by having the folding mirror 1115 be reflective over a first spectral range and transmissive over a second spectral range.
• the first image plane 1116 is positioned to be below and parallel to the optical axis
• the second image plane 1116’ is positioned to be below and perpendicular to the optical axis.
• optical system 1100 can use a sensor for the image plane in a larger package.
• a CMOS sensor or a sensor with a ROIC tends to have larger package so that it can embrace more circuits or components that may help minimize readout noise, crosstalk, and blooming.
• FIG 12 another embodiment of an all-reflective optical system 1200 having multiple image planes and a folding mirror 1215 is shown.
• the optical system 1200 may have the same or similar features and/or functions as the optical system 1100.
• the optical system 1200 includes a primary mirror 1204, a secondary mirror 1205, a tertiary mirror 1213, a quaternary mirror 1214, a folding mirror 1215 and a first image plane 1216, which may have the same or similar features and/or functions as, respectively, the primary mirror 1104, the secondary mirror 1105, the tertiary mirror 1113, the quaternary mirror 1114, the folding mirror 1115 and the first image plane 1116 of the optical system 1100, and vice versa.
• the first image plane 1216 is positioned at a shorter distance to the optical axis than the first image plane 1116 is to its respective optical axis.
• the optical system 1200 has a second image plane 1216’ similar to the first image plane 1216.
• the first image plane 1216 can be dedicated to a first spectral range and the second image plane 1216’ can be dedicated to a second spectral range.
• the quaternary mirror 1214 is behind the primary mirror 1204 at a distance greater than in the respective corresponding components of the optical system 1100.
• the tertiary mirror 1213 is positioned behind the primary mirror 1204 at a greater distance than in the respective corresponding components of the optical system 1100.
• the folding mirror 1215 receives rays from the quaternary mirror 1214 and reflects them to the first image plane 1216, which is positioned below the folding mirror 1215.
• the optical system 1200 enables simultaneous multi-color imaging by having the folding mirror 1215 be reflective over the first spectral range and transmissive over the second spectral range.
• the first image plane 1216 is positioned to be below and parallel to the optical axis
• the second image plane 1216’ is positioned to be below and perpendicular to the optical axis.
• the second image plane 1216’ is positioned to be closer to the optical axis than the second image plane 1116’ is close to its respective optical axis.
• the optical system 1200 is designed to utilize smaller pixel sensors for the image plane.
• An advantage of the optical system 1200 is that it can utilize up-to-date sensors, including commercial or MIL- STD sensors.
• the optical system 1300 may have the same or similar features and/or functions as the optical system 1000.
• the optical system 1300 includes a primary mirror 1304, a secondary mirror 1305, a tertiary mirror 1313, a quaternary mirror 1314, a folding mirror 1315 and a first image plane 1316, which may have the same or similar features and/or functions as, respectively, the primary mirror 1004, the secondary mirror 1005, the tertiary mirror 1013, the quaternary mirror 1014, the folding mirror 1015 and the first image plane 1016 of the optical system 1000, and vice versa.
• the first image plane 1316 is positioned at a shorter distance to the optical axis than the first image plane 1016 is to its respective optical axis.
• the optical system 1300 has a second image plane 1316’ similar to the first image plane 1316.
• the first image plane 1316 can be dedicated to a first spectral range and the second image plane 1316’ can be dedicated to a second spectral range.
• the quaternary mirror 1314 is behind the primary mirror 1304 at a distance greater than in the respective corresponding components of the optical system 1000.
• the tertiary mirror 1313 is positioned behind the primary mirror 1304 at a greater distance than that in the respective corresponding components of the optical system 1000.
• the folding mirror 1315 receives rays from the quaternary mirror 1314 and reflects them to the first image plane 1316, which is positioned above the folding mirror 1315.
• the optical system 1300 enables simultaneous multi-color imaging by having the folding mirror 1315 be reflective over the first spectral range and transmissive over the second spectral range.
• the first image plane 1316 is positioned to be above and parallel to the optical axis
• the second image plane 1316’ is positioned to be below and perpendicular to the optical axis.
• the second image plane 1316’ is positioned to be closer to the optical axis than the second image plane 1016’ is close to its respective optical axis.
• FIG. 14A a cross-sectional perspective view of a camera system 1400 having an optical system is illustrated.
• a box 1410 illustrates enclosing of the camera and can be a mechanical interface to a satellite BUS.
• a metering structure 1418 shown as a cone shaped structure, maintains a distance between a primary mirror 1404 and a secondary mirror 1405. The metering structure 1418 may maintain this distance within one micrometer when a temperature changes by 1 oC degree.
• the radius of the cylindrical structure 1408 can be defined by a radius from the portion of the optical axis extending between the primary mirror 1404 and the secondary mirror 1405.
• the inner surface of the curved sidewall of the cylindrical structure 1408 can be a limit of the specific distance from the optical axis for the image plane 316 described above.
• dimensions of the camera are 200 mm x 200 mm x 250 mm. Depending on the focal length of an optical system, the dimensions may range from 75 mm x 75 mm x 100 mm, designed for 5 m resolution at 500 km, to dimensions of 750 mm x 750 mm x 1000 mm, designed for 0.25 m resolution at 500 km.
• the overall volumetric envelope of the camera system may be less than 0.01m3, less than 0.008m3, less than 0.006m3, less than 0.004m3, less than 0.003m3, less than 0.001m3, or from .0005 m3 to 0.01m3.
• the form factor is defined as the ratio of a distance between the secondary and tertiary mirror to a focal length of the optical system. The distance between the secondary and tertiary mirror may be measured along the optical path.
• the optical system can be implemented in a form factor having the values described above, for example of less than 0.2, less than 0.15, or less than 0.1. For the prior art, the form factor is known to be more than 0.25.
• the optical system can provide imaging resolution better than 1 m, 0.5 m, or 0.25 m at 500 km altitude.
• the optical system can also be capable of imaging resolution better than 0.1 m in an elliptical orbit.
• the form factor can be in a range between 0.04 and 0.09. Examples of focal lengths, distances between the secondary mirror and the tertiary mirror for each focal length and a corresponding form factor of the system are provided in Table 2.
• a metering structure 1448 shown as a cone shaped structure, keeps a distance between a primary mirror 1434 and a secondary mirror 1435 to the design within +/- one micrometer when a temperature changes by one Celsius degree.
• temperature sensors and heaters can be installed on the metering structure.
• Payload control electronics reads the data from the temperature sensors and control the heaters to keep the metering structure 1448 within a specified range so that the focus of the camera system is on aggregated sensors.
• a ring structure 1440 is a supporting structure for the primary mirror 1434 and supports the primary mirror kinematically so as to minimize structural distortion that may be induced during assembly. Also, the ring structure 1440 can be an interface to a satellite BUS, which can eliminate the need of a box-type enclosure, such as the enclosure 1410 shown in Figure 14A.
• a partial cross-sectional perspective view of an optical system 1460 for cameras is illustrated.
• a metering structure 1478 shown cone- shaped, maintains a distance between a primary mirror 1464 and a secondary mirror 1465, which may be maintained in some embodiments within ⁇ one micrometer when a temperature of the metering structure 1478 changes by one Celsius degree.
• the supporting structure 1470 shown as a ring-shaped structure, for the primary mirror 1464 supports a primary mirror kinematic mounting structure 1472 so as to minimize structural distortion that may be induced during assembly.
• the supporting structure 1470 can be an interface to a satellite BUS.
• the radius of the supporting structure 1470 can be defined by a physical radius from the optical axis of the primary mirror 1464.
• the inner surface of the ring structure 1470 can be a limit of the specific distance from the optical axis for the image plane 316 described above.
• a diameter of the primary mirror 1464 which is about 7 % of the focal length of the optical system, determines a width and height of the camera system.
• Figure 14D is a partial cross-sectional perspective view of an optical system 1480 showing the cylindrical housing 1408 having a radius 1486 equal to the radius of the primary mirror 1404.
• the imaging plane may be located a radial distance from the optical axis that is no more the radius 1486.
• the housing 1408 may thus also have the same or nearly the same radius as the primary mirror for space savings.
• the optical axis extends between vertices of the primary and secondary mirrors.
• the performance of the optical system 100 and the optical system 150 was analyzed to assess its design Modulation Transfer Function (MTF), tolerance MTF, and its distortion. Even though MTF and distortion is a way to evaluate optical performance of the system, they also indicate how the quality of resulting images will be.
• MTF Modulation Transfer Function
• the MTF of panchromatic band is lower than other big camera systems, which cannot be avoided due to its smaller aperture size. Despite the lower MTF values, image quality can be enhanced by post processing on ground and also benefits by having a smaller anti-aliasing effect.
• the graphs present the optical design MTF and tolerance MTF, respectively, of the panchromatic band for the optical system 100.
• the Nyquist frequencies at which the MTF values are estimated are 100 mm / cycle for the panchromatic band and 25 mm / cycle for the multispectral bands. For tolerancing, sensitivity of each component is studied with assembly and alignment logics considered.
• the graphs present the optical design MTF and tolerance MTF, respectively, of the near-infrared (NIR) band for the optical system 100.
• the graphs present the optical design MTF and tolerance MTF, respectively, of the blue band for the optical system 100.
• the estimated MTF values of optical system 100 are summarized in Table 3.
• the design MTF is higher than 11 % and tolerance value is slightly above 10 %.
• the design values are greater than 57 % and tolerance values are better than 51 %.
• MTF drop is higher in multispectral bands because those are located away from optical axis with their lower sampling frequency reflected.
• Table 3 [0134] Referring to Figures 18A and 18B, the graphs present the analysis results of optical design MTF and tolerance MTF, respectively, of the panchromatic band for the optical system 150.
• the Nyquist frequencies are 100 mm / cycle for the panchromatic band and 25 mm/ cycle for the multispectral bands.
• the graphs present the optical design MTF and tolerance MTF, respectively, of the NIR band for the optical system 150.
• the graphs present the optical design MTF and tolerance MTF, respectively, of the blue band for the optical system 150.
• the estimated MTF values of the optical system 150 are summarized in Table 4. The design MTF of the panchromatic band is higher than 15 % and the tolerance value is greater than 14 %. For the multispectral bands, the results are different from the optical system 100.
• the MTF drops are strange and get much harsher than for the optical system 100.
• the lowest multispectral MTF value is just above 40 % at the outer field and surprisingly at the near- infrared band, which is located closer to optical axis.
• the tolerance values are managed to be higher than 35 %.
• Table 4 [0137] Referring to Figures 21A and 21B, the distortion performance of the optical system 100 and the optical system 150, respectively, is illustrated.
• the distortion magnitude of the optical system 150 is 0.08 micrometer, higher than that of the optical system 100, 0.02 micrometer at the edge, due to its larger field of view.
• the optical system 100 has performance better than other camera systems in constellation operation as shown in Table 5.
• the optical system 100 is designed to have a ground sample distance of 0.9-meter and a swath-width of 10.8 kilometer at 500 kilometer altitude, which are comparable to or better than those of SKYSAT. It should be also highlighted that the optical system 100 can operate a panchromatic band and a near-infrared band simultaneously on the fly, which are optimized compatible with other remote sensing missions and the other cameras identified in Table 5 are lacking.
• the benefits of the optical system 150 over DOVE cameras are better resolution, diverse spectral bands and shorter in axial direction as shown in Table 6.
• the optical system 150 has a ground sample distance of 1.85 meters, which is half resolution of DOVE or PLANETSCOPE.
• the optical system 150 can be equipped with the customized spectral bands, which are essential to extract meaningful spectral information.
• the optical system is based on the 4-mirror all-reflective optical design and is free from chromatic aberration and distortion. Being free from chromatic aberration helps the optical system go beyond the visible spectral range so it can support imaging in the infrared and UV spectral range. Being distortion-free helps the optical system to support TDI imaging in orbit and precision metrics in post-processing. [0141] Some of the prior art, especially the less expensive solutions, still relies heavily on a combination of lenses and mirrors in a catadioptric design, so that limits its application, or its optical design needs be revised from the beginning to adapt to a different spectral range.
• the catadioptric design does not easily embrace TDI imaging, especially for a wider field of view imaging because of inherent or residual aberrations.
• the form factor is smaller compared to bigger, more massive systems of the prior systems.
• the optical system described herein has a much smaller form factor compared to the prior art. It is quite small so that it can be installed on a small flying object, including CUBESAT, minisatellite, airplane, UAV, drone, or balloon. It can also be onboard flying objects as a secondary or tertiary payload, which helps provide diverse missions or more opportunities for missions.
• the optical system is small and lightweight so that it helps reduce launch cost and increase the opportunity of a launch compared to the prior art.
• the optical system can be developed at a lower cost so that it is more affordable than the prior art. In developing the optical system, smaller test equipment and facilities can be used due to the smaller aperture size. Also, the optical system is lightweight, and it can be transported with lower logistics costs. [0143] The developing process of the optical system can be automated more efficiently compared to the prior art. Developing the prior art, which is quite bigger, always mandates labor resources, leading to an increasing budget. For the optical system with its smaller aperture size and being lightweight, iterative or repetitive processes or procedures can be automated, even with affordable equipment.
• the processes may include optical alignment, optical measurements (such as wavefront error, modulation transfer function, a focal length, a field of view, instantaneous field of view, distortion, signal to noise ratio), and those under various conditions.
• optical system can sustain stability in operation because of shorter physical distance among mirrors.
• the optical system is based on the 4-mirror optical design and provides design flexibility that is backed by a degree of freedom of the optical design. With a minimal modification of the optical design, it can be adapted to provide imaging in the modes of starring, scanning or pushbroom, video, stereo, BRDF (Bidirectional Reflectance Distribution Function), HDR (High Dynamic Range), Polarimetric, or low-light.
• BRDF Bidirectional Reflectance Distribution Function
• HDR High Dynamic Range
• Polarimetric or low-light.
• the optical system based on the 4-mirror optical design, can support panchromatic, multispectral, hyperspectral, infrared, and UV imaging with minimal design modification, mainly due to different pixel sizes.
• the optical system has a degree of freedom of optical design and can support super-resolution, high dynamic range, polarimetric, and other remote sensing or scientific imaging.
• the optical system can support planetary or deep space missions, which mandates a small form factor for payload selection.
• the optical system can embrace diverse missions because of its affordability and launch opportunity, which may include AI-based imaging.
• the optical system can be used for a precision star sensor and a stellar sensor.
• the optical system based on the 4-mirror optical design, can support simultaneous multi-color imaging.
• the optical system can include, for example, but is not limited to, panchromatic + RGB + near-infrared, visible + infrared (near-infrared, shortwave infrared, mid-wave infrared, or longwave infrared), visible + visible, infrared + infrared, UV + visible, or UV + infrared imaging.
• the optical system being of the small form factor, can be onboard the satellites of a non-imaging mission, like communication satellites (for example, Starlink of SpaceX).
• the optical system can also be installed on other imaging satellites, quasi-imaging satellites, like SAR mission, or scientific mission satellites.
• the optical system 1500 may include any or all features of the various four mirror designs described herein with respect to the systems 100 to 1300 (as shown in Figs. 1A, 1C and 3-13).
• the optical system 1500 may be further configured for additional multispectral imaging with additional auxiliary mirrors.
• the systems 100 to 1300 are designed in certain embodiments for higher resolution imaging in wide spectral bands, including the visible band and near-infrared band. Simultaneously with the visible and near-infrared imaging, the systems 100 to 1300 are capable of moderate resolution imaging in shortwave infrared, midwave infrared, and longwave infrared, when equipped with customized sensors of the pixel size that is tailored to the focal length ratio with respect to the one for visible band imaging.
• the systems numbered from 100 to 1300 are capable of two spectral-range imaging simultaneously: for example, visible-near infrared and shortwave infrared, visible-near infrared and midwave infrared, shortwave infrared and midwave infrared, midwave infrared and longwave infrared, and etc.
• the system 1500 may add more spectral ranges than the systems 100 to 1300, using auxiliary mirrors including beam splitters and additional tertiary and quaternary mirrors of a four-mirror system.
• the optical system 1500 includes a concave primary mirror 1504 having a central aperture 1510, where the primary mirror may have one of a parabolic, non-parabolic conical or aspherical surface.
• a smaller convex secondary mirror 1505 faces the primary mirror 1504 and has an aspherical surface.
• a concave tertiary mirror 1513 is arranged behind (behind is with respect to an incoming direction of light), the primary mirror 1504, where the tertiary mirror may have one of a parabolic, non-parabolic conical or aspherical surface.
• a concave quaternary mirror 1514 is arranged near the central aperture 1510 of the primary mirror 1504, where the quaternary mirror may have one of a spherical, parabolic, non-parabolic conical or aspherical surface.
• the quaternary mirror 1514 is positioned near the central aperture 1510, facing the tertiary mirror 1513, to accommodate folding mirrors 1522 and 1532 (described below) outside the incident beam radius that is defined by a radius of the primary mirror 1504.
• the primary mirror 1504, the tertiary mirror 1513 and the quaternary mirror 1514 each have positive power or focal length, and the secondary mirror 1505 has negative power.
• An image plane 1516 having one or more aggregated sensors that convert light into electrical signals is positioned behind the primary mirror 1504.
• the image plane 1516 is positioned at a specific distance from an optical axis that is defined by a mechanical symmetry around a line through the vertices of the primary and the secondary mirrors, and line which may define the “optical axis.”
• the optical system 1500 uses a beam splitter 1521 and 1531 to separate specific spectral ranges of the rays destined for the tertiary mirror 1513.
• the beam splitters, 1521 and 1531 are positioned between the primary mirror 1504 and the secondary mirror 1505 and have the opposite tilt angles with respect to each other.
• Each beam splitter may have a tilt angle ranging from 65 to 115 degrees with respect to the optical axis.
• 1521 and 1531, tertiary mirrors, 1523 and 1533, and quaternary mirrors, 1524 and 1534 focus light rays to image planes, 1526 and 1536.
• the image planes, 1526 and 1536, having one or more aggregated sensors that convert light into electrical signals are positioned close to the primary mirror 1504.
• the sets of the auxiliary mirrors including folding mirrors 1522 and 1532, the tertiary mirrors 1523 and 1533, the quaternary mirrors 1524 and 1534, and the image planes 1526 and 1536 are positioned at a specific distance from and around the optical axis in a circumferential direction.
• the image plane 1516 includes one or more sensors, which may be aggregated in an orderly manner.
• An entrance pupil of the optical system 1500 may be positioned near the primary or the secondary mirrors 1504, 1505.
• An intermediate focus is formed around a vertex of the primary mirror 1504, positioned between the primary and the secondary mirrors 1504, 1505, or between the primary mirror 1504 and the tertiary mirror 1513.
• An exit pupil or Lyot stop may be positioned near the quaternary mirror 1514, between the tertiary and the quaternary mirrors 1513, 1514, or between the quaternary mirror 1514 and the image plane 1516.
• the rays reflected by the secondary mirror 1505 impinge on the beam splitters, 1521 and 1531 that redirect the rays to the folding mirrors, 1522 and 1532.
• the rays are reflected by the tertiary mirrors, 1523 and 1533, thirdly and finally the quaternary mirrors, 1524 and 1534, so that the rays reach the image planes 1526 and 1536.
• intermediate focuses are formed near the folding mirrors 1522 and 1532.
• An exit pupil or Lyot stop may be positioned near the quaternary mirrors, 1524 and 1534, between the tertiary mirrors 1523 / 1533 and the quaternary mirrors 1524 / 1534, or between the quaternary mirror 1524 / 1534 and the image plane 1526 / 1536.
• FIG. 23 a schematic of an embodiment of a cata-dioptric and reflective system 1600 is shown.
• the optical system 1600 is based on a four mirror design such as the systems numbered from 100 to 1300 and the system 1500.
• the baseline design of the system 1600 has a primary mirror 1604, a secondary mirror 1605, a tertiary mirror 1613, a quaternary mirror 1614, a folding or beam splitter 1615, and image planes 1616 / 1616’.
• the optical system 1600 is different from the systems 100 to 1300 and 1500 by being capable of additional multispectral imaging with a focal length reducer or optimizer 1623 and 1633 that consists of lenses of spherical or aspherical surfaces.
• the systems 100 to 1300 and the system 1500 are capable of the highest resolution imaging in visible and near-infrared spectral range, the systems utilize customized or tailored sensors to take images simultaneously in shortwave infrared, midwave infrared and longwave infrared.
• the optical system 1600 is capable of multispectral imaging with off-the-shelf or ready-made sensors, and not customized or tailored pixel sensors. To accomplish this, the system 1600 has a series of splitting mirrors that separate the rays of a specific spectral range and then transmit or reflect them.
• the optical system 1600 includes a concave primary mirror 1604 having a central aperture 1610, where the primary mirror may have one of a parabolic, non-parabolic conical or aspherical surface.
• a smaller convex secondary mirror 1605 faces the primary mirror 1604 and has an aspherical surface.
• a concave tertiary mirror 1613 is arranged behind (behind is with respect to an incoming direction of light) the primary mirror 1604, where the tertiary mirror may have one of a parabolic, non-parabolic conical or aspherical surface.
• a concave quaternary mirror 1614 is arranged near the central aperture 1610 of the primary mirror 1604, where the quaternary mirror may have one of a spherical, parabolic, non- parabolic conical or aspherical surface.
• the quaternary mirror 1614 is positioned near the central aperture 1610, facing the tertiary mirror 1613, to accommodate folding mirrors 1622 and 1632 (described below) outside the incident beam radius that is defined by a radius of the primary mirror 1604.
• the primary mirror 1604, the tertiary mirror 1613 and the quaternary mirror 1614 each have positive power or focal length, and the secondary mirror 1605 has negative power.
• An image plane 1616 having one or more aggregated sensors that convert light into electrical signals is positioned behind the primary mirror 1604.
• the image plane 1616 is positioned at a specific distance from an optical axis that is defined by a mechanical symmetry around a line through the vertices of the primary and the secondary mirrors, and which line may define the “optical axis.”
• the optical system 1600 uses beam splitters 1621 and 1631 to separate specific spectral ranges of the rays destined for the tertiary mirror 1513.
• the beam splitters, 1621 and 1631 are positioned between the primary mirror 1604 and the secondary mirror 1605 and have opposite tilt angles with respect to each other.
• Each splitter may have a tilt angle ranging from 65 to 115 degrees with respect to the optical axis.
• groups of lenses 1623 and 1633 focus light rays to the image planes 1626 and 1636.
• the groups of lenses 1623 and 1633 with the image planes 1626 and 1636 are positioned at a specific distance from and around the optical axis in a circumferential direction.
• the groups of lenses, 1623 and 1633 act as focal length optimizers to adjust and optimize the focal lengths to match its optical resolution to the pixel size of an off- the shelf or ready-made sensor.
• the focal length of the first lens group 1623 may be different from the focal length of the second lens group 1633.
• the lenses in 1623 and 1633 are radiation hardened or resistant; for aerial application, commercial lenses are used in combination with radiation resistant lenses for best performance.
• CTE matching with lens materials a combination of titanium, ceramics, aluminum, Kovar®, and designed composite structures are manufactured by additive manufacturing technology.
• the image plane 1616 includes one or more sensors, which may be aggregated in an orderly manner.
• An entrance pupil of the optical system 1600 may be positioned near the primary or the secondary mirrors 1604, 1605.
• An intermediate focus is formed around a vertex of the primary mirror 1604, positioned between the primary and the secondary mirrors 1604, 1605, or between the primary mirror 1604 and the tertiary mirror 1613.
• An exit pupil or Lyot stop may be positioned near the quaternary mirror 1614, between the tertiary and the quaternary mirrors 1613, 1614, or between the quaternary mirror 1614 and the image plane 1616.
• the rays reflected by the secondary mirror 1605 impinge on the beam splitters, 1621 and 1631 that redirect the rays to folding mirrors 1622 and 1632. Then the rays go through the focal length optimizer 1623 and 1633 and reach the image planes 1626 and 1636.
• the beam splitters, 1621 and 1631 With the beam splitters, 1621 and 1631, intermediate focuses are formed near the folding mirrors 1622 and 1632.
• An exit pupil or Lyot stop may be positioned in the lens group 1623 and 1633, before the image plane 1626 / 1636.
• the optical system 1700 includes a primary mirror 1704, a secondary mirror 1705, a tertiary mirror 1713 (not shown for simplicity), a quaternary mirror 1714 (not shown for simplicity), and an image plane 1716 (not shown for simplicity).
• the primary mirror 1704, the secondary mirror 1705, the tertiary mirror 1713, the quaternary mirror 1714 and the image plane 1716 may have the same or similar features and/or functions as, respectively, the primary mirror 1604, the secondary mirror 1605, the tertiary mirror 1613, the quaternary mirror 1614 and the image plane 1616 of the optical system 1600, and vice versa.
• the optical system 1700 includes a focal length optimizer 1733, 1733’ or 1733” to match its optical resolution to the pixel size of an off-the shelf or ready-made sensor.
• the focal length optimizers, 1733, 1733’ and 1733” may have the same or similar features and/or functions as the focal length optimizer 1623 and 1633 of the optical system 1600, and vice versa.
• the focal length optimizers can be positioned to have a tilt angle ranging 65 to 115 degrees with respect to an “optical axis,” which may be defined by a mechanical symmetry around a line through the vertices of the primary and the secondary mirrors, and which line may define the “optical axis”.
• a series of beam splitters may be placed along the optical axis so that additional multispectral imaging may be possible.
• the beam splitters may be positioned to have a tilt angle ranging 65 to 115 degrees with respect to the optical axis.
• folding mirrors 1732, 1732’and 1732’’ direct the rays through the focal length optimizers 1733, 1733’, and 1733” and reach image planes 1736, 1736’ and 1736’’.
• the optical system 1800 includes a primary mirror 1804, a secondary mirror 1805, a tertiary mirror (not shown for clarity), a quaternary mirror (not shown for clarity), and an image plane (not shown for clarity).
• the primary mirror 1804, the secondary mirror 1805, the tertiary mirror, the quaternary mirror and the image plane may have the same or similar features and/or functions as, respectively, the primary mirror 1604, the secondary mirror 1605, the tertiary mirror 1613, the quaternary mirror 1614 and the image plane 1616 of the optical system 1600, and vice versa.
• the optical system 1800 includes a focal length optimizer 1833 to match its optical resolution to the pixel size of an off-the shelf or ready-made sensor.
• the focal length optimizer 1833 may have the same or similar features and/or functions as the focal length optimizer 1623 and 1633 of the optical system 1600, and vice versa.
• the focal length optimizers can be positioned to have a tilt angle ranging 65 to 115 degree with respect to the “optical axis,” which may be defined by a mechanical symmetry around a line through the vertices of the primary and the secondary mirrors, and which line may define the “optical axis.” Adjusting a position and a tilt angle of a beam splitter 1831 and a folding mirror 1832, one or more beam splitters may be placed along the optical axis so that additional multispectral imaging may be possible.
• the beam splitter may be positioned to have a tilt angle ranging 65 to 115 degrees with respect to the optical axis.
• the focal length optimizer 1833 is different from the focal length optimizers 1623 and 1633 of the optical system 1600 in that the focal length optimizer 1833 has an additional branch of lenses to further separate spectral range from the rays going through the focal length optimizer 1833.
• another beam splitter 1835 is placed in the focal length optimizer 1833 and enables additional multispectral imaging simultaneously.
• the focal length optimizer 1833 may be different from the focal length optimizers 1623 and 1633 of the optical system 1600 in that, instead of a beam splitter, the focal length optimizer 1833 may utilize a reflective polarizer 1835 that separates s-polarized rays from p-polarized rays.
• FIGS. 26A and 26B schematics of other embodiments of a cata-dioptric and reflective system 1900 and 2000, respectively, are shown.
• the optical systems 1900 and 2000 include, respectively, primary mirrors 1904 / 2004, secondary mirrors 1905 / 2005, tertiary mirrors (not shown for clarity), quaternary mirrors (not shown for clarity), and image planes (not shown for clarity).
• the primary mirrors 1904 / 2004, the secondary mirrors 1905 / 2005, the tertiary mirrors, the quaternary mirrors, and the image planes may have the same or similar features and/or functions as, respectively, the primary mirror 1604, the secondary mirror 1605, the tertiary mirror 1613, the quaternary mirror 1614 and the image plane 1616 of the optical system 1600, and vice versa.
• the optical systems 1900 / 2000 includes focal length optimizers 1933 / 2033 respectively, to match its optical resolution to the pixel size of off-the shelf or ready-made sensors.
• the focal length optimizers 1933 / 2033 may have the same or similar features and/or functions as the focal length optimizers 1623 and 1633 of the optical system 1600, and vice versa.
• the focal length optimizers can be positioned to have a tilt angle ranging 65 to 115 degree with respect to an “optical axis,” which may be defined by a mechanical symmetry around a line through the vertices of the primary and the secondary mirrors, and which line may define the “optical axis.” Adjusting a position and a tilt angle of beam splitters 1931 / 2031 and folding mirrors 1932 / 2032 respectively, a series of beam splitters may be placed along the optical axis so that additional multispectral imaging may be possible.
• the beam splitters 1931 / 2031 may be positioned to have a tilt angle ranging 65 to 115 degrees with respect to the optical axis. After the beam splitters 1931 / 2031, folding mirrors 1932 / 2032 direct the rays through the focal length optimizers 1933 /2033 to reach image planes 1936 / 2036. [0179] However, the focal length optimizers 1933 / 2033 are different from focal length optimizers 1623 and 1633 in that the focal length optimizers 1933 / 2033 have filter wheels or sliders to use spectral filters or calibration targets. [0180] The filter wheels or sliders 1934 may have narrow-band spectral filters within the spectral range that is defined by the beam splitter 1931.
• the filter wheels or sliders 2034 may have calibration targets that may include transmitting or diffusive targets at a different transmission level or reflectance, respectively.
• a set of spectral diodes may be a reference light source and installed on the lens barrel in a circumferential direction.
• FIGs 27A and 27B schematics of other embodiments of a cata-dioptric and reflective system 2100 and 2200, respectively, are shown.
• the optical systems 2100 and 2200 include, respectively, primary mirrors 2104 / 2204, secondary mirrors 2105 / 2205, tertiary mirrors (not shown for clarity), quaternary mirrors (not shown for clarity), and image planes (not shown for clarity).
• the primary mirrors 2104 / 2204, the secondary mirrors 2105 / 2205, the tertiary mirrors, the quaternary mirrors, and the image planes may have the same or similar features and/or functions as, respectively, the primary mirror 1604, the secondary mirror 1605, the tertiary mirror 1613, the quaternary mirror 1614 and the image plane 1616 of the optical system 1600, and vice versa.
• the optical systems 2100 / 2200 includes focal length optimizers 2133 / 2233 respectively, to match its optical resolution to the pixel size of off-the shelf or ready-made sensors.
• the focal length optimizers 2133 / 2233 may have the same or similar features and/or functions as the focal length optimizers 1623 and 1633 of the optical system 1600, and vice versa.
• the focal length optimizers can be positioned to have a tilt angle ranging 65 to 115 degree with respect to an “optical axis,” which may be defined by a mechanical symmetry around a line through the vertices of the primary and the secondary mirrors, and which line may define the “optical axis.” Adjusting a position and a tilt angle of beam splitters 2131 / 2231 and folding mirrors 2132 / 2232, respectively, a series of beam splitters may be placed along the optical axis so that additional multispectral imaging may be possible.
• the beam splitters 2131 / 2231 may be positioned to have a tilt angle ranging 65 to 115 degrees with respect to the optical axis. After the beam splitters 2131 / 2231, folding mirrors 2132 / 2232 direct the rays through the focal length optimizers 2133 /2233 to reach image planes 2136 / 2236. [0185] However, the focal length optimizers 2133 / 2233 are different from the focal length optimizers 1623 and 1633 in that the focal length optimizers 2133 / 2233 have filter wheels or sliders 2134 / 2234 to use polarizers or hyperspectral filters.
• the filter wheel or slider 2134 or 2134’ may include polarizers at 0, 45, 90, 135 degrees or at other polarization angles for polarimetric imaging. In certain embodiments, the filter wheel or slider 2134’ may be located between the beam splitter 2131 and folding mirror 2132. [0187]
• the filter wheel or slider 2234 may include a Fabry Perot interferometer for hyperspectral imaging, or the filter wheel or slider 2234’ may include a linear variable filter for hyperspectral imaging. In certain embodiments, the filter wheel or slider 2234’ may be located between the beam splitter 2231 and folding mirror 2232.
• FIG. 28A a schematic of another embodiment of a cata- dioptric and reflective system 2300 is shown.
• the optical system 2300 includes a primary mirror 2304, a secondary mirror 2305, a tertiary mirror 2313, a quaternary mirror 2314, a folding mirror or beam splitter 2315, and image planes 2316 / 2316’.
• the primary mirror 2304, the secondary mirror 2305, the tertiary mirror 2313, the quaternary mirror 2314, the folding mirror / beam splitter 2315, and the image planes 2316 / 2316’ may have the same or similar features and/or functions as, respectively, the primary mirror 1604, the secondary mirror 1605, the tertiary mirror 1613, the quaternary mirror 1614, the folding mirror / beam splitter 1615, and the image planes 1616 / 1616’ of the optical system 1600, and vice versa.
• the optical system 2300 includes focal length optimizers 2323 / 2333 respectively, to match its optical resolution to the pixel size of off-the shelf or ready-made sensors.
• the focal length optimizers 2323 / 2333 may have the same or similar features and/or functions as the focal length optimizer 1623 and 1633 of the optical system 1600, and vice versa.
• the focal length optimizers can be positioned to have a tilt angle ranging 65 to 115 degree with respect to the
• An “optical axis” may be defined by a mechanical symmetry around a line through the vertices of the primary and the secondary mirrors, and which line may define the “optical axis.” Adjusting a position and a tilt angle of beam splitters 2321 / 2331 and folding mirrors 2322 / 2332 respectively, a series of beam splitters may be placed along the optical axis so that additional multispectral imaging may be possible.
• the beam splitters 2321 / 2331 may be positioned to have a tilt angle ranging 65 to 115 degrees with respect to the optical axis.
• folding mirrors 2322 / 2332 direct the rays through the focal length optimizers 2323 /2333 to reach image planes 2326 / 2336.
• the optical system 2300 is different from the system 1600 in that the folding mirrors, 2315, 2322, and 2332, may function as a scanning mirror or forward motion compensator.
• the scanning mirror is used to cover the field of view of the optical system 2300 with a smaller number of sensors than the system is designed to have.
• the forward motion compensator may stabilize the line of sight of the optical system 2300 or the instantaneous field-of-view (IFOV) of the image sensors that are positioned at the image planes 2316, 2316’, 2326, and 2336.
• IFOV instantaneous field-of-view
• the folding mirrors 2315, 2322, and 2332 may help in reducing image blur that is caused by an unstable motion of the platform, such as spacecraft, fighters, planes, drones, UAVs, or balloons.
• the folding mirrors, 2315, 2322, and 2332 may be used for low-light level imaging, casting away replacing time delay integration (TDI) sensors that have been used for such a mission.
• TDI time delay integration
• the folding mirrors, 2315, 2322, and 2332 may help in maintaining the line of sight of the optical system 2300 or the instantaneous field-of-view (IFOV) of the image sensors so that the sensors may collect more light for a given time period.
• IFOV instantaneous field-of-view
• This approach may replace pitch maneuvering of a platform, like a spacecraft, drone, or UAV and gimbals.
• the folding mirrors, 2315, 2322, and 2332 may be used to generate pixel shift images.
• FIGS 29A and 29B a schematic of another embodiment of a cata-dioptric and reflective system 2400 is shown.
• Figure 29A illustrates a perspective view of the optical system 2400 and
• Figure 29B illustrates a front view facing a primary mirror 2404 and a set of focal length optimizers 2423, 2433, 2443, 2453, 2463, 2473, 2483, and 2493 described below.
• the optical system 2400 includes the primary mirror 2404, a secondary mirror 2405, a tertiary mirror (not shown for clarity), a quaternary mirror (not shown for clarity), and an image plane (not shown for clarity).
• the primary mirror 2404, the secondary mirror 2405, the tertiary mirror, the quaternary mirror and the image plane may have the same or similar features and/or functions as, respectively, the primary mirror 1604, the secondary mirror 1605, the tertiary mirror 1613, the quaternary mirror 1614 and the image plane 1616 of the optical system 1600, and vice versa.
• the optical system 2400 includes the focal length optimizers 2423, 2433, 2443, 2453, 2463, 2473, 2483, and 2493 to match its optical resolution to the pixel size of an off-the shelf or ready-made sensors respectively.
• the focal length optimizers 2423, 2433, 2443, 2453, 2463, 2473, 2483, and 2493 may have the same or similar features and/or functions as the focal length optimizer 1623 and 1633 of the optical system 1600, and vice versa.
• the focal length optimizers 2423, 2433, 2443, 2453, 2463, 2473, 2483, and 2493 may be positioned to have a tilt angle ranging 65 to 115 degree with respect to and in the circumferential direction of an optical axis, which was defined by a mechanical symmetry around a line through the vertices of the primary and the secondary mirrors, and which line may define the “optical axis.” Adjusting a position and a tilt angle of beam splitters (not specifically referenced for simplicity) 2421, 2431, 2441, 2451, 2461, 2471, 2481, and 2491, and folding mirrors (not all of them referenced for simplicity) 2422, 2432, 2442, 2452, 2462, 2472, 2482, and 2492, the focal length optimizers 2423, 2433, 2443, 2453, 2463, 2473, 2483, and 2493 may find their proper position along the circumference whose diameter is larger than the primary mirror 2404.
• the beam splitters 2421, 2431, 2441, 2451, 2461, 2471, 2481, and 2491 may be positioned to have a tilt angle ranging 65 to 115 degrees with respect to and in the circumferential direction of the optical axis.
• folding mirrors 2422, 2432, 2442, 2452, 2462, 2472, 2482, and 2492 direct the rays through the focal length optimizers 2423, 2433, 2443, 2453, 2463, 2473, 2483, and 2493 to reach image planes 2426, 2436, 2446, 2456, 2466, 2476, 2486, and 2496 (not all of them referenced for simplicity).
• the focal length optimizers 2423, 2433, 2443, 2453, 2463, 2473, 2483, and 2493 may have the same or similar features and/or functions as the focal length optimizer 1833 (Fig.25) of the optical system 1800, and vice versa.
• the focal length optimizers 2423, 2433, 2443, 2453, 2463, 2473, 2483, and 2493 may have the same or similar features and/or functions as the focal length optimizer 1933 / 2033 of the optical system 1900 (Fig. 26A) / 2000 (Fig. 26B), and vice versa.
• the focal length optimizers 2423, 2433, 2443, 2453, 2463, 2473, 2483, and 2493 may have the same or similar features and/or functions as the focal length optimizer 2133 / 2233 of the optical system 2100 (Fig. 27A) / 2200 (Fig. 27B), and vice versa.
• the folding mirrors 2415, 2422, 2432, 2442, 2452, 2462, 2472, 2482, and 2492 may have the same or similar features and/or functions as the folding mirrors, 2315, 2322, and 2332, of the optical system 2300 (Fig.28A), and vice versa.
• Numbers preceded by a term such as “approximately”, “about”, “up to about,” and “substantially” as used herein include the recited numbers, and also represent an amount or characteristic close to the stated amount or characteristic that still performs a desired function or achieves a desired result.
• the terms “approximately”, “about”, and “substantially” may refer to an amount that is within less than 10% of, within less than 5% of, within less than 1% of, within less than 0.1% of, and within less than 0.01% of the stated amount or characteristic.

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