WO2021162669A1

WO2021162669A1 - Device for reduction of off-axis aberrations of optical systems

Info

Publication number: WO2021162669A1
Application number: PCT/UA2020/000075
Authority: WO
Inventors: Roman Duplov
Original assignee: Roman Duplov
Priority date: 2020-08-03
Filing date: 2020-08-03
Publication date: 2021-08-19

Abstract

The invention discloses the device for reduction of off-axis aberrations of imaging optical systems in order to increase the practically useful field of view. This device can be used along with various optical systems, preferably simple ones. It can be placed in front of the optical system as attachable device or embedded in the optical system. The idea is to reduce off-axis aberrations by reduction of the effective aperture for incident off-axis beams depending on the angle of incidence. The device works as angle-dependent aperture stop for the subsequent optical system. This is achieved by the use of special baffles which can be implemented in different ways. For example, these baffles can be implemented in the form of a set of coaxial thin cylindrical tubes of decreasing diameter placed one inside another.

Description

DEVICE FOR REDUCTION OF OFF-AXIS ABERRATIONS OF OPTICAL SYSTEMS

Technical Field

The present invention relates to imaging optical systems. More particularly, this invention relates to imaging optical systems with significant residual off-axis aberrations. The invention is particularly suitable for use in simple optical systems with low number of optical surfaces.

Background Art

Most imaging optical systems comprise of lenses, mirrors, or both. Usually optical sys tems suffer from various optical aberrations which cause unwanted blurring of the image. These aberrations limit the optical resolution of optical systems and the practically useful field of view (FOV). Large optical systems usually require especially good correction of aberrations. The typical sample of a large optical system is the astronomical telescope. The aberrations in astronomical telescopes can be corrected either with the use of combi nations of mirrors in reflecting telescopes, combinations of lenses in refracting telescopes or combinations of both lenses and mirrors in catadioptric telescopes.

However, it is very expensive to make large mirrors. Manufacturing cost of the large lenses is even higher. For this reason the design of a telescope should utilize a minimal possible number of lenses and mirrors. Generally, refracting telescopes are more expensive when compared with reflecting ones because of higher cost of optical materials and expenses in production. Therefore, the reflecting and catadioptric telescopes are most popular ones among professional and amateur astronomers. The Newtonian reflector and various Cassegrain reflecting telescopes are probably the simplest astronomical telescopes. These types of telescopes are widely used in amateur astronomy.

The Newtonian telescope has only one curved optical surface, namely concave parabolic mirror. This telescope has excellent pin-point images in the center of the field of view but optical resolution quickly degrades with the field. So the field of view of the Newtonian telescope is small. The classical Cassegrain reflecting telescope has two curved optical surfaces. The primary concave mirror of the classical Cassegrain telescope is parabolic and the secondary convex mirror is hyperbolic.

However, both the Newtonian and classic Cassegrain telescopes have relatively small field of view due to significant off-axis optical aberrations. The most limiting aberration for these telescopes is coma. In order to correct the coma aberration these telescopes should be modified or equipped with expensive special coma correctors. Typical modi fications of the Newtonian or Cassegrain telescopes require the use of additional lenses. It makes these modified telescopes significantly more expensive and it results in various additional limitations in use.

In 1930 Bernhard Schmidt invented a new telescope which is also referred as the Schmidt camera. His famous telescope became a popular professional astronomical cam era. The Schmidt camera comprises of concave spherical primary mirror and Schmidt aspheric corrector plate. The details of Schmidt design can be found in many textbooks, for example in Modern Optical Engineering, Warren J. Smith, McGraw-Hill, 4rd edition (2008), pp.515-517.

A spherical mirror is probably the cheapest mirror in production. Although the Schmidt corrector plate is not easy in production, it still can be relatively inexpensive when compared with various lenses of similar size. Schmidt camera has extremely wide field of view, along with good optical resolution. This became possible because corrector plate is placed in the center of curvature of the primary mirror. The Schmidt corrector plate works not only as corrector of the spherical aberration but the plate edge is the aperture diaphragm of the telescope. A diaphragm is a thin opaque structure with an opening (aperture) at its center. This aperture diaphragm stops some harmful edge rays from reaching the primary mirror. These specific rays can cause coma and other field optical aberrations of image but they are effectively stopped by the aperture diaphragm, located in the center of curvature of spherical mirror. This is because any line through the center of the curvature of spherical mirror may be considered to be the effective optical axis. For this reason incident off-axis beams are symmetrical and image quality is practically uniform for wide range of angles of incidence.

In other words, the placement of the aperture diaphragm in the center of curvature of concave spherical mirror results in blocking of some unwanted ray beams which degrade the images. The location of the aperture diaphragm in this proper place is the key idea of Schmidt camera.

Since this specifically placed aperture diaphragm stops some rays, it inevitably reduces energy of incident light at all angles, including near on-axis field. The diameter of the primary mirror of Schmidt camera usually is significantly larger than the diameter of the corrector plate. It means that Schmidt corrector reduces the aperture of the telescope. This is important disadvantage because the expensive primary mirror should have much more diameter than aperture of the telescope. So the partial loss of light energy and optical resolution is the price for obtaining the wider field of view.

Thus Bernhard Schmidt presented a new idea of reduction of optical off-axis aber rations in optical systems. Schmidt did not try to correct aberrations by collecting all rays of off-axis incident beams in the focal points, as usual. He just removed those rays of off-axis incident beams which produce coma and some other off-axis aberrations. Preventing most harmful rays from reaching the image surface is very simple way to re duce off-axis aberrations in optical systems, since it does not require additional expensive optical elements like lenses or mirrors. For obtaining this outstanding result Bernhard Schmidt used only single plain diaphragm located in the proper position. It appears to be enough for removing coma as well other most limiting field aberrations for this telescope.

However Schmidt telescope have some important disadvantages. First of all, this optical system has refracting corrector plate. For this reason Schmidt camera is usually significantly more expensive when compared with the Newtonian telescope and classical Cassegrain telescope of the same aperture. In addition the Schmidt camera has a very long tube. The tube of Schmidt telescope is about twice longer than the tube of the Newtonian telescope of similar focal length. The long tube is important drawback of Schmidt camera because long telescopes require much more expensive mount and dome. Moreover, the aperture of Schmidt camera is significantly reduced by aperture diaphragm for complete field of view, as was mentioned above.

Actually Bernhard Schmidt used the new idea of reduction of optical off-axis aber rations only for very specific symmetrical optical system. The present invention utilizes the general idea of removing unwanted rays for reducing off-axis aberrations but use it in a more general and technically different way. Due to this difference the present inven tion can be used for various optical systems and it has some other advantages over the Schmidt design. Disclosure of Invention

Technical Problem An object of the present invention is to provide an inexpensive optical device for reduction of the off-axis aberrations of imaging optical systems in order to improve the off-axis optical resolution and increase the field of view.

Technical Solution An optical system usually has openings or structures that limit the ray bundles, these structures are called stops. These structures may typically be the edge of a lens or mirror, or a fixture, or a diaphragm placed in the optical path to limit the the light admitted by the optical system. The aperture stop is the stop that primarily determines the ray cone and brightness at the image point. The size of the aperture stop determines the illumination at the image. The diameter of the aperture stop is often called aperture. Usually the aperture of an optical system is approximately the same for most incident beams of light. However, the device of the present invention decreases aperture for off-axis beams significantly and approximately gradually, depending on the angle of incidence. In this context we consider incident beams of parallel rays. Since the actual aperture depends on the angle of incidence of beam it is called the effective aperture. The angle of incidence is the angle between an incident ray and optical axis of the system. The angle of incidence is zero if an incident ray is parallel to the optical axis of the optical system.

The field of view of simple optical systems is usually limited by coma or other off- axis aberrations. The idea of the invention is to reduce off-axis aberrations by gradual decrease of the effective aperture of incident off-axis beams with increase in angle of incidence. This result can be obtained with the device of the invention. The device works as the aperture stop for the optical system but the effective aperture of this stop depends on the angle of incidence of the beam. It reduces the effective aperture for off-axis beams. The extent of this reduction depends on the angle of incidence of the beam. It leads to reduction of aperture-dependent off-axis aberrations. The result is better optical resolutions for off-axis images and increasing of FOV of acceptable quality of images. This device only slightly affects nearly on-axis beams but significantly reduces effective aperture for off-axis beams. Beams are the bundles of rays from the object limited by the aperture stop. These bundles are sometimes referred as pencils of ray. The parts of beams can be also considered as beams. In Schmidt camera the aperture is limited by the edge of Schmidt corrector plate. This edge is the aperture stop of this camera. It affects both nearly on-axis and off-axis beams in similar way. By contrast, the device of the present invention affects the incident oblique beams in different way, depending on the angle of incidence.

What is most important is that the aperture stop of conventional optical systems acts like two-dimensional element. It usually limits rays by some edges. In the present invention the three-dimensional structure is used. It limits rays by surfaces but not just edges. This provides an optical designer with additional degree of freedom to control aperture-dependent off-axis aberrations. This structure is located in front of the optical system and works as angle-dependent aperture stop. The effective aperture is sometimes unsymmetrical, i.e. it can be different for tangential and sagittal planes as well as other sections of the incident beams. So the value of the effective aperture can be different for meridional rays and skew rays. It depends on the specific form of said three-dimensional structure. This fact affects the performance of the device of the invention. This is reason for using various forms of said three-dimensional structure.

According to the invention, this three-dimensional structure has a number of parts which block some part of outer rays of incident off-axis beams. Generally, it blocks greater part of rays for beams with greater angle of incidence. Therefore, the effective aperture for off-axis beams is gradually declined with increase in angle of incidence. The rate of reduction of effective aperture with angle of incidence depends on the specific task. The reduction of the effective aperture for various angles of incidence should be chosen in order to reduce the most harmful off-axis aberrations for the specific optical system. For instance, the limiting optical aberration for the Newtonian telescope is usually the third- order coma. This aberration increases linearly with the angle of incidence. On the other hand, this aberration is proportional to the square of the aperture. So, the choice of rate of reduction of the effective aperture allows to reduce spot size of the off-axis images of the Newtonian telescope according to the application conditions. For example, the rate of increase in spot size with the angle in this telescope can be significantly reduced if the reduction of the effective aperture is approximately proportional to the square root of the angle of incidence. In other embodiments the reduction of the effective aperture is approximately proportional to the cubic root of the angle of incidence or other function of the angle of incidence, depending on the specific optical system and intended application.

The device of the invention which is implemented in some form of three-dimensional structure is herein called baffles because the baffles are often used in astronomical tele scopes for blocking unwanted rays. Although the baffles are usually used in astronomy in order to block the scattered rays of light, they used in the present invention for blocking some direct rays from the far objects. In this invention, the dependence of effective aperture on angle of incidence of oblique beams is obtained by the specific design of the baffles. These baffles can be implemented in different forms. Various embodiments of the baffles have different advantages in optical performance, in weight or size, or in cost.

In order to demonstrate the key idea of the present invention, the extremely simplified example is presented with reference to Figs. 1-4. This extremely simplified example consists the baffles in the form of three thin coaxial cylindrical tubes of different heights located inside of the tube of the Newtonian telescope. These tubes are placed one inside another. The separations between baffles, i.e. distances between internal surface of outer cylindrical tube and external surface of the next inner cylindrical tube, are the same.

These baffles can be made from thin film of metal, plastic or other material. This material should be black enough to prevent reflections of rays. Primarily, the blackening of baffles is necessary in order to prevent direct reflections of rays which are blocked by the baffles. The secondary goal is the reduction of scattered rays because they reduce the contrast of image. The internal surfaces of the telescope tubes are commonly also blackened.

The parallel incident on-axis beam of rays can freely pass through the baffles to the primary mirror of this Newtonian telescope, although very small part of the rays is blocked by thin faces of the baffles. The great part of nearly on-axis rays also reach the primary mirror. So the baffles only slightly affect the images in the central part of the field of view. It means that the effective aperture for the nearly on-axis beams is practically equal to the full aperture of the telescope. However, these baffles very significantly affect incident off-axis beams and, therefore, the off-axis part of the field of view. The heights of the baffles and the separation between them are specifically chosen to block some outer rays of off-axis beams in order to reduce the effective aperture for these beams. The general rule is that the beams with more angle of incidence should have more reduced effective aperture. It is not always necessary to block absolutely all outer rays for the beams with reduced effective aperture. The passing of small amount of light from the outer parts of beams are usually still acceptable if energy of this light is small enough.

The outer cylindrical baffle is the longest one. This baffle partly blocks rays for relatively small angles of incidence. This baffle also blocks all rays at greater angles of incidence. So the effective aperture for beams at some small angle of incidence is reduced. The next baffles are shorter and, therefore, additionally block rays for more greater angles of incidence. In result, the effective aperture for beams of greater angles of incidence is progressively reduced. Note that in this specific case of extremely simplified example, the telescope tube works as the most outer baffle. Usually it’s not the case. This is done here to simplify the explanation and drawings. On the other hand, the tube is still usually needed in order to ensure that the all incident beams in field of view come through the aperture stop before entering in the subsequent optical system.

The figures Fig. 1, Fig. 2, Fig. 3, Fig. 4 are intended for illustration of the idea of using baffles for reduction of relative aperture for off-axis beams. These drawings are schematic illustrations of the meridional cross-section of the Newtonian telescope without secondary mirror, which is equipped with the optical device according to the extremely simplified example of the present invention. These drawings are not to scale and the proportions of certain parts are exaggerated to better illustrate the idea of the invention. The off-axis rays and reflected rays are depicted at highly exaggerated angles.

Reference should now be made to Fig. 1, which is schematic illustration of the cross-section of the Newtonian telescope without flat secondary mirror but with three cylindrical baffles. These baffles are the thin hollow circular cylinders that are placed one inside another. The concave parabolic mirror 1 located inside of the cylindrical tube where 2 and 3 are the cross-sections of the telescope tube, the upper and bottom ones respectively. Three thin cylindrical baffles of different heights are placed inside the telescope tube. These baffles and the parabolic mirror of the telescope are placed coaxially, i.e. the axis of symmetry of these baffles and optical axis of the parabolic mirror are coincident.

The upper cross-sections of the outer, middle and inner baffles are designated as 4, 6 and 8 respectively. The bottom cross-sections of these baffles are designated as 5, 7 and 9. The outer cylindrical baffle is the longest one. Its cross-sections are designated as 4 and 5. The cross-sections of the middle baffle are designated as 6 and 7. The middle baffle is shorter when compared with the outer baffle. The inner baffle is the shortest one. Its upper and bottom cross-sections are designated as 8 and 9 respectively.

The parallel rays 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 and 20 represent on- axis incident beam from a very far object. The object is so distant that these rays are practically parallel. Fig. 1 represents the incident beam for the center of FOV. The rays travel from left to right. All these rays passes through the set of the baffles, reflect from the concave parabolic mirror and are focused at the focal point 21. Therefore, the effective aperture for the on-axis beam is approximately equal to the diameter of the mirror of the telescope.

Reference should now be made to Fig. 2 which depicts the same device but with slightly off-axis incident beam. The parallel oblique rays 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 and 32 represent slightly off-axis incident beam.

In this case not all rays reach the focus. As can be seen from Fig. 2 the most outer rays 22, 31 and 32 are blocked by the outer baffle and the telescope tube. For this reason, only eight rays from incident eleven rays participate in forming of the focal point. Note that the baffle blocks most outer part of the beam for this angle of incidence. It means that the effective aperture for this beam is reduced, it is approximately equal to the diameter of the outer baffle. It results to significant reduction of coma for this angle of oblique incident beam because the value of third-order coma aberration is proportional to the square of the effective aperture. This improves the optical resolution for off-axis images because coma is the limiting off-axis aberration for the Newtonian telescope. The ratio of the separation distance between the telescope tube and the outer baffle to the height of the outer baffle approximately defines the tangent of the angle of incidence of completely blocked rays. The rays at smaller angles of incidence are only partly blocked.

Reference should now be made to Fig. 3 which depicts the same device but with moderately off-axis incident beam from a very far object. The angle of incidence of this oblique beam is greater when compared with incident beam of Fig. 2. The parallel oblique rays 33, 34, 35, 36, 37, 38, 39, 40, 41, 42 and 43 represent moderately off-axis incident beam. In this case even more rays are blocked by both the outer and middle baffles. Only six rays pass through the baffles structure to the focal point. So the baffles block greater part of outer rays of the beam at the moderate angle of incidence when compared with the previous case of the slightly off-axis incident beam.

The rays 33, 34, 41, 42 and 43 are blocked by the outer baffle, the middle baffle and the telescope tube. It means that the effective aperture of this telescope is reduced even more than in the previous case of Fig. 2, it is roughly equal to the diameter of the middle baffle. For this reason the coma for this moderately oblique beam is reduced more significantly when compared with the slightly oblique beam.

Reference should now be made to Fig. 4 which depicts the same device but with highly off-axis incident beam. The parallel oblique rays 44, 45, 46, 47, 48, 49, 50, 51, 52, 53 and 54 represent highly off-axis incident beam.

The incident rays 44, 45, 46, 51, 52, 53 and 54 are blocked by the telescope tube, the outer, middle and inner baffles. Only four central rays 47, 48, 49 and 50 reach the common focus. In this case the effective aperture is greatly reduced. So the outer, middle and inner baffles progressively reduce the effective aperture for oblique beams of small, middle and high angles of incidence, by blocking the outer parts of the incoming beams of light.

The effective aperture for this highly off-axis incident beam is even less than the diameter of the inner baffle. For this reason the off-axis aberrations are significantly reduced for this beam, which represents the far off-axis part of the field of view. In contrast to Schmidt camera, this device almost does not obstruct the light at central part of the field of view. The incoming light energy from the object is significantly reduced for off-axis images only. The removing of some harmful off-axis rays makes the off-axis images sharper but fainter. This effect is similar to vignetting, i.e. a reduction of an image's brightness towards the periphery compared with the image center. In the Schmidt camera, the partial loss of energy of incident light is almost the same, in the first approximation, for all field of view. By contrast, in the present invention, the incident rays are only insignificantly obstructed for nearly on-axis beams. So, the central part of the field of view is almost fully illuminated by the incident beams. It means that the each point image in the central part of FOV is illuminated by almost the full aperture of the primary mirror. However, the partial blocking of the off-axis beams is progressively increasing, so the illumination of the off-axis images is progressively decreasing. This vignetting can be visible as saturation of off-axis objects or reduction of star's brightness at the edge of the field of view. The vignetting can be compensated by the image processing as it’s often done in modern digital photographic cameras. The description of this extremely simplified example applies for meridional rays. The propagation of skew rays through this type of baffles is more complex.

To summarize, the general requirements for the baffles are enumerated below.

1. The baffles should approximately gradually reduce the effective aperture for oblique beams depending on the angle of incidence. 2. The baffles generally reduce the effective aperture to a greater extent for beams with greater angle of incidence.

3. The mathematical function of reduction of effective aperture with angle of inci dence depends on the specific task. Usually, the function should be chosen in order to reduce the most harmful off-axis aberrations for the specific optical system. 4. The baffles reduce the effective aperture by blocking mainly outer (i.e., peripheral) parts of the incident off-axis beams with increase the angle of incidence, meaning that the chief ray (also known as the principal ray) passes through the center of the aperture stop and, therefore, is the most inner ray of the beam.

5. Sometimes it is acceptable for baffles to pass small amount of light from the outer parts of oblique beams to the optical system if the energy of this light is small enough.

This is because the really small amount of light usually worsen the optical resolution insignificantly.

6. Preferably the effective aperture should be reduced in a symmetrical manner, i.e. the amount of reduction should be as similar as possible for tangential and sagittal planes of the incident beams as well as the skew rays. In this case, the improvement of the optical resolution is optimal.

A lot of various designs of the baffles satisfy these requirements. Some of these designs are presented below but the present invention is not limited to the aforementioned embodiments, as it will be obvious that various alternative implementations are possible. The more complicated example of the baffles is shown in Fig. 5. Fig. 5 is the perspective cut-away view of the set of 33 coaxial thin hollow circular cylinders that are placed one inside another. All these baffles have the same height and they are aligned with each other, including the external baffle 55 and the internal baffle 56. Fig. 6 shows the perspective cut-away view of the same baffles coaxially aligned with the concave parabolic primary mirror of the Newtonian telescope. The light travels from left to right through the baffles 57 to the parabolic primary mirror 58. In this specific case, all baffles have the same height. However, the distances between some baffles are different. The separation distances between inner baffles are significantly greater that the separation distances between peripheral baffles. In general case, both the heights of the baffles and the separation distances can be different.

In this embodiment, the baffles comprises a set of coaxial thin hollow circular cylinders that are placed one inside another. The term "circular cylinder" as used herein means the right circular cylinder. The geometric term "hollow circular cylinder" as used herein means the circular cylindrical shell, i.e. a three-dimensional region bounded by two substantially right circular cylinders having the same axis and two parallel annular bases perpendicular to the cylinders’ common axis. In other embodiments, the baffles can have different shapes instead of circular cylinders. For example, the baffles can have polygonal shape in cross section. The baffles can be made from tin, tinfoil, foil, plastic film or other materials. Alternatively, the baffles can be manufactured using 3D printers from plastic or metal, or in other ways. The baffles can be manufactured with relatively rough tolerances in shape when compared with lenses and mirrors but with sufficient accuracy for the specific applications. Usually, the baffles are co-axial to the optical system. The tolerances of axis alignment are often rough, i.e optical axis of the baffles sometimes can be shifted with respect of the axis of the optical system, without significant effect on optical performance. However, the axis of symmetry of these baffles should be substantially parallel to the optical axis. In some embodiments, the baffles can be non- symmetrical, such baffles have not the axis of symmetry. The surfaces of the walls of these baffles still should be substantially parallel to the optical axis of the associated optical system. The baffles are not necessary to be concentric, they can be spiral in cross-section for example. This is a sample of the non-symmetrical baffles.

In another embodiment, the baffles are implemented in the form of flat, thin and long strip (band) rolled into roughly cylindrical three-dimensional structure, that is spiral in cross-section, with significant separation distances between turns, as it is shown Fig. 7. This drawing depicts perspective view of such baffles. The terms "band" and "strip" as used herein mean the wide enough and long strip of some flat and thin material like foil or tin. This spiral band can have the same width but the separation distance between successive turns of the spiral is increasing from the external part to the internal one, as can be seen in Fig. 7. Alternatively, the separation distance between successive turns of the spiral band can be fixed but the width is decreasing from the external part to the internal one. In other cases, both the width and the separation distance can be different for at least some turns of the spiral-shaped strip. The height of the baffles herein means the width of the band. The baffles in the form of spiral band may sometimes be cheaper in the production. This spiral band can be manufactured from a number of pieces of the material.

Although the baffles, which are implemented in the form of a spiral bent or a set of coaxial tubes, effectively block unwanted rays of incidents beams in the meridional plane, they are not so efficient for some part of skew rays. In order to illustrate this fact, Fig. 8 and Fig. 9 are presented. Fig. 8 shows the view of the baffles, which are implemented in form of the set of 33 coaxial thin hollow circular cylinders that are placed one inside another, as they are seen from the distant on-axis point. Fig. 9 shows the same baffles as they are seen from the distant slightly off-axis point. Due to the principle of reversibility, a light ray will follow exactly the same path if its direction of travel is reversed. Therefore, the view of Fig. 9 shows those parts of incident beam which are blocked by the baffles. These parts of the beam are black in Fig. 9 because they are shadowed by the walls of the baffles. Since Fig. 8 represents view of the baffles from the distance on-axis point we can see only the black edges of the baffles. The black area 59 in Fig. 9 shows a part of the outer baffles where all incoming beams are completely blocked. On the contrary, the white area 60 shows a part of the outer baffles where incoming beams are almost not blocked. As can be seen in Fig. 9, the black areas are concentrated around some vertical line (not depicted) which actually represents the meridional plane. On the other side, the white areas tend to be placed on left and right parts of the drawing, nearby the plane that is perpendicular to the meridional plane. This illustrates the fact that skew rays are blocked by cylindrical baffles less effectively than the rays nearby the meridional plane. This fact is drawback of these types of baffles because this feature make reduction of the effective aperture less effective for skew rays when compared with meridional ones. It results in the specific triangle-like shape of the spot diagrams as it shown in the illustrative embodiments below.

Thus, the optical performance of both the cylindrical and the spiral baffles is not always good enough. For this reason, more complex baffles can be useful if the better off-axis optical resolution is required. These baffles should have another design to fix the above problem.

In order to fix this problem, the baffles can be made in the form of an array of long columnar cells of appropriate heights and sizes. These columnar cells look like tubes. These tubes may have various shapes in cross-section. The general rule is that the shape should be more or less central-symmetrical because, in this case, the rays are blocked in a similar way in both meridional and sagittal planes. This is not strict rule, it’s only approximate. Such tubes can completely block skew rays of incoming beams at specific angles of incidence and at all angles greater than this specific one. The relatively long tubes block beams with relatively small angles of incidence. The tubes with relatively small size in cross-section also block beams with relatively small angles of incidence. So, the angle-dependent aperture stop can be made in the form of the array of these tubes of appropriate heights and sizes. The most important advantage of cell baffles is that the meridional and skew rays are blocked approximately equally. It improves off-axis optical resolution. It is often advantageous to make these baffles with the columnar cells of the same shape and size but different heights.

So, in another embodiment the baffles can be made in a form of honeycomb-style three-dimensional structure. This kind of baffles can be made in the form of an array of hollow columnar cell formed between thin walls that are parallel to the optical axis of the associated optical system. These cells can be considered as generalized cylinders. Since generalized cylinder is a ruled surface this cylinder can be described as the set of points swept by a moving straight line. In this invention the generalized cylinders of baffles are positioned so as the mentioned straight line is substantially parallel to the optical axis of the subsequent optical system. Due to columnar cell shape these baffles are more efficient in blocking of rays in the both tangential and sagittal planes, as well as of skew rays. These columnar cells can have different shapes in cross-section. They can be hexagonal, polygonal or of other shape, or various combination of cells of different shapes. However, these cells should not be too elongated in shape to avoid big difference in the blocking of rays in tangential and other planes. For example, these cells can be hexagonal in shape with the same cell size but some of them have different heights depending on the position of the particular cells. Fig. 10 shows perspective cut-away view of the exemplary baffles with the hexagonal cells of the same size but the different heights. The cells size of these baffles is greatly exaggerated for better illustration because small cells are hard to see in the drawing. Fig. 11 shows the cross-section of similar baffles with hexagonal shaped cells of the same size. The cell size of these baffles is also exaggerated. Although baffles for some optical systems may have relatively large cell size, many applications require relatively small cell sizes when compared to the full aperture of optical system. The baffles for astronomical telescopes may comprise thousands of columnar cells. As can be seen in Fig. 10, the outer baffles are greater in height than the internal ones. The reduction of the effective aperture in these baffles is obtained in the similar way as in the case of the extremely simplified baffles described above, i.e. the height and position of the cells should be properly chosen in order to reduce the relative aperture of the incident off-axis beams depending on angle of incidence. The difference is in the much more symmetrical blocking of skew rays by hexagonal-shaped cells when compared with cylindrical or spiral baffles. This is because the relatively small parts of the incident beams can be blocked independently. For this reason, the skew rays are blocked approximately in the same way as the meridional ones. The result is a smaller spot size for off-axis images. This improves the off-axis optical resolution of the optical systems. It is an important advantage of this embodiment. In addition, the geometry of honeycomb often allows to minimize the amount of used material in order to get durable baffles with minimal weight and material cost. This kind of baffles can be manufactured with the use of 3D printing technology or in other ways.

Moreover, the cross-sections of these cells are not necessary to be regular in shape. Actually, these columnar cells can be generalized cylinders of various shapes in cross- section. The term "cylinder" as used herein refers to what can be called a cylindrical surface. A cylinder is defined as a surface consisting of all the points on all the lines which are parallel to a given line and which pass through a fixed plane curve in a plane not parallel to the given line. Such cylinders have, at times, been referred to as generalized cylinders. Through each point of a generalized cylinder there passes a unique line that is contained in the cylinder. Thus, this definition may be rephrased to say that a cylinder is any ruled surface spanned by a one-parameter family of parallel lines. These parallel lines should be substantially parallel to the optical axis of the optical system. The circular cylinder is just a particular case of a generalized cylinder.

The choice of the specific embodiment depends on purpose of the optical system, available materials, cost of materials, manufacturing capabilities and manufacturing cost of the specific optical device according to the present invention.

The device of the present invention is placed in front of the optical system, i.e. the incoming light pass through the device before entering into the optical system. Actually, this device works as angle-dependent aperture stop for the subsequent optical system.

In the illustrative embodiments which are presented in this description, the typical thickness of baffles is 0.1 mm. The weight of these baffles can be several kilograms if they are made from a metal of this thickness. The weight can be reduces to less than one kilogram by replacing a metal by some plastics. Alternatively, the thickness of the baffles can be reduced. The reasons to use relatively thick walls of baffles can be durability and costs of work with very thin materials.

On the other hand, there is possibility to reduce weight of baffles by perforating the holes of significant size in the baffles . The perforated baffles are often only slightly less durable than unperforated ones. In the description of this invention it is generally assumed that baffles are made from thin material like foil, tin, plastic film or 3D printed structures, without any holes. However, the holes in baffles can be used whenever necessary. Since the device of the invention usually comprises many baffles, it is sometimes possible to choose the positions and sizes of the holes in the baffles so that baffles still block almost all unwanted rays despite the presence of the holes. As it was already mentioned above, it is sometimes acceptable if the baffles pass small amount of unwanted light from the outer parts of oblique beams if the energy of this light is small enough. For this reason it is possible to chose proper perforation for various baffles so that don’t seriously worsen the optical performance. The numbers, positions, shapes and sizes of the perforated holes depend on the design of specific baffles. Thus, the perforated baffles can be both durable and lightweight simultaneously.

Advantageous Effects

The present invention improves the off-axis optical resolution of various optical sys- terns. Due to this improvement the practically useful linear field of view of some opti cal systems can be significantly increased. This advantageous effect can be useful for various optical systems including, but not limited to, telescopes, aspheric lenses, some moderate-field and wide-field optical systems. The present invention improves off-axis optical resolution of optical systems without the use of additional expensive optical ele ments like lenses or mirrors. The present invention utilizes only simple and inexpensive elements made from cheap materials like metal foil, tin, plastic or other materials. The present invention is also inexpensive in production because it dees not require so tight tolerances in manufacturing as lenses and mirrors.

The optical device of the present invention is universal and can be used with various types of optical systems. This device can be made either as detachable or fixed, depending on the application requirements. The fixed devices often may be completely embedded into a housing of optical systems.

Description of Drawings

Fig. 1 is a schematic illustration of the cross-section of the Newtonian telescope with three coaxial cylindrical baffles for the case of on-axis incident beam.

Fig. 2 is the optical system of Fig. 1 for the case of slightly off-axis incident beam. Fig. 3 is the optical system of Fig. 1 for the case of moderately off-axis incident beam. Fig. 4 is the optical system of Fig. 1 for the case of highly off-axis incident beam.

Fig. 5 is a perspective cut-away view of the baffles which are presented in the illustrative Embodiment 1.

Fig. 6 is a perspective cut-away view of the baffles of Fig. 5 coaxially aligned with the primary concave mirror of the Newtonian telescope.

Fig. 7 is a perspective view of the baffles which are implemented in the form of the spiral bent.

Fig. 8 is a view of the baffles which are implemented in the form of the set of 33 coaxial thin hollow circular cylinders that are placed one inside another as they are seen from the distant on-axis point.

Fig. 9 is the device of Fig. 8 as it is seen from the distant slightly off-axis point.

Fig. 10 is a perspective cut-away view of the hexagonal baffles. Fig. 11 is a cross-section of another baffles which are implemented in the form of hexagonal shaped cells.

Fig. 12 is an on-axis spot diagram of the 200 mm F/5 Newtonian telescope.

Fig. 13 is a spot diagram of the telescope of Fig. 12 at 0.1 degree off-axis.

Fig. 14 is a spot diagram of the telescope of Fig. 12 at 0.2 degree off-axis.

Fig. 15 is a spot diagram of the telescope of Fig. 12 at 0.3 degree off-axis.

Fig. 16 is a spot diagram of the telescope of Fig. 12 at 0.4 degree off-axis.

Fig. 17 is a spot diagram of the telescope of Fig. 12 at 0.5 degree off-axis.

Fig. 18 is an on-axis spot diagram of the 200 mm F/5 Newtonian telescope which is coaxially aligned with the device of the illustrative Embodiment 1.

Fig. 19 is a spot diagram of the optical system of Fig. 18 at 0.1 degree off-axis.

Fig. 20 is a spot diagram of the optical system of Fig. 18 at 0.2 degree off-axis.

Fig. 21 is a spot diagram of the optical system of Fig. 18 at 0.3 degree off-axis.

Fig. 22 is a spot diagram of the optical system of Fig. 18 at 0.4 degree off-axis.

Fig. 23 is a spot diagram of the optical system of Fig. 18 at 0.5 degree off-axis.

Fig. 24 is an on-axis spot diagram of the 200 mm F/5 Newtonian telescope coaxially aligned with the device of the illustrative Embodiment 2.

Fig. 25 is a spot diagram of the optical system of Fig. 24 at 0.1 degree off-axis.

Fig. 26 is a spot diagram of the optical system of Fig. 24 at 0.2 degree off-axis.

Fig. 27 is a spot diagram of the optical system of Fig. 24 at 0.3 degree off-axis.

Fig. 28 is a spot diagram of the optical system of Fig. 24 at 0.4 degree off-axis.

Fig. 29 is a spot diagram of the optical system of Fig. 24 at 0.5 degree off-axis.

Fig. 30 is a perspective cut-away view of the baffles which are presented in the illustrative Embodiment 3.

Fig. 31 is a perspective cut-away view of the 200 mm F/8 Dall-Kirkham telescope which is coaxially aligned with the device of the illustrative Embodiment 3.

Fig. 32 is an on-axis spot diagram of the 200 mm F/8 Dall-Kirkham telescope.

Fig. 33 is a spot diagram of the telescope of Fig. 32 at 0.05 degree off-axis.

Fig. 34 is a spot diagram of the telescope of Fig. 32 at 0.1 degree off-axis.

Fig. 35 is a spot diagram of the telescope of Fig. 32 at 0.15 degree off-axis.

Fig. 36 is a spot diagram of the telescope of Fig. 32 at 0.2 degree off-axis.

Fig. 37 is a spot diagram of the telescope of Fig. 32 at 0.25 degree off-axis. Fig. 38 is an on-axis spot diagram of the 200 mm F/8 Dall-Kirkham telescope which is coaxially aligned with the with the device of the illustrative Embodiment 3.

Fig. 39 is a spot diagram of the optical system of Fig. 38 at 0.05 degree off-axis.

Fig. 40 is a spot diagram of the optical system of Fig. 38 at 0.1 degree off-axis.

Fig. 41 is a spot diagram of the optical system of Fig. 38 at 0.15 degree off-axis.

Fig. 42 is a spot diagram of the optical system of Fig. 38 at 0.2 degree off-axis.

Fig. 43 is a spot diagram of the optical system of Fig. 38 at 0.25 degree off-axis.

Fig. 44 is an on-axis spot diagram of the optical system of Fig. 38.

Fig. 45 is a spot diagram of the optical system of Fig. 38 at 0.1 degree off-axis.

Fig. 46 is a spot diagram of the optical system of Fig. 38 at 0.2 degree off-axis.

Fig. 47 is a spot diagram of the optical system of Fig. 38 at 0.3 degree off-axis.

Fig. 48 is a spot diagram of the optical system of Fig. 38 at 0.4 degree off-axis.

Fig. 49 is a spot diagram of the optical system of Fig. 38 at 0,5 degree off-axis. Best Mode for Carrying Out the Invention

The device of this invention is used in conjunction with an optical system. The present invention is most useful for simple optical systems because such systems usu ally have large off-axis optical aberrations and, therefore, the narrow field of view. For this reason the invention is especially useful for simplest astronomical telescopes like the Newtonian, classical Cassegrain or Dall-Kirkham. On the other hand, the manufacturing of the optical device according to the present invention is even easier for the imaging systems with extended field of view, including wide-angle ones, rather than simple as tronomical telescopes. This is because the baffles for blocking of beams at great angles of incidence can be made much shorter when compared with the baffles for blocking of beams at small angles, i.e. the device of this invention can be much more compact. In addition, such baffles usually have looser manufacturing tolerances, so they are cheaper in manufacturing. For this reason, the device of the invention for use with moderate-angle or wide-angle imaging optical systems can be made cheap, lightweight and compact. This reason makes the present invention useful for imaging optical systems with wide field of view, especially if the optical device of this invention is made detachable. So, the off-axis optical resolution of complex optical systems can also be improved with the present invention.

In order to demonstrate the use of the present invention in conjunction with optical systems, three illustrative embodiments are presented. While the present invention is described herein with reference to the embodiments for particular applications, it should be understood that the invention is not limited thereto.

These illustrative embodiments are described below with reference to the accompany ing drawings and tables. The specifications for all baffles of the illustrative embodiments are summarized in Tables I through III. Every row of these tables lists, in order from the outer side, the number of the cylindrical baffle in the device, the radius of external cylindrical surface of this baffle, the thickness of the baffle, the separation distance be tween the inner cylindrical surface of the baffle and the external cylindrical surface of neighboring inner baffle, the height of the baffle. The edges of the baffles are aligned on the object side of the device. So, all heights are also measured from the object edge of the baffles.

Embodiment 1

The first illustrative embodiment of the device of the present invention comprises 33 baffles in the form of a set of coaxial thin hollow circular cylinders of decreasing diameter that are placed one inside another. These hollow cylinders have the same thickness and height but different diameters. Some groups of the baffles have the same separation distances. The baffles should be made black enough in order to prevent reflections as well as to reduce scattering of the light. The perspective cut-away view of this device is shown in Fig. 5.

The full aperture of this device is 200 mm. The specifications for this illustrative embodiment are summarized in Table I. As can be seen from Table I, the first group of twelve baffles comprises the baffles which are designated by the numbers from 1 to 12. The baffles in the first group have the same separation distances of 1.0 mm. These baffles block only small part of incident nearly on-axis rays. However the blocking of off-axis rays is progressively increasing with the increasing of the angle of incidence. These baffles effectively block oblique rays in meridional plane with angles of incidence greater than approximately 11.5 minutes of arc. This angle is directly calculated from the specifications, as explained below. The ratio of the separation distance (1.0 mm) to the height of the baffles (300 mm) in this group yields the tangent of this angle, i.e. tangent is approximately equal to 0.003333. So, the angle is equal to arctangent of 0.003333, it yields approximately 0.191 degree or 11.5 minutes of arc. Note that although this group of baffles effectively block meridional rays, the significant part of skew rays is not blocked. As it was already mentioned, this is common feature of baffles made in the form of a set of coaxial thin hollow cylinders that are placed one inside another or in the form of spiral bent. In order to get better control of skew rays the baffles should be made in the more complex form an array of hollow columnar cells formed between thin walls. In this case the asymmetry of the effective aperture for beams in tangential and sagittal planes is small. It results in more symmetrical reduction of effective aperture for skew rays of incident beams. For this reason the better off-axis optical resolution can be obtained.

So, these twelve baffles of the first group reduce effective aperture in tangential plane for beams greater than 11.5 minutes of arc approximately by 13 percent of the full aperture, taking into account the thickness of each baffle. It means that the effective aperture for these beams in the tangential plane is 87% of the full aperture. It results in reduction of third-order meridional coma for this beam approximately by 25% when compared with the beam of the full aperture. However, due to less effective blocking of the skew rays the reduction of third-order coma aberration is smaller than reduction of size of coma in meridional plane.

The calculations of effective apertures for various angles are approximate for all em bodiments here. This is just estimation for illustration but not exact values.

The next group comprises seven baffles, which are designated by the numbers from 13 to 19 in Table I. Each pair of neighboring baffles is separated by the distance of 1.3 m . The baffles of this group pass some part of meridional oblique rays at angle of incidence of 11.5 arcminutes but block meridional oblique rays with angles of incidence greater than approximately 14.9 minutes of arc. However, some part of skew rays at this angle still pass through these baffles. Thus, this set of the baffles additionally reduce the effective aperture for meridional rays at angles greater than 14.9 minutes of arc by 10% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays is about 23% of the full aperture. Thus, the effective aperture for meridional rays is approximately 77% of the full aperture at angles greater than 14.9 minutes of arc.

The next group comprises four baffles, which are designated by the numbers from 20 to 23. Each pair of neighboring baffles is separated by the distance of 1.6 mm. The baffles of this group additionally block meridional oblique rays with angles of incidence greater than approximately 18.3 minutes of arc. Some part of skew rays at this angle still pass through these baffles. This is correct for all other baffles of this embodiment. This set of the baffles additionally reduce the effective aperture for meridional rays by 7% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is about 30% of the full aperture. Thus, the effective aperture for meridional rays is approximately 70% of the full aperture.

The next group comprises three baffles, which are designated by the numbers from 24 to 26. Each pair of neighboring baffles is separated by the distance of 1.8 mm. The baffles of this group additionally block meridional oblique rays with angles of incidence greater than approximately 20.6 minutes of arc. This set of baffles additionally reduce the effective aperture for meridional rays by 6% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is about 36% of the full aperture. Thus, the effective aperture for meridional rays is 64% of the full aperture for this angle.

The next group comprises only two baffles, which are designated by the numbers 27 and 28. The baffles are separated by the distance of 2.1 m . The baffles of this group additionally block meridional oblique rays with angles of incidence greater than approximately 24.1 minutes of arc. These baffles additionally reduce the effective aper ture for meridional rays approximately by 4% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is about 40% of the full aperture. Thus, the effective aperture for this angle in meridional plane is 60% of the full aperture.

The next baffle is designated by the number 29. The separation distance between neighboring baffles 29 and 30 is 2.4 mm. This baffle additionally block meridional oblique rays with angles of incidence greater than approximately 27.5 minutes of arc. The baffle additionally reduce the effective aperture for meridional rays approximately by 2.5% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is about 42.5% of the full aperture. Thus, the effective aperture for meridional rays at this angle is 57.5% of the full aperture.

The next baffle is designated by the number 30. The separation distance between neighboring baffles 30 and 31 is 2.6 mm. This baffle additionally block meridional oblique rays with angles of incidence greater than approximately 29.8 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is about 45% of the full aperture. Thus, the effective aperture for meridional rays at this angle is 55% of the full aperture.

The next baffle is designated by the number 31. The separation distance between neighboring baffles 30 and 31 is 2.9 mm. This baffle additionally block meridional oblique rays with angles of incidence greater than approximately 33.2 minutes of arc. The baffle additionally reduce the effective aperture for meridional rays by 3% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is about 48% of the full aperture. Thus, the effective aperture for for meridional rays at this angle is 52% of the full aperture.

The next baffle is designated by the number 32. The separation distance between neighboring baffles 31 and 32 is 3.2 mm. This baffle additionally block meridional oblique rays with angles of incidence greater than approximately 36.7 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture is approximately 51% of the full aperture. Thus, the effective aperture for meridional rays at this angle is about 49% of the full aperture.

The last baffle is designated by the number 33. Thus, taking into account the reduction by all baffles, the cumulative reduction of effective aperture for meridional rays is more than half of the full aperture. It results in reduction various off-axis aberrations. The extent of reduction depends on the angle of incidence of beam and the particular off-axis aberration. For example, since third-order tangential coma is proportional to square of the aperture, the reduction of the spot size in tangential plane for this device can be approximately 3.5 times at 0.5 degree (i.e. 30 minutes of arc) off-axis. However, the reduction of root mean square (RMS) spot size is lower because cylindrical baffles block skew rays less effective than meridional ones.

The device of the present invention is intended for use together with an optical system. Basically there are two different ways to use this invention. The first way is using of the device as add-on to the existing optical device. For example, the device of the invention can be placed in front of the telescope in order to reduce the off-axis aberrations of this telescope and increase the field of view. The device should be positioned substantially coaxially with the telescope. In this case an optical system can be used with attached device of the invention or without it depending on the specific task demands. It is often suitable to use the device according to the present invention as external extension for the optical devise, i.e. add-on. Another way is using of the invention as fully or partially embedded device. The device of the invention can be embedded into the optical device during manufacturing process. In the case of a telescope, this device can be embedded into the telescope’s tube in order to make the telescope more compact.

The optical device according to the first illustrative embodiment can be used with various telescopes of appropriate aperture. For example, consider using this device with the Newtonian telescope. Reference should now be made to Fig. 6, which is the perspective cut-away view of the optical device of the first illustrative embodiment which is placed in front of the Newtonian telescope. This specific Newtonian telescope has concave parabolic primary mirror but it is not equipped with a flat secondary mirror because it is dedicated for astrophotography only. The Newtonian telescope with a flat secondary mirror has actually the same optical performance. This astronomical telescope has the aperture of 200 mm, the effective focal length of 1000 mm and the f-number of f/5. The device and the telescope are placed coaxially, i.e. the axis of symmetry of the device and optical axis of the telescope are substantially coincident. The distance between the rear end of this device and the primary mirror is 700 millimeters. This distance can usually be changed without significant affecting the optical performance of the system while the device does not obscure the rays that are reflected from the primary mirror. This device can be placed completely inside the tube of the telescope. The device of the invention often can be placed inside tubes or other housings of optical systems.

In order to illustrate the improvement in the off-axis optical resolution and increasing of the field of view, the spot diagrams are shown in Figs. 12-23. Figs. 12-17 show the spot diagrams of the Newtonian telescope for six different angles from zero to 0.5 degree off-axis with the step of 0.1 degree, respectively. The circles at every spot diagram indicate Airy disk at the full aperture. The diameter of Airy disk for this telescope at the full aperture is of 6.7 micrometers. Figs. 18-23 show the spot diagrams of the same Newtonian telescope used in conjunction with the optical device of this first illustrative embodiment. Fig. 18, Fig. 19, Fig. 20, Fig. 21, Fig. 22 and Fig. 23 present on-axis spot diagram and the spot diagrams at 0.1 degree (i.e. 6 arcminutes), 0.2 degree (i.e. 12 arcminutes), 0.3 degree (i.e. 18 arcminutes), 0.4 degree (i.e. 24 arcminutes), 0.5 degree (i.e. 30 arcminutes) off-axis respectively. Note that these spot diagrams of Fig. 18-23 are scaled-up twice when compared with the Figs. 12-17 since the spots are much smaller. The diameter of Airy disk in Figs. 18-23 is also 6.7 micrometers but depicted twice as large as in Figs. 12-17 due to the scaling. As can be seen from these spot diagrams, the present invention significantly improves the off-axis optical resolution of this Newtonian telescope. The spot sizes for the middle and edge parts of the field of view are significantly reduced if the present invention is used. For example the spot size in meridional plane at 0.3 degree off-axis in Fig. 21 is reduced in approximately 2 times when compared with the spot size in Fig. 15 at the same angle. The RMS spot size is also reduced approximately in 2 times at this angle. The spot size in meridional plane at 0.5 degree off-axis in Fig. 23 is reduced in approximately 3.5 times when compared with the spot size in Fig. 17 at the same angle. The RMS spot size is reduced approximately in 2.5 times at this angle. Further, the RMS radius of spot at 0.5 degree in Fig. 23 is about 10.9 micrometers but RMS radius of spot at 0.2 degree in Fig. 14 is about 11.4 micrometers. It means that the Newtonian telescope used in conjunction with the optical device of the first illustrative embodiment has very close optical resolution at 0.5 degree as the same Newtonian telescope without the device at 0.2 degree off-axis. Thus, in this case the device of this embodiment extends the field of view of acceptable optical resolution of the Newtonian telescope approximately in 2.5 times.

Although the baffles of this illustrative embodiment are implemented in the form of a set of coaxial thin hollow cylinders that are placed one inside another, they can also be implemented in other forms. For example, these baffles can be replaced by the long wide spiral band with thickness 0.1 mm and height of 300 mm. The separation distances of the successive step of this band could be made close to the specifications in Table I. The optical performance of this spiral baffles can be similar to the baffles in the form of a set of coaxial thin hollow cylinders. Such replacement can be sometimes cheaper in manufacturing.

As can be seen in Figs. 18-23, the image spots have specific nearly triangular shape. By contrast, the image spots of the Newtonian telescope without the use of this embodiment have typical coma shape. This difference is caused by the use of cylindrical baffles. As it was repeatedly mentioned above, this type of baffles has different effective apertures for the tangential and sagittal planes as well as skew rays. For this reason the reduction of image spot sizes is better for the tangential plane. This feature somewhat limits the reduction of the off-axis aberrations when the cylindrical or spiral baffles are used in this invention. If the better reduction of off-axis aberrations for skew rays is needed, the more complex baffles can be used. For example, hexagonal cell baffles can additionally improve off-axis optical resolution.

Embodiment 2

The second illustrative embodiment of this invention is similar to the first one but it has more baffles. The device comprises 50 blackened baffles in the form of a set of coaxial thin hollow circular cylinders of decreasing size that are placed one inside another. These hollow cylinders have the same thickness and height but different diameters. All these baffles have the same height and are aligned with each other. Some groups of the baffles have the same separation distances. The baffles should be made black enough in order to prevent reflections as well as to reduce scattering of the light. The minimal separation distance between baffles in this embodiment is 0.5 mm. This is twice smaller than the minimal separation distance in the first illustrative embodiment. The smaller separation distances allow to increase number of baffles as well as to reduce height of the baffles to 200 mm instead of 300 mm in the first illustrative embodiment. The shorter baffles make the device more compact. It is especially useful if the device is used as attachable add-on device for a telescope. Thus, the device of this embodiment has overall advantage in performance over the device of the first embodiment but it is more complex.

The full aperture of this device is 200 mm. The specifications for this illustrative embodiment are summarized in Table II. As can be seen from this table, the first group of twenty two baffles comprises baffles with numbers from 1 to 22. The baffles in the first group have the same separation distances of 0.5 mm. These baffles block only small part of incident nearly on-axis rays but the blocking of off-axis rays is progressively increasing with the increasing of the angle of incidence. These baffles effectively block oblique meridional rays with angles of incidence greater than approximately 8.6 minutes of arc. This angle is calculated from the specifications from Table II, as explained below. The ratio of the separation distance (0.5 mm) to the height of the baffles (200 mm) yields the tangent of this angle, this tangent is approximately equal to 0.0025. So, the angle is equal to arctangent of 0.0025, it yields approximately 0.143 degree or 8.6 minutes of arc. Thus, the incident meridional rays are completely blocked by these baffles if angles of incidence greater than 8.6 minutes of arc. The rays with lower angles of incidence partially pass. In addition, the some part of skew rays with angles greater than 8.6 minutes of arc is also blocked by these baffles.

These twenty two baffles of the first group reduce the effective aperture in the tan- gential plane for beams greater than 8.6 minutes of arc by 13 percent of the full aperture, taking into account the thickness of each baffle. It means that the effective aperture for meridional rays at angles of incidence greater than 8.6 arcminutes is 87% of the full aperture. It results in reduction of meridional third-order coma at this angle approximately by 25% when compared with the beam of the full aperture. However, due to less effective blocking of the skew rays the reduction of third-order coma aberration is smaller than reduction of size of third-order coma in meridional plane.

The next group comprises ten baffles, which are designated by the numbers from 23 to 32 in Table II. Each pair of neighboring baffles is separated by the distance of 0.7 mm. These baffles additionally block meridional oblique rays with angles of incidence greater than approximately 12 minutes of arc. However, some part of skew rays at this angle still pass through these baffles. This is also correct for other baffles of this embodiment. This set of baffles additionally reduce the effective aperture for meridional rays by 8% of the full aperture. Taking into account the reduction by the previous group of baffles, the cumulative reduction of the effective aperture for meridional rays is 21% of the full aperture. Thus, the effective aperture for meridional rays is 79% of the full aperture at angles more than 12 arcminutes.

The next group comprises five baffles, which are designated by the numbers from 33 to 37. Each pair of neighboring baffles is separated by the distance of 0.9 mm. These baffles additionally block meridional oblique rays with angles of incidence greater than approximately 15.5 minutes of arc. This set of baffles additionally reduce the effective aperture for meridional rays by 5% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is 26% of the full aperture. Thus, the effective aperture for meridional rays is 74% of the full aperture at this angle.

The next group comprises four baffles, which are designated by the numbers from 38 to 41. Each pair of neighboring baffles is separated by the distance of 1.0 mm. These baffles additionally block meridional oblique rays with angles of incidence greater than approximately 17.2 minutes of arc. This set of baffles additionally reduce the effective aperture for meridional rays approximately by 4.5% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is 30.5% of the full aperture. Thus, the effective aperture for meridional rays is 69.5% of the full aperture at this angle.

The next group comprises three baffles, which are designated by the numbers 42, 43 and 44. Each pair of neighboring baffles is separated by the distance of 1.2 mm. These baffles additionally block meridional oblique rays with angles of incidence greater than approximately 20.6 minutes of arc. This set of baffles additionally reduce the effective aperture for meridional rays by 4% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is 34.5% of the full aperture. Thus, the effective aperture for meridional rays is 65.5% of the full aperture at this angle.

The next group comprises only two baffles, which are designated by the numbers 45 and 46. The neighboring baffles are separated by the distance of 1.4 mm. These baffles additionally block meridional oblique rays with angles of incidence greater than approximately 24.1 minutes of arc. This set of baffles additionally reduce the effective aperture for meridional rays by 3% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is 37.5% of the full aperture. Thus, the effective aperture for meridional rays is 62.5% of the full aperture at this angle

The next baffle is designated by the number 47. The separation distance between neighboring baffles 47 and 48 is 1.6 mm. This baffle additionally block meridional oblique rays with angles of incidence greater than approximately 27.5 arcminutes. It additionally reduce the effective aperture for meridional rays by almost 2% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is approximately 39% of the full aperture. Thus, the effective aperture for meridional rays at this angle is about 61% of the full aperture.

The next baffle is designated by the number 48. The separation distance between neighboring baffles 48 and 49 is 1.8 mm. This baffle additionally block meridional oblique rays with angles of incidence greater than approximately 31 arcminutes. It additionally reduce the effective aperture for meridional rays by almost 2% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is 41% of the full aperture. Thus, the effective aperture for meridional rays at this angle is approximately 59% of the full aperture.

The next baffle is designated by the number 49. The separation distance between neighboring baffles 49 and 50 is 1.9 mm. This baffle additionally block meridional oblique rays with angles of incidence greater than approximately 32.7 arcminutes. It additionally reduce the effective aperture for meridional rays by 2% of the full aperture. Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is 43% of the full aperture. Thus, the effective aperture for meridional rays at this angle is approximately 57% of the full aperture.

The last baffle of the device of second illustrative embodiment is designated by the number 50 in Table II. Thus, taking into account the reduction by all baffles, the cumulative reduction of effective aperture for meridional rays is close to half of the full aperture. It results in reduction of various off-axis aberrations. The extent of reduction depends on the angle of incidence of the beam and the particular off-axis aberration. For example, since third-order tangential coma is proportional to square of the aperture, the reduction of the spot size in tangential plane for this device can be approximately 3 times at 0.5 degree off-axis. However, the reduction of root mean square (RMS) spot size is less than could be because the cylinder baffles block skew rays less effective than meridional ones.

In order to illustrate the performance of the optical device according to the second illustrative embodiment, consider using this device with the Newtonian telescope.

This Newtonian telescope has same design as the telescope in the first illustrative embodiment. The device and the Newtonian telescope are placed coaxially, i.e. the axis of symmetry of the device and optical axis of the Newtonian telescope are substantially coincident. The distance on the optical axis between the rear end of the device and the primary mirror of the Newtonian telescope is 800 mm. This distance can usually be changed without significant affecting the performance of the system while the device does not obscure the rays that are reflected from the concave primary mirror. This device can be placed completely inside the tube of the telescope.

In order to illustrate the improvement in the off-axis optical resolution and increasing of the field of view, the spot diagrams are shown in Figs. 24-29. Figs. 24-29 show the spot diagrams of the 200 mm F/5 Newtonian telescope used in conjunction with the optical device of this second illustrative embodiment. Fig. 24, Fig. 25, Fig. 26, Fig. 27, Fig. 28 and Fig. 29 present on-axis spot diagram and spot diagrams at 0.1 degree (i.e. 6 arcminutes), 0.2 degree (i.e. 12 arcminutes), 0.3 degree (i.e. 18 arcminutes), 0.4 degree (i.e. 24 arcminutes), 0.5 degree (i.e. 30 arcminutes) off-axis respectively. Note that these spot diagrams are scaled-up twice when compared with the Figs.12-17 since these spots are much smaller. The diameter of Airy disk in Figs. 24-29 is also 6.7 micrometers but depicted twice as large as in Figs. 12-17 due to scaling.

As can be seen from these spot diagrams, the present invention significantly improve off-axis optical resolution of this Newtonian telescope. The spot sizes for the middle and edge parts of the field of view are significantly reduced if the present invention is used. For example, the RMS spot size at 0.3 degree off-axis in Fig. 27 is reduced approximately in 2.1 times when compared with spot size in Fig. 15. The improvement of the optical resolution in the middle part of FOV is even better than in the first illustrative embodiment. The spot size in meridional plane at 0.5 degree off-axis in Fig. 29 is reduced in approximately 2.7 times when compared with the spot size in Fig. 17 at the same angle. The RMS spot size is reduced approximately in 2.6 times at this angle. Moreover, the RMS radius of spot at 0.5 degree in Fig. 29 is about 11 micrometers. This is very close to RMS radius of spot at 0.2 degree in Fig. 14, which is about 11.4 micrometers. So, the FOV of acceptable optical resolution of the Newtonian telescope is extended in 2.5 times by the device of this illustrative embodiment.

The device of this embodiment can be used with various types of telescopes with aperture of 200 mm and FOV similar to this Newtonian telescope.

Embodiment 3

The third illustrative embodiment of the invention has different design when com- pared with the first and second ones. This embodiment illustrates using the baffles of the different heights, in contrast with the baffles of the same height in the previous em bodiments. Despite the separation distances are the same for all baffles in this specific embodiment, it is usually optimal to choose different separation distances, at least some of them.

Another feature of this illustrative embodiment of the invention is a different strategy of reduction of the effective aperture when compared with the previous illustrative em bodiments. In this embodiment, the effective aperture is reduced faster with the angle of incidence than in the first and second embodiments. The third illustrative embodiment of the device according to the present invention comprises 51 baffles in the form of the set of coaxial thin hollow circular cylinders that are placed one inside another. These hollow cylinders have the same thickness and separation distances but different heights. Some groups of the baffles have the same heights. The baffles should be made black enough in order to prevent reflections as well as to reduce scattering of the light. The perspective cut-away view of this device is shown in Fig. 30 The external group of nineteen baffles is designated as 61 These baffles are longest ones. The next group of eighteen baffles is designated as 62 The most internal baffle is designated as 63 This is the shortest baffle.

The full aperture of this device is 200 mm. The specifications for this illustrative embodiment are summarized in Table II I. The separation distances for all baffles is 1.0 mm. The first group comprises nineteen baffles which are designated by the numbers from 1 to 19 in Table III. The height of each baffle in this group is 400 mm. These baffles block only small part of incident nearly on-axis rays but the blocking of off-axis rays is progressively increasing with the increasing of the angle of incidence. These baffles effectively block oblique rays in meridional plane with angles of incidence greater than approximately 8.6 minutes of arc. This angle is directly calculated from the specifications, as explained below. The ratio of the separation distance (1.0 mm) to the height of the baffles (400 mm) in this group yields the tangent of this angle, this tangent is approximately equal to 0.0025. So, the angle is equal to arctangent of 0.0025, it yields approximately 0.143 degree or 8.6 minutes of arc. Note that although this group of baffles effectively block meridional rays, the significant part of skew rays is not blocked. These nineteen baffles of the first group reduce the effective aperture in tangential plane for beams greater than 8.6 minutes of arc, approximately by 21 percent of the full aperture, taking into account the thickness of each baffle. It means that the effective aperture for the beam in the tangential plane at this angle is 79% of the full aperture.

The next group comprises eighteen baffles, which are designated by the numbers from 20 to 37 in Table III. Each baffle in his group is 380 mm in height. The baffles of this group additionally block meridional oblique rays with angles of incidence greater than approximately 9.1 minutes of arc. However, some part of skew rays at this angle still pass through these baffles. This is also correct for all baffles of this embodiment. In contrast with the previous illustrative embodiments, the second group of the baffles reduces relative aperture almost in the same extent as the first group. This set of baffles additionally reduce the effective aperture for meridional rays at angles greater than 9.1 minutes of arc by approximately 20% of the full aperture. Taking into account the reduction by the previous group of the baffles, the cumulative reduction of the effective aperture for meridional rays is about 41% of the full aperture. Thus, the effective aperture for meridional rays at 9.1 arcminutes is 59% of the full aperture.

The next baffle is designated by the number 38, this baffle is 363 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 9.5 minutes of arc. It additionally reduces the effective aperture at this angle by 1.1%. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is approximately 42% of the full aperture. Thus, the effective aperture for meridional rays at 9.5 arcminutes is 58% of the full aperture.

The every next baffle with numbers from 39 to 50 of Table III also reduces effective aperture for meridional oblique rays by 1.1% due to the same separation distances.

The next baffle is designated by the number 39, this baffle is 348 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 9.9 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 43% of the full aperture. Thus, the effective aperture for meridional rays at 9.9 arcminutes is 57% of the full aperture.

The next baffle is designated by the number 40, this baffle is 333 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 10.3 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 44% of the full aperture. Thus, the effective aperture for meridional rays at 10.3 arcminutes is 56% of the full aperture.

The next baffle is designated by the number 41, this baffle is 320 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 10.7 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 45% of the full aperture. Thus, the effective aperture for meridional rays at 10.7 arc minutes is 55% of the full aperture.

The next baffle is designated by the number 42, this baffle is 308 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 11.2 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 46% of the full aperture. Thus, the effective aperture for meridional rays at 11.2 arcminutes is 54% of the full aperture.

The next baffle is designated by the number 43, this baffle is 297 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 11.6 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 47% of the full aperture. Thus, the effective aperture for meridional rays at 11.2 arcminutes is approximately 53% of the full aperture.

The next baffle is designated by the number 44, this baffle is 286 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 12 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 48% of the full aperture. Thus, the effective aperture for meridional rays at 12 arcminutes is 52% of the full aperture.

The next baffle is designated by the number 45, this baffle is 276 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 12.5 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 49.5% of the full aperture. Thus, the effective aperture for meridional rays at 12.5 arcminutes is 50.5% of the full aperture.

The next baffle is designated by the number 46, this baffle is 267 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 12.9 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 51% of the full aperture. Thus, the effective aperture for meridional rays at 12.9 arcminutes is 49% of the full aperture.

The next baffle is designated by the number 47, this baffle is 259 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 13.3 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 52% of the full aperture. Thus, the effective aperture for meridional rays at 13.3 arcminutes is 48% of the full aperture.

The next baffle is designated by the number 48, this baffle is 251 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 13.7 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 53% of the full aperture. Thus, the effective aperture for meridional rays at 13.7 arcminutes is 47% of the full aperture.

The next baffle is designated by the number 49, this baffle is 244 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 14.1 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 54% of the full aperture. Thus, the effective aperture for meridional rays at 14.1 arcminutes is 46% of the full aperture.

The next baffle is designated by the number 50, this baffle is 237 mm in height. This baffle additionally blocks meridional oblique rays with angles of incidence greater than approximately 14.5 minutes of arc. Taking into account the reduction by the previous baffles, the cumulative reduction of the effective aperture for meridional rays at this angle is 55% of the full aperture. Thus, the effective aperture for meridional rays at 14.5 arcminutes is 45% of the full aperture. The last baffle of the device of the third illustrative embodiment is designated by the number 51 in Table III. Thus, taking into account the reduction by all baffles, the cumulative reduction of effective aperture for meridional rays is 55 percent of the full aperture. It results in reduction of various off-axis aberrations. The extent of the reduction depends on the angle of incidence of beam and the particular off-axis aberration. For example, since third-order tangential coma is proportional to square of the aperture, the reduction of the spot size in tangential plane for this device can be almost 5 times at 0.25 degree off-axis.

Taking into account the reduction by the previous baffles, the cumulative reduction of effective aperture for meridional rays is about 55% of the full aperture. Thus, the effective aperture for meridional rays at this angle of incidents is approximately 45% of the full aperture.

In order to illustrate the performance of the optical device of the third illustrative embodiment, consider using this device with the Dall-Kirkham telescope. The Dall-Kirkham telescope is a Cassegrain-type reflector. It is relatively easy to make and collimate this telescope. Unfortunately, this telescope's design has large coma aberration. This is reason why the present invention may be advantageously used with the Dall-Kirkham telescope.

Reference should now be made to Fig. 31, which is the perspective cut-away view of the Dall-Kirkham telescope in conjunction with the optical device according to third illustrative embodiment. The set of the baffles is designated as 64. The primary concave mirror is designated as 65 and the secondary convex mirror designated as 66. The device and the telescope are placed coaxially. The distance between the rear end of the device and the primary mirror of this Dall-Kirkham telescope is 260 mm. This distance can usually be changed without affecting the performance of the system while the device does not obscure the rays that are reflected from the primary mirror. The sample design of the Dall-Kirkham telescope is described in the book "Telescope Optics" by Harrie Rutten and Martin van Venrooij (Willmann-Bell, 1988) at page 70. The design of the present Dall-Kirkham telescope is very close to the sample design mentioned above, with minor changes. The aperture of this astronomical telescope is 200 mm, the effective focal length is about 1600 mm and the f-number is f/8. The focal length of the concave elliptical primary mirror of this telescope is 600 mm and Schwarzschild constant (also known as conic constant) is -0.613. The radius of curvature of the convex spherical secondary mirror is 628 mm. The distance between the secondary and primary mirrors is 404 mm.

In order to show the improvement in the off-axis optical resolution and increasing of the field of view by this device, a number of spot diagrams are presented below. Figs.

32-37 show the spot diagrams of the Dall-Kirkham telescope for six different angles from zero to 0.25 degree off-axis with the step of 0.05 degree, respectively. The circles at every spot diagram indicate Airy disk at the full aperture. The diameter of Airy disk for this telescope is 10.8 micrometers. Figs. 38-43 show the spot diagrams of the same Dall-Kirkham telescope used in conjunction with the device of this embodiment at the same angles. Note that these spot diagrams are scaled-up twice when compared with the Figs. 32-37 because the spots are smaller. The diameter of Airy disk in Figs. 38-43 is also 10.8 micrometers but depicted twice as large as in Figs. 32-37 due to scaling respectively. As can be seen from these spot diagrams, the present invention significantly improve off-axis optical resolution of this Dall-Kirkham telescope. The spot sizes for both the middle and edge parts of the field of view are significantly reduced if the present invention is used. For example the RMS spot size at 0.25 degree off-axis at Fig. 43 is reduced approximately in 2.5 times when compared with spot size of the Dall-Kirkham telescope are shown in Fig. 37. In addition, Figs. 44-49 show spots diagrams of the Dall- Kirkham telescope with these baffles at 0.0 degree, 0.1 degree, 0.2 degree, 0.3 degree, 0.4 degree and 0.5 degree off-axis respectively. These diagrams illustrate the performance of this embodiment at extended field of view. The RMS spot size at 0.5 degree off-axis of the Dall-Kirkham telescope with the use of the device of this embodiment is reduced approximately in 3.2 times when compared with spot size of the Dall-Kirkham telescope without the device. Actually, the RMS spot radius at 0.5 degree off-axis of Dall-Kirkham telescope with the baffles is close to the RMS spot radius at 0.15 degree off-axis of the Dall-Kirkham telescope without the baffles. This is significant improvement.

This is just a sample of the optical device of the invention with using the baffles of the different heights. This specific device can not be completely embedded into the tube of this specific Dall-Kirkham telescope due to the large size. However, the separation distances of this type of baffles can be reduced if needed. In this case, the device can be compact enough to fit the telescope’s tube.

Although all three illustrative embodiments have some groups of baffles of the same heights, it is usually not optimal. These illustrative embodiments describe the simple de vices of the invention, just for illustration. In general case, there are optimal distributions for heights of every baffle and the separations between them, depending on the specific optical system and the application’s requirements.

Another important advantage of the baffles of the different heights is possibility to choose the same separation distance because the baffles, which are made in the form of an array of hollow columnar cells formed between thin walls, can have the same cell’s size. For example, these cells can be hexagonal shaped. A hexagon can be inscribed in a circle. For this reason, the size of hexagonal cell can be defined as the diameter of this circle. The diameter of this circle is usually referred to as honeycomb cel! diameter. The hexagon shaped baffle structure can be used in the device similar to the third illustrative embodiment. The perspective cut-away view of the honeycomb baffles with highly exaggerated cells is shown in Fig. 10. The cells are columnar and hexagonal in cross section. In the case of the third illustrative embodiment, the honeycomb cell diameter should be approximately 1 mm. The thickness and heights of the columnar cells should be as specified in Table III. The baffles in the form of an array of hollow columnar hexagon cells formed between thin walls, that are parallel to the optical axis of the subsequent optical system, can provide with significant better off-axis optical resolution due to much better blocking of skew rays.

Table I

Table I - continued

Table II

Table II - continued

Table III

Table III - continued

Claims

1. A device for reduction of off-axis aberrations of imaging optical systems, comprising a set of thin hollow cylinders of decreasing diameter that are placed one inside another; wherein said set of thin hollow cylinders is placed in front of a subsequent optical system, walls of said thin hollow cylinders are configured so as to be substantially parallel to the optical axis of the subsequent optical system, heights of said thin hollow cylinders and separation distances between them are selected so as to block to some extent beams of incident oblique rays depending on angles of incidence of said beams with respect to the optical axis of said optical system, the heights of said thin hollow cylinders and the separation distances between them are selected so as the extent of the blocking of said beams increases with increase in the angles of incidence of said beams, the heights of said thin hollow cylinders and the separation distances between them are selected so as to block peripheral parts of said beams to the significantly greater extent than internal parts of said beams.

2. The device of claim 1, wherein said device is placed inside a housing of the optical system.

3. The device of claim 1, wherein said device is configured to be removably attached to the optical system.

4. The device of claim 1, wherein at least one said thin hollow cylinder has perforation.

5. The device of claim 1, wherein said thin hollow cylinders are substantially circular in cross-section.

6. A device for reduction of off-axis aberrations of imaging optical systems, comprising a flat, thin and long strip that is twisted into three-dimensional structure; wherein said three-dimensional structure is spiral in cross-section, parts of said strip have significant separation distances between turns of said three- dimensional structure, said three-dimensional structure is placed in front of a subsequent optical system, said strip is configured so as to be substantially parallel to the optical axis of the subse quent optical system, heights and said separation distances are selected so as to block to some extent beams of incident oblique rays depending on angles of incidence of said beams with respect to the optical axis of said optical system, the heights and said separation distances between said turns of said three-dimensional structure are selected so as the extent of the blocking of said beams of incident oblique rays increases with increase in the angles of incidence with respect to the optical axis of said optical system, the heights and the separation distances between said turns of said three-dimensional structure are selected so as to block peripheral parts of said beams of incident oblique rays to the significantly greater extent than internal parts of said beams.

7. The device of claim 6, wherein said device is placed inside a housing of the optical system.

8. The device of claim 6, wherein said device is configured to be removably attached to the optical system.

9. The device of claim 6, wherein said strip has perforation.

10. The device of claim 6, wherein the separation distance between said turns of said three-dimensional structure is substantially constant.

11. The device of claim 6, wherein said strip has substantially the same height along the length of said strip.

12. A device for reduction of off-axis aberrations of imaging optical systems, comprising an array of hollow columnar cells formed between thin walls; wherein said array of hollow columnar cells is placed in front of an optical system, said walls are configured so as to be substantially parallel to the optical axis of the subsequent optical system, heights and cell sizes of said cells are selected so as to block to some extent beams of incident oblique rays depending on angles of incidence of said beams with respect to the optical axis of said optical system, the heights and cell sizes of said cells are selected so as the extent of the blocking of said beams increases with increase in the angles of incidence with respect to the optical axis of said optical system, the heights and the cell sizes of said cells are selected so as to block peripheral parts of said beams to the significantly greater extent than internal parts of said beams.

13. The device of claim 12, wherein said device is placed inside a housing of the optical system.

14. The device of claim 12, wherein said device is configured to be removably attached to the optical system.

15. The device of claim 12, wherein at least one of said cells has perforation.

16. The device of claim 12, wherein at least one of said cells is substantially hexagonal in cross section.