CN217467333U

CN217467333U - Broadband coaxial four-reflector imaging optical system

Info

Publication number: CN217467333U
Application number: CN202221030840.9U
Authority: CN
Inventors: 王士铭; 刘贯领; 王英瑞
Original assignee: Tianjian Aurora Beijing Technology Development Co ltd
Current assignee: Tianjian Aurora Beijing Technology Development Co ltd
Priority date: 2022-04-29
Filing date: 2022-04-29
Publication date: 2022-09-20
Anticipated expiration: 2032-04-29

Abstract

The utility model relates to the technical field of optics, in particular to coaxial four speculum imaging optical system of broadband, including speculum and receiver, the center is equipped with the first speculum in hole, the second speculum, the center is equipped with the third speculum in hole, the center is equipped with the fourth speculum center in hole and all sets up on same axis, and the second speculum is installed on the reflection light path of first speculum, and the third speculum is placed in the central hole of first speculum, and the fourth speculum is installed on third speculum reflection light path, still includes the fifth speculum of slope placement on the central axis, image on the receiver after the fifth speculum reflects; the system is compact in structure, suitable for being used under various conditions, capable of solving the contradiction of small and light-weight, long-focus and high-resolution application requirements of the space camera, and meanwhile, the system is appropriate in center blocking, free of chromatic aberration, wide in working waveband range, good in imaging quality and capable of achieving low-distortion and high-quality imaging effects.

Description

Broadband coaxial four-reflector imaging optical system

Technical Field

The utility model relates to the field of optical technology, in particular to coaxial four speculum imaging optical system of broadband.

Background

With the development of space remote sensing detection and aerospace technology, the requirements on a space optical system are higher and higher, and the requirements are expressed in the following points: higher spatial resolution, wider spectral range, ability to observe all day, etc. In response to these requirements, the optical system used in this field generally has the characteristics of large aperture, long focal length, wide wavelength band, compact structure, and the like.

The main forms of optical systems are refractive, catadioptric and reflective systems. To improve resolution, spatial cameras generally require a large aperture, a long focal length, a compact structure, and a light weight. The large-aperture lens of the refraction system has higher material and processing cost and heavy weight, and meanwhile, in order to correct various aberrations, the number of the lenses is inevitably increased, and the requirements of light weight and compact structure are difficult to meet, so the large-aperture space optical system basically does not adopt the mode; although the conventional refraction-reflection system avoids the disadvantage of refraction, the optical cylinder length is generally 1/2-1/3, and the structure is not compact enough.

The reflective optical system has been developed for over 400 years, has the advantages of no chromatic aberration, good environmental adaptability, large aperture, wide wavelength band and other optical performances, and is more and more widely applied to modern optical instruments and equipment, particularly space optical systems. The mirror blank material adopted by the reflecting system has the characteristics of small density, low thermal expansion coefficient, high thermal conductivity, large elastic modulus and the like, can effectively reduce the total weight of the optical system, and can ensure the stability of the optical performance in the environment with a large temperature change range.

Historically, reflective optical systems have undergone two major stages of development, a single mirror optical system, represented by a newton telescope, and a two mirror optical system, represented by a cassegrain system. The aberration correction capability of the reflective optical system is greatly improved by the increase of the number of the reflectors and the asphericization of the reflector surface type in the system, but the two-mirror reflective optical system can only better correct spherical aberration and coma aberration, and further improvement of the optical performance is limited.

As early as 1905, Schwarzschild has demonstrated that from an engineering practical point of view, a two-mirror reflective optical system cannot simultaneously correct four seidel aberrations of spherical aberration, coma, astigmatism and field curvature, but when a third, focal-power mirror is added to the system, all the third-order aberrations can be eliminated with different mirror combinations. In more than 100 years, schemes of three-mirror reflective optical systems with various structural forms are continuously proposed by researchers, abundant structures are evolved, and the reflective optical systems enter the development era of three-mirror structures.

Compared with the two reflectors, the three reflectors are easier to control stray light outside a visual field, but because the image surface is usually inside the system, a plane mirror is usually required to be added for leading out the image surface, and secondary blocking of light rays is required to be avoided. If a strabismus field structure is adopted, secondary blocking can be avoided, but residual coma, astigmatism and distortion of the marginal field of view are large, and imaging quality is affected; if the real image of the primary mirror and the secondary mirror and the inclined plane reflector near the exit pupil of the three mirrors are utilized to fold the plane reflector, so that the real image of the primary mirror and the secondary mirror passes through the central hole of the plane reflector without blocking, and the light rays reflected by the three mirrors do not pass through the central hole of the plane reflector but pass through the annular aperture of the plane reflector without blocking, the method needs to increase the magnification of the three mirrors in order to reduce the total length of the system, and a plurality of plane mirrors for folding and rotating the light paths are needed to be added for leading out an image surface, so that the system is more complicated.

SUMMERY OF THE UTILITY MODEL

An object of the utility model is to overcome the aforesaid not enough, provide a coaxial four speculum imaging optical system of broadband, provide a coaxial four speculum imaging optical system, compact structure uses under the suitable multiple condition, solves the contradiction of the small-size lightweight of space camera, long focus high resolution application demand, and simultaneously, this system center blocks suitably, and no colour difference, the working wave band scope is wide, and imaging quality is good, has realized the high-quality formation of image effect of low distortion.

The utility model provides a specific technical scheme as follows:

a broadband coaxial four-reflector imaging optical system comprises reflectors and a receiver, wherein the centers of a first reflector, a second reflector, a third reflector and a fourth reflector are arranged on the same axis, the first reflector and the second reflector are provided with holes in the centers, the fourth reflector is provided with holes in the centers, the centers of the fourth reflector are arranged on the same axis, the second reflector is arranged on a reflection light path of the first reflector, the third reflector is arranged in the central hole of the first reflector, the fourth reflector is arranged on the reflection light path of the third reflector, the broadband coaxial four-reflector imaging optical system also comprises a fifth reflector which is obliquely arranged on a central axis, an included angle between the surface normal of the fifth reflector and the central axis is 40-50 degrees, and the fifth reflector is imaged on the receiver after being reflected;

the first reflector is an even-order aspheric reflector, the second reflector is a plane mirror, the third reflector is an ellipsoidal reflector, and the fourth reflector is an even-order aspheric reflector.

In some embodiments, the first mirror is an aperture stop.

In some embodiments, the fifth mirror surface normal makes an angle of 45 ° with the central axis.

In some embodiments, the structural parameters are:

the curvature radius of the first reflector is-343.124 mm, the coefficient of the quadric surface is-0.178, the outer aperture diameter is 256mm, and the diameter of the central hole is 122 mm;

the curvature of the second reflector is a plane reflector, and the outer aperture is 94 mm;

the curvature radius of the third reflector is-84.335 mm, the coefficient of the quadric surface is-0.083, the outer aperture diameter is 108mm, and the diameter of the central hole is 46 mm;

the curvature radius of the fourth reflector is-75.375 mm, the coefficient of the quadric surface is 1.638, the outer caliber is 33mm, and the diameter of the central hole is 15.4 mm;

the fifth reflecting mirror comprises a plane mirror and a spectroscope, wherein the spectroscope realizes dual-channel and waveband imaging.

In some embodiments, the even aspheric coefficients of the first and fourth mirrors are as follows:

reflecting mirror	2 degree item	Item 4	Item of 6	8 items
					First reflector	4.165×10 ^-5	1.947×10 ^-9	5.896×10 ^-15	—
Fourth reflector	-7.23×10 ^-4	-8.655×10 ^-8	-2.778×10 ^-10	-3.655×10 ^-13

In some embodiments, the first mirror center to the second mirror center distance is 120.732 mm; the second mirror center to the third mirror center distance 128.793 mm; the third mirror center to fourth mirror center distance is 71.155 mm; the fourth mirror center to fifth mirror center distance is 120.476 mm; the distance between the fifth reflector and the center of the receiver is 98.999 mm.

In some embodiments, the second mirror has an obscuration ratio of 0.37 from the first mirror.

In some embodiments, the clear aperture of the system is 240 mm-260 mm.

In some embodiments, the working band of the system is 400-1100 nm; the maximum field of view is 1.04 degrees in the meridian direction multiplied by 1.26 degrees in the sagittal direction; the diagonal field of view is 1.64 degrees; focal length 1600mm, F number 6.3.

In some embodiments, the radius of the light spot RMS is 0.680-3.575 μm within the field of view; the diffraction limit of the visible light band is 325lp/mm, and the full field transfer function is close to the diffraction limit.

Has the advantages that:

the broadband coaxial four-reflector imaging optical system has the advantages that through the coaxial arrangement of the first reflector to the fourth reflector, each optical surface is rotationally symmetrical relative to the main optical axis, and the coefficients of the secondary curved surfaces of the first reflector, the third reflector and the fourth reflector are smaller, so that the difficulty of production and processing is greatly reduced; the blocking ratio of the second reflector to the first reflector is reasonable; the fourth reflector is provided with a hollow hole, so that secondary shielding of light rays is effectively reduced; all optical elements are coaxially arranged, so that the assembly and adjustment difficulty during system integration is reduced.

The light reflected by the fourth reflector is naturally led out through a central hole of the third reflector, so that the installation, adjustment and use of the receiver are facilitated; the fifth reflector which is obliquely arranged is arranged, so that the axial length is reduced, and the small and compact structure advantage is realized; and the fifth reflecting mirror is replaced by a light splitting plate, and the plurality of spectral detectors can be imaged respectively.

The residual aberration is corrected by using the fourth reflector with the smaller caliber and the concave surface for reflection, the aberration correction and the balance of the system are better realized by means of the even aspheric surface and the ellipsoidal surface, the full field distortion is smaller, the imaging quality is better, the problems of high image quality, long focal length, compact structure and the like are solved, the optical lens can be used for space equipment for ground target detection, information acquisition and the like, and the optical lens has important practical significance and application prospect.

Drawings

The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the application and together with the description serve to explain the application and not to limit the application.

In the drawings:

fig. 1 is a schematic structural diagram of a technical scheme of the broadband coaxial four-mirror imaging optical system of the present invention;

fig. 2 is a Modulation Transfer Function (MTF) curve of the broadband coaxial four-mirror imaging optical system of the present invention at 20 ℃ in the visible light band, and both the mirror and the outer structural member are aluminum products;

fig. 3 is a Modulation Transfer Function (MTF) curve of the broadband coaxial four-mirror imaging optical system of the present invention at a near-infrared band of 20 ℃, the mirrors and the outer structural member are aluminum products;

FIG. 4 is a dot-column diagram of the broadband coaxial four-reflector imaging optical system of the present invention at 20 ℃ in the visible light band, wherein the reflector and the outer structural member are both made of aluminum;

fig. 5 shows the field curvature and distortion curve of the broadband coaxial four-reflector imaging optical system of the present invention at 20 ℃ in the visible light band, and the reflector and the outer structural member are both aluminum products;

FIG. 6 shows a distortion grid of the broadband coaxial four-reflector imaging optical system of the present invention at 20 ℃ in the visible light band, wherein the reflector and the outer structural member are made of aluminum;

FIG. 7 is a Modulation Transfer Function (MTF) curve of the broadband coaxial four-mirror imaging optical system of the present invention at-40 ℃ in the visible light band, wherein the mirrors and the outer structure are made of aluminum;

FIG. 8 is a dot-column diagram of the broadband coaxial four-reflector imaging optical system of the present invention at-40 deg.C of visible light band, the reflector and the outer structural member are both aluminum products;

fig. 9 is a Modulation Transfer Function (MTF) curve of the broadband coaxial four-mirror imaging optical system of the present invention at a visible light band of 60 ℃, the mirrors and the outer structural member are aluminum products;

fig. 10 is a dot-column diagram of the broadband coaxial four-reflector imaging optical system of the present invention at 60 ℃ in the visible light band, and both the reflector and the outer structural member are made of aluminum;

fig. 11 is a schematic structural diagram of the technical scheme of the present invention in which the fifth reflecting mirror is a spectroscope in the broadband coaxial four-reflecting mirror imaging optical system.

In the figure:

a first mirror 101; a second mirror 102; a third mirror 103; a fourth mirror 104; a fifth mirror 105; a near-infrared receiver 106; a visible light receiver 107.

Detailed Description

As used in the specification and in the claims, certain terms are used to refer to particular components. As one skilled in the art will appreciate, manufacturers may refer to a component by different names. The description and claims do not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms "include" and "comprise" are used in an open-ended fashion, and thus should be interpreted to mean "include, but not limited to,"; the description which follows is a preferred embodiment for carrying out the present application, but is made for the purpose of illustrating the general principles of the application and not for the purpose of limiting the scope of the application; the protection scope of the present application shall be subject to the definitions of the appended claims.

As shown in fig. 1-11, the utility model adopts the following technical scheme:

the center of a first reflector 101, a second reflector 102, a third reflector 103 and a fourth reflector 104 which are provided with holes at the centers are arranged on the same axis, the second reflector 102 is arranged on a reflection light path of the first reflector 101, the third reflector 103 is arranged in the central hole of the first reflector 101, the fourth reflector 104 which is provided with holes at the center is arranged on a reflection light path of the third reflector 103, the imaging optical system also comprises a fifth reflector 105 which is obliquely arranged on the central axis, the included angle between the surface normal of the fifth reflector 105 and the central axis is 40-50 degrees, the fifth reflector 105 is imaged on a receiver after being reflected, and the receiver comprises a near infrared receiver 106 and a visible light receiver 107.

The first reflecting mirror 101 is an even-order aspheric reflecting mirror, the second reflecting mirror 102 is a plane mirror, the third reflecting mirror 103 is an ellipsoidal reflecting mirror, and the fourth reflecting mirror 104 is an even-order aspheric reflecting mirror.

The utility model discloses a first embodiment, as shown in fig. 1-10:

as shown in fig. 1, a broadband coaxial four-mirror imaging optical system includes mirrors and a receiver, where centers of a first mirror 101, a second mirror 102, a third mirror 103, and a fourth mirror 104 are all disposed on the same axis, the first mirror 102, the second mirror 102, the third mirror 103, and the fourth mirror 104 are disposed on the same axis, the second mirror 102 is installed on a reflection light path of the first mirror 101, the third mirror 103 is disposed in a central hole of the first mirror 101, the fourth mirror 104 is installed on a reflection light path of the third mirror 103, and the broadband coaxial four-mirror imaging optical system further includes a fifth mirror 105 obliquely disposed on the central axis, a surface normal of the fifth mirror 105 forms an angle of 40 ° to 50 ° with the central axis, the fifth mirror 105 is a flat mirror, and images on the receiver 106 after reflection.

The first reflecting mirror 101 is an even-order aspheric reflecting mirror, the second reflecting mirror 102 is a plane mirror, the third reflecting mirror 103 is an ellipsoidal reflecting mirror, and the fourth reflecting mirror 104 is an even-order aspheric reflecting mirror. The first reflector 101 is an aperture diaphragm; the incident light sequentially passes through the first reflector 101, the second reflector 102, the third reflector 103, and the fourth reflector 104, and is reflected by the fifth reflector 105, which refracts the light, to be imaged on the receiver 106. The whole optical system has a compact structure, and secondary shielding of light rays is effectively reduced; all optical elements are coaxially arranged, so that the assembly and adjustment difficulty during system integration is reduced.

In an example, the surface normal of the fifth mirror 105 is preferably at an angle of 45 ° to the central axis.

In an example, the structural parameters are:

the curvature radius of the first reflector 101 is-343.124 mm, the coefficient of the quadric surface is-0.178, the curvature radius of the first reflector is-343.124 mm, the coefficient of the quadric surface is-0.178, the outer aperture is 256mm, and the diameter of the central hole is 122 mm;

the fifth mirror 105 is a flat mirror, and is reflected and imaged on the receiver 106.

In an example, even aspheric coefficients of the first and

fourth mirrors

101, 104 are as follows:

reflecting mirror	2 degree item	Item 4	Item of 6	Term of 8 orders
					First reflector 101	4.165×10-5	1.947×10-9	5.896×10-15	—
Fourth reflector 104	-7.23×10-4	-8.655×10-8	-2.778×10-10	-3.655×10-13

In an example, the first mirror 101 center to the second mirror 102 center distance 120.732 mm; the distance 128.793mm from the center of the second mirror 102 to the center of the third mirror 103; the distance 71.155mm from the center of the third mirror 103 to the center of the fourth mirror 104; the distance 120.476mm from the center of the fourth mirror 104 to the center of the fifth mirror 105; the fifth mirror 105 is located at a distance of 98.999mm from the center of the receiver 106.

The shielding ratio of the second mirror 102 to the first mirror 101 is 0.37.

The clear aperture of the system is 240mm to 260mm, preferably 254 mm.

In an example, the working band of the system is 400-1100 nm; the maximum field of view is 1.04 degrees in the meridian direction multiplied by 1.26 degrees in the sagittal direction; the diagonal field of view is 1.64 degrees; focal length 1600mm, F number 6.3.

In an example, the radius of the light spot RMS is 0.680-3.575 μm in the field range; the diffraction limit of the visible light band is 325lp/mm, and the full field transfer function is close to the diffraction limit.

In the example, the first mirror 101 to the fourth mirror 104 are coaxially disposed, the optical surface is rotationally symmetric with respect to the main axis due to the even power, and the coefficients of the optical surface shown in the following table are changed in the design stage to achieve the purpose of correcting and balancing the aberration and reduce the processing difficulty.

FIG. 2 is a transfer function curve of the system in the visible light band, the multi-color diffraction MTF, data 0.4861-0.6563 μm; dough making: like this.

FIG. 3 shows the transfer curve of the system in the near infrared band (0.7-1.1 μm), the multi-color diffraction MTF, and the data 0.7000-1.1000 μm; dough making: an image; it can be seen that in the two wave bands, the transfer function is close to the diffraction limit, the imaging quality is good, and the diffraction limit of the transfer function curve of the near infrared wave band is lower than that of the visible light wave band.

Fig. 4 is a dot diagram of the system in the visible light band, and the RMS (root mean square) radius of each field is: 0.68 μm, 3.443 μm, 3.536 μm, 3.512 μm, 3.512 μm, 3.575 μm, the radius of airy disk is 4.514 μm; GEO (geometric) radius, 0.888 μm, 9.014 μm, 10.139 μm, 10.499 μm, 10.499 μm, 10.499 μm, 10.499 μm, zoom bar 40, reference: the chief ray. Therefore, each field of view spot is in the Airy spot, the imaging quality is good, the energy is concentrated, the radius of the Airy spot is increased along with the wavelength, and the dot sequence diagrams of other wave bands are not listed.

FIG. 5 is a field curvature/F-Tan (theta) distortion curve of the optical system of the present invention at 20 ℃ in the visible light band, the reflector is made of aluminum; the maximum field of view is 0.817 degrees, wavelengths 0.486, 0.588, 0.656 in mm.

FIG. 6 shows a distortion grid in the visible band at 20 ℃ with an aluminum mirror. The field of view is 1.1553w, 1.1553h mm,

the maximum distortion of the full field of view is not more than 0.55%, the zoom is 1.000X, the wavelength is 0.5876 mu m, the final imaging deformation is small, and the image distortion basically does not exist.

FIG. 7 is a MTF curve at-40 ℃ for the present system with mirrors made of aluminum, data 0.4861 to 0.6563 μm, area: an image;

FIG. 8 is a dot-column diagram at-40 ℃ for the present system where the mirrors are all aluminum; RMS (root mean square) radii for each field are: 0.679 μm, 3.438 μm, 3.531 μm, 3.507 μm, 3.570 μm; GEO radius, 0.887 μm, 9.001 μm, 10.125 μm, 10.484 μm, 10.484 μm, 10.484 μm, 10.484 μm, zoom bar 40, reference: the chief ray.

FIG. 9 is an MTF curve at 60 ℃ for the present system with mirrors made of aluminum, data 0.4861 to 0.6563 μm, area: an image; under a wider temperature environment of-40-60 ℃, the temperature-sensitive adhesive has no obvious change and good thermal stability, and can meet the use requirements of different temperature environments.

FIG. 10 is a dot alignment chart at 60 ℃ for the present system where the mirrors are all aluminum. The RMS (root mean square) radius of each field is: 0.681 μm, 3.446 μm, 3.540 μm, 3.516 μm, 3.516 μm, 3.578 μm, 3.578 μm; GEO radius, 0.889 μm, 9.022 μm, 10.149 μm, 10.509 μm, 10.509 μm, 10.509 μm, 10.509 μm, zoom bar 40, reference: a chief ray; under a wider temperature environment of-40-60 ℃, the temperature-sensitive adhesive has no obvious change and good thermal stability, and can meet the use requirements of different temperature environments.

The utility model discloses the second embodiment:

as shown in fig. 11, a broadband coaxial four-mirror imaging optical system includes mirrors and a receiver, a first mirror 101 with a hole at the center, a second mirror 102, a third mirror 103 with a hole at the center, and a fourth mirror 104 with a hole at the center all disposed on the same axis, the second mirror 102 is mounted on the reflection light path of the first mirror 101, the third mirror 103 is placed in the central hole of the first mirror 101, the fourth reflecting mirror 104 is installed on the reflected light path of the third reflecting mirror 103, and further includes a beam splitter 105 obliquely placed on the central axis, an included angle between the normal line of the surface of the spectroscope 105 and the central axis is 40-50 degrees, the spectroscope 105 is a spectroscope and divides light into two beams which are respectively imaged on two receivers, and the receivers comprise a near infrared receiver 106 and a visible light receiver 107.

The first reflecting mirror 101 is an even-order aspheric reflecting mirror, the second reflecting mirror 102 is a plane mirror, the third reflecting mirror 103 is an ellipsoidal reflecting mirror, and the fourth reflecting mirror 104 is an even-order aspheric reflecting mirror. The first reflector 101 is an aperture diaphragm; after the incident light sequentially passes through the first reflecting mirror 101, the second reflecting mirror 102, the third reflecting mirror 103, and the fourth reflecting mirror 104, the beam splitter 105 serving as a beam splitter splits the incident light into two beams, which are respectively imaged on the near-infrared receiver 106 and the visible light receiver 107, and the dot-and-column diagram of this embodiment is not described again.

The surface of the spectroscope adopts a specific optical film layer, such as a 400-740 nm high-reflection and 850-1200 nm high-transmission optical film system, so that light with a wave band of 400-740 nm can be distributed to the receiver 106, and light with a wave band of 850-1200 nm can be distributed to the receiver 107, light with a wider wave band range can be imaged respectively, and the requirement of the space optical field on multispectral imaging can be met.

The foregoing description shows and describes the preferred embodiments of the present application, but as aforementioned, it is to be understood that the application is not limited to the forms disclosed herein, but is not to be construed as excluding other embodiments and is capable of use in various other combinations, modifications, and environments and is capable of changes within the scope of the application as described herein, commensurate with the above teachings, or the skill or knowledge of the relevant art. And that modifications and variations may be effected by those skilled in the art without departing from the spirit and scope of the application, which is to be protected by the claims appended hereto.

Claims

1. The utility model provides a coaxial four speculum imaging optical system of broadband, includes speculum and receiver, its characterized in that: the center of a first reflector, a second reflector, a third reflector and a fourth reflector is arranged on the same axis, the first reflector and the second reflector are provided with holes, the third reflector is arranged on a reflection light path of the first reflector, the third reflector is arranged in the central hole of the first reflector, the fourth reflector is arranged on a reflection light path of the third reflector, the fifth reflector is obliquely arranged on the central axis, an included angle between the surface normal of the fifth reflector and the central axis is 40-50 degrees, and the fifth reflector is reflected and imaged on a receiver;

2. The broadband on-axis four-mirror imaging optical system according to claim 1, characterized in that: the first reflector is an aperture diaphragm.

3. The broadband on-axis four-mirror imaging optical system according to claim 2, characterized in that: and the included angle between the surface normal of the fifth reflector and the central axis is 45 degrees.

4. The broadband on-axis four-mirror imaging optical system according to claim 1, characterized in that: the structural parameters are as follows:

5. The broadband coaxial four-mirror imaging optical system according to claim 4, wherein even-order aspheric coefficients of the first mirror and the fourth mirror are as follows:

2 degree item Item 4 Item of 6 8 items First reflector 4.165×10 ^-5 1.947×10 ^-9 5.896×10 ^-15 — Fourth reflector -7.23×10 ^-4 -8.655×10 ^-8 -2.778×10 ^-10 -3.655×10 ^-13

6. The broadband on-axis four-mirror imaging optical system of claim 5, wherein the first mirror center to the second mirror center distance is 120.732 mm; the second mirror center to the third mirror center distance 128.793 mm; the third mirror center to fourth mirror center distance is 71.155 mm; the distance from the center of the fourth mirror to the center of the fifth mirror is 120.476 mm; the distance between the fifth reflector and the center of the receiver is 98.999 mm.

7. The broadband on-axis four-mirror imaging optical system according to claim 6, wherein the second mirror has an obscuration ratio of 0.37 for the first mirror.

8. The broadband on-axis four-mirror imaging optical system according to claim 7, wherein the clear aperture of the system is 240mm to 260 mm.

9. The broadband coaxial four-mirror imaging optical system according to claim 8, wherein the operating wavelength band of the system is 400-1100 nm; the maximum field of view is 1.04 degrees in the meridian direction multiplied by 1.26 degrees in the sagittal direction; the diagonal field of view is 1.64 degrees; focal length 1600mm, F number 6.3.

10. The broadband coaxial four-mirror imaging optical system according to claim 9, wherein in a field of view range, a light spot RMS radius is 0.680-3.575 μm; the diffraction limit of the visible light band is 325lp/mm, and the full field transfer function is close to the diffraction limit.