An unobstructed long-focal-length star sensor optical system

Abstract

The invention discloses an unobstructed long-focal-length star sensor optical system, comprising an aperture stop, a front lens group, a reflector group, a rear lens group and an image plane, wherein the front lens group comprises a first lens and a second lens arranged in sequence from front to back, the aperture stop is located in front of the first lens, the reflector group comprises a primary reflector and a secondary reflector, and the rear lens group comprises a third lens and a fourth lens arranged in sequence from front to back; the invention adopts a catadioptric structure based on an aperture off-axis, effectively shortens the size of the long-focal-length star sensor optical system, solves the problem of central obstruction caused by a coaxial catadioptric system, and improves the energy concentration performance of detecting star light signals.

Description

Technical Field

[0001] The invention relates to the technical field of optical systems, and more particularly to an unobstructed long-focal-length star sensor optical system. Background Art

[0002] Among the known inertial navigation devices, star sensors are one of the most accurate measuring instruments, with a measurement accuracy of sub-second or even higher. Since star sensors use optical systems to detect the position and spectrally stable distribution of stellar light signals in space, the measurement accuracy does not drift over time, providing a stable three-axis attitude angle information output for long-term high-precision flight of spacecraft, and thus have been widely used in the field of high-precision autonomous navigation.

[0003] As the core device of the star sensor, the star sensor optical system is the key component for the star sensor to achieve high signal-to-noise ratio star spectrum energy collection and high-precision star center of mass position detection. The star sensor optical system detects stars with weak energy and wide spectral distribution, which belongs to point target detection.

[0004] The main parameters of the star sensor optical system include focal length, field of view, relative aperture, imaging spectrum, and single star measurement accuracy. The focal length of the star sensor optical system is inversely proportional to the single star measurement accuracy. The longer the focal length, the higher the measurement accuracy. The focal length of the current mainstream star sensor optical system generally does not exceed 50mm, and most of them are concentrated in the range of 20mm to 30mm. The detection field of view is relatively large, the detection spectrum range generally does not exceed 300nm, the single star measurement accuracy is not high, and the star detection capability is relatively limited. In order to pursue higher star detection accuracy, the use of long-focal-length optical systems is an effective means. With the development of technologies in the fields of high-resolution stereo mapping cameras, space astronomical observation telescopes, and space-guided weapon systems, there is a demand for sub-second or even higher-precision star sensors to meet the key performance requirements of high-precision ground positioning, long-term image stabilization observation, or autonomous navigation of long-duration flight attitudes in the application system. The core technology is to use a long-focal-length star sensor optical system to improve the resolution of a single pixel, and then use a subdivision algorithm to further improve the accuracy of the centroid resolution. However, when the focal length of the star sensor optical system approaches or reaches the meter level, the pure transmission optical system not only has a long system size, but also has difficulty in correcting secondary spectral aberrations under a wide spectrum, and cannot realize the collection of wide-spectrum star light signals. It cannot meet the application requirements of the space platform in terms of size or performance. Although the reflective optical system can realize light path folding and obtain a compact optical system layout design, it needs to use aspherical surfaces to correct aberrations, which are difficult to manufacture and assemble, and are not conducive to reducing costs.

[0005] Further research found that the coaxial catadioptric optical system can effectively solve the above-mentioned design contradictions and achieve a high-quality and lightweight design; however, due to the obstruction of the secondary reflector, the central obstruction causes the central Airy disk diffraction energy to shift to the secondary peak, resulting in a decrease in energy concentration performance. Under the premise of consistent relative aperture, even if the optical system achieves the diffraction-limited image quality, the obstructed optical system cannot achieve the same star light signal gathering ability as the unobstructed optical system, resulting in a decrease in the performance of the star sensor optical system. Summary of the invention

[0006] The technical problem to be solved by the present invention is that the central obstruction of the existing star sensor optical system causes a decrease in energy concentration.

[0007] The present invention provides an unobstructed long-focal-length star sensor optical system, which adopts a catadioptric structure with an off-axis aperture, greatly reduces the size of the optical system, and is beneficial to star detection.

[0008] The solution of the present invention to solve its technical problem is:

[0009] An unobstructed long-focal-length star sensor optical system comprises an aperture stop, a front lens group, a reflector group, a rear lens group and an image plane, wherein the front lens group comprises a first lens and a second lens sequentially arranged from front to back, the aperture stop is located in front of the first lens, the reflector group comprises a primary reflector and a secondary reflector, the primary reflector is located behind the second lens, the secondary reflector is below the second lens, the rear lens group comprises a third lens and a fourth lens sequentially arranged from front to back, the rear lens group is located behind the secondary reflector, and the image plane is located behind the fourth lens;

[0010] The first lens is a meniscus lens with negative power, the second lens is a biconvex lens with positive power, and the third lens and the fourth lens are both meniscus lenses with negative power;

[0011] The distance between the center of the aperture stop and the optical axis of the optical system is the off-axis amount h, the entrance pupil diameter is D, and the height of the upper edge light of the secondary reflector from the optical axis is h B2 , where h, D and h B2 satisfy:

[0012] 10mm≤h-(D / 2+h B2 )≤35mm;

[0013] The incident light passes through the aperture stop, the first lens and the second lens in sequence to the primary reflector, and the primary reflector reflects the incident light to the secondary reflector. The light beam is reflected again on the secondary reflector to form reflected light. The reflected light passes through the third lens and the fourth lens in sequence and forms an image on the image plane.

[0014] The present invention adopts a catadioptric optical system structure based on off-axis aperture, which can obtain a design result in which the length of the optical system is much smaller than the focal length, avoids the problem of central obstruction in the coaxial catadioptric optical system, and improves the energy concentration performance.

[0015] As a further improvement of the above technical solution, the optical power of the first lens is The optical power of the second lens is The optical power of the optical system is in and satisfy:

[0016]

[0017]

[0018] As a further improvement of the above technical solution, the combined optical power of the reflector group is The optical power of the optical system is in and satisfy:

[0019]

[0020] As a further improvement of the above technical solution, the combined focal length of the rear lens group is The optical power of the optical system is in and satisfy:

[0021]

[0022] As a further improvement of the above technical solution, the distance from the aperture stop to the image plane is the total length L of the optical system, and the focal length of the optical system is f, then L and f satisfy:

[0023] L / f≤0.34.

[0024] As a further improvement of the above technical solution, the first lens, the second lens, the third lens and the fourth lens are all made of the same crown glass.

[0025] As a further improvement of the above technical solution, both the primary reflector and the secondary reflector are spherical.

[0026] The first lens, the second lens, the third lens and the fourth lens are all spherical lenses.

[0027] The optical system of the present invention has reasonable focal power distribution, all lenses and reflectors are spherical, and the processing, manufacturing and assembly tolerances are relatively loose, which reduces the difficulty of processing and assembly, and is conducive to improving the manufacturability and assembly yield of the long-focal-length star sensor optical system.

[0028] As a further improvement of the above technical solution, the front surface curvature radius of the first lens is -311.8mm, the rear surface curvature radius is -561.2mm, the center thickness is 8mm, and the lens aperture is φ75mm; the front surface curvature radius of the second lens is 2911.5mm, the rear surface curvature radius is -958.6mm, the center thickness is 17mm, and the lens aperture is φ75mm; the main reflector is a concave reflector with a curvature radius of -598.6mm and a light aperture of The secondary reflector is a convex reflector with a curvature radius of -273.8 mm and a light aperture of The third lens has a front surface curvature radius of -47.1 mm, a rear surface curvature radius of -66.5 mm, a center thickness of 8 mm, and a clear aperture of The fourth lens has a front surface curvature radius of 32.6 mm, a rear surface curvature radius of 21.2 mm, a center thickness of 15 mm, and a clear aperture of

[0029] As a further improvement of the above technical solution, the distance between the rear surface of the first lens and the front surface of the second lens is 0.1mm; the distance between the rear surface of the second lens and the front surface of the primary reflector is 215mm, and the distance between the front surface of the primary reflector and the rear surface of the secondary reflector is 215mm; the distance between the front surface of the third lens and the rear surface of the secondary reflector is 178.5mm; the distance between the rear surface of the third lens and the front surface of the fourth lens is 0.1mm, and the distance from the rear surface of the fourth lens to the image plane is 18.3mm.

[0030] The present invention realizes the design of a star sensor optical system with a focal length close to the meter level, and the detection spectrum range reaches 650nm, which improves the measurement accuracy of star positions and the collection of starlight energy. Under the same detection aperture, the ability to detect stars can be increased by more than 1 times.

[0031] The present invention adopts a catadioptric structure based on off-axis aperture, effectively shortens the size of the optical system of a long focal length star sensor, solves the center obstruction problem caused by a coaxial catadioptric system, and improves the energy concentration performance of detecting star light signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly describes the drawings required for use in the description of the embodiments. Obviously, the drawings described are only part of the embodiments of the present invention, not all of the embodiments, and those skilled in the art can also obtain other design solutions and drawings based on these drawings without creative work.

[0033] Figure 1 Schematic diagram of the composition structure of the optical system of this embodiment;

[0034] Figure 2 It is a comparison of the energy concentration between an unobstructed optical system and an obstructed optical system;

[0035] Figure 3 is the optical transfer function curve of the optical system of this embodiment;

[0036] Figure 4 It is the energy concentration curve of the optical system of this embodiment. DETAILED DESCRIPTION

[0037] The following will clearly and completely describe the concept, specific structure and technical effects of the present invention in combination with the embodiments and drawings, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by technicians in this field without creative work are all within the scope of protection of the present invention. In addition, all the connection relationships mentioned in the text do not refer to the direct connection of components, but refer to the fact that a better connection structure can be formed by adding or reducing connecting accessories according to the specific implementation situation. The various technical features in the invention can be combined interchangeably without conflicting with each other.

[0038] Example 1, reference Figure 1 , an unobstructed long focal length star sensor optical system, comprising an aperture stop 800, a front lens group, a reflector group, a rear lens group and an image plane 700, wherein the front lens group comprises a first lens 100 and a second lens 200 arranged in sequence from front to back, the aperture stop 800 is located in front of the first lens 100, the reflector group comprises a primary reflector 300 and a secondary reflector 400, the primary reflector 300 is located behind the second lens 200, the secondary reflector 400 is below the second lens 200, the rear lens group comprises a third lens 500 and a fourth lens 600 arranged in sequence from front to back, the rear lens group is located behind the secondary reflector 400, and the image plane 700 is located behind the fourth lens 600;

[0039] The first lens 100 is a meniscus lens with negative power, the second lens 200 is a biconvex lens with positive power, and the third lens 500 and the fourth lens 600 are both meniscus lenses with negative power;

[0040] The incident light passes through the aperture stop 800, the first lens 100 and the second lens 200 in sequence and is emitted to the primary reflector 300. The primary reflector 300 reflects the incident light to the secondary reflector 400. The light beam is reflected again at the secondary reflector 400 to form reflected light. The reflected light passes through the third lens 500 and the fourth lens 600 in sequence and is imaged on the image plane 700.

[0041] The first lens 100 and the second lens 200 form a double separated lens, and the third lens 500 and the fourth lens 600 form a double separated lens.

[0042] refer to Figure 1 , where O is the optical axis of the optical system, h is the distance between the center of the aperture stop 800 and the optical axis, D is the entrance pupil diameter of the optical system, and h B2 is the height of the upper edge light of the secondary reflector 400 from the optical axis.

[0043] The distance between the center of the aperture stop 800 and the optical axis of the optical system is the off-axis distance h, the entrance pupil diameter is D, and the height of the upper edge light of the secondary reflector 400 from the optical axis is h B2 , where h, D and h B2 satisfy:

[0044] 10mm≤h-(D / 2+h B2 )≤35mm.

[0045] In order to avoid blocking light, it is key to reasonably select and design the aperture off-axis amount of the optical system. Under the premise of ensuring the compact design of the optical system, on the one hand, the height of the light on the secondary reflector 400 should be reduced as much as possible. On the other hand, the distance between the center of the aperture stop 800 and the optical axis, that is, the off-axis amount, should ensure that the light will not overlap with the secondary reflector 400 when passing through the front lens group; in addition, the interval between the rear lens group and the secondary reflector 400 should not be designed to be too large, otherwise it will make it difficult to correct optical aberrations and the volume of the optical system perpendicular to the optical axis will also increase.

[0046] The present invention adopts a catadioptric optical system structure based on off-axis aperture, which can obtain a design result in which the length of the optical system is much smaller than the focal length, avoids the problem of central obstruction in the coaxial catadioptric optical system, and improves the energy concentration performance.

[0047] As a preferred embodiment, the optical power of the first lens 100 is The optical power of the second lens 200 is The optical power of the optical system is in and satisfy:

[0048]

[0049]

[0050] The combined optical power of the front lens group is Close to zero.

[0051] As a preferred embodiment, the combined optical power of the reflector group is The optical power of the optical system is in and satisfy:

[0052]

[0053] As a preferred embodiment, the combined optical power of the rear lens group is The optical power of the optical system is in and satisfy:

[0054]

[0055] As a preferred embodiment, both the primary reflector 300 and the secondary reflector 400 are spherical.

[0056] As a preferred embodiment, the first lens 100, the second lens 200, the third lens 500 and the fourth lens 600 are all made of the same crown glass.

[0057] The first lens 100 , the second lens 200 , the third lens 500 and the fourth lens 600 are all spherical lenses.

[0058] In order to reduce the processing and manufacturing costs of the optical system and obtain a cost-effective design solution, the reflector and lens of the present invention both adopt spherical surface shapes, and conventional processing technology can be used to realize the manufacturing and detection of all optical elements.

[0059] This optical system adopts a catadioptric optical system structure based on off-axis aperture, which not only avoids the difficulty of correcting wide-spectrum chromatic aberration, especially secondary spectrum, in a pure transmission optical system under long focal length design, but also can obtain a design result in which the optical system length is much smaller than the focal length; it also avoids the problem of central obstruction in a coaxial catadioptric optical system, and improves the energy concentration performance.

[0060] When working, the starlight signal passes through the front lens group, which is a double-separated lens with a combined optical power close to zero, and the light propagation direction remains basically unchanged; then the light signal is gathered through the spherical primary reflector 300 and the spherical secondary reflector 400, and the reflector group bears the main optical power of the optical system. Since the reflectors are all spherical, a large number of aberrations such as spherical aberration and coma are generated, and these aberrations are mainly corrected by the front lens group. When the optical power of the front lens group is close to zero, even if the same glass material is used, the axial chromatic aberration and vertical chromatic aberration generated are very small, and the reflector does not produce chromatic aberration, thereby realizing the spectrum design of ultra-wide spectrum. The double-separated lens of the rear lens group corrects the residual spherical aberration, coma, chromatic aberration and distortion aberration.

[0061] The optical path of the optical system of the present invention is highly folded at a long focal length, thereby realizing a compact design; avoiding the use of optical elements with large chromatic aberration, realizing the design of low chromatic aberration and secondary spectrum, and obtaining spectrum detection in an ultra-wide spectrum band; adopting a full-spherical optical element, greatly reducing the processing and manufacturing costs; avoiding the problem of decreased energy concentration of the central Airy disk caused by central obstruction, and improving the aggregation ability of stellar light signals.

[0062] As a preferred embodiment, the distance from the aperture stop 800 to the image plane 700 is the total length L of the optical system, and the focal length of the optical system is f, then L and f satisfy:

[0063] L / f≤0.34.

[0064] As a preferred embodiment, the first lens 100 has a front surface curvature radius of -311.8 mm, a rear surface curvature radius of -561.2 mm, a center thickness of 8 mm, and a lens aperture of φ75 mm; the second lens 200 has a front surface curvature radius of 2911.5 mm, a rear surface curvature radius of -958.6 mm, a center thickness of 17 mm, and a lens aperture of φ75 mm; the main reflector 300 is a concave reflector with a curvature radius of -598.6 mm and a light aperture of The secondary reflector 400 is a convex reflector with a curvature radius of -273.8 mm and a light aperture of The third lens 500 has a front surface curvature radius of -47.1 mm, a rear surface curvature radius of -66.5 mm, a center thickness of 8 mm, and a light aperture of The fourth lens 600 has a front surface curvature radius of 32.6 mm, a rear surface curvature radius of 21.2 mm, a center thickness of 15 mm, and a light aperture of

[0065] As a preferred embodiment, the distance between the rear surface of the first lens 100 and the front surface of the second lens 200 is 0.1 mm; the distance between the rear surface of the second lens 200 and the front surface of the primary reflector 300 is 215 mm, and the distance between the front surface of the primary reflector 300 and the rear surface of the secondary reflector 400 is 215 mm; the distance between the front surface of the third lens 500 and the rear surface of the secondary reflector 400 is 178.5 mm; the distance between the rear surface of the third lens 500 and the front surface of the fourth lens 600 is 0.1 mm, and the distance from the rear surface of the fourth lens 600 to the image plane 700 is 18.3 mm.

[0066] The specific parameters achieved by the optical system of this embodiment are:

[0067] Full spherical optical system; focal length 800mm; entrance pupil diameter The field of view angle is 1.5°; the spectral range is 450nm~1100nm; the image quality is close to the diffraction limit, and the average MTF of the whole field of view is better than 0.42@50lp / mm; the total length of the optical system (the distance from the aperture diaphragm 800 to the image plane 700 of the optical system) is 270mm, and the ratio of the total length to the focal length is 0.338.

[0068] When the optical system of the present invention matches a CMOS detector with a pixel size of 5.5 μm, the single pixel resolution accuracy reaches 1.38″.

[0069] The present invention realizes the design of a star sensor optical system with a focal length close to the meter level, and the detection spectrum range reaches 650nm, which improves the measurement accuracy of star positions and the collection of starlight energy. Under the same detection aperture, the ability to detect stars can be increased by more than 1 times.

[0070] The present invention adopts a catadioptric structure based on a spherical optical element, which effectively shortens the size of the optical system of a long-focal-length star sensor, can meet the requirements of various spacecraft platforms with stringent requirements on weight and size, and resolves the design contradiction between lightness and smallness and high precision.

[0071] The optical system of the present invention has reasonable focal power distribution, all lenses and reflectors are spherical, and the processing, manufacturing and assembly tolerances are relatively loose, which reduces the difficulty of processing and assembly, and is conducive to improving the manufacturability and assembly yield of the long-focal-length star sensor optical system.

[0072] refer to Figure 2 , Figure 2The energy concentration curves of the optical system with and without central obstruction are compared when the relative aperture is the same and the image quality of the optical system reaches the diffraction limit. When the relative aperture is F / 10.6 and the obstruction ratio (the ratio of the obstructed spot area to the entrance pupil area at the entrance pupil position) takes a typical value of 16%, P1 is the energy concentration curve without obstruction, and P2 is the energy concentration curve with obstruction. It can be seen that in the case of obstruction, The energy concentration within the diameter range reaches 73.5%; in the absence of obstruction, The energy concentration within the diameter range reaches 86.5%, and the energy concentration performance is improved by more than 17.7% compared with the case with obstruction.

[0073] refer to Figure 3 , Figure 3 The optical transfer function curve distribution of the entire optical system in the embodiment of the present invention is characterized. The average optical transfer function value of the optical system reaches above 0.42 at 50lp / mm, and the imaging quality is excellent.

[0074] refer to Figure 4 , Figure 4 The energy concentration curve distribution of the optical system in the embodiment of the present invention is characterized. Except for the edge field of view, The energy concentration within the range reaches more than 80%, and the stellar light signals are well concentrated.

[0075] The present invention provides a high-precision star sensor optical system solution suitable for space application platforms. The focal length of the optical system reaches 800mm, and the spectral range is 450nm to 1100nm. The optical system has the characteristics of compact spatial layout, strong light collection ability and high energy concentration.

[0076] The preferred embodiments of the present invention are specifically described above, but the invention is not limited to the embodiments. Those skilled in the art may make various equivalent modifications or substitutions without violating the spirit of the invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. An unobstructed long focal length star sensor optical system, characterized in that it comprises an aperture stop, a front lens group, a reflector group, a rear lens group and an image plane, wherein the front lens group comprises a first lens and a second lens arranged in sequence from front to back, the aperture stop is located in front of the first lens, the reflector group comprises a primary reflector and a secondary reflector, the primary reflector is located behind the second lens, the secondary reflector is located below the second lens, the rear lens group comprises a third lens and a fourth lens arranged in sequence from front to back, the rear lens group is located behind the secondary reflector, and the image plane is located behind the fourth lens; The first lens is a meniscus lens with negative power, the second lens is a biconvex lens with positive power, and the third lens and the fourth lens are both meniscus lenses with negative power; The distance between the center of the aperture stop and the optical axis of the optical system is the off-axis amount h, the entrance pupil diameter is D, and the height of the upper edge light of the secondary reflector from the optical axis is h B2 , where h, D and h B2 satisfy: 10mm≤h-(D / 2+h B2 )≤35mm; The incident light passes through the aperture stop, the first lens, and the second lens in sequence to the primary reflector. The primary reflector reflects the incident light to the secondary reflector. The light beam is reflected again at the secondary reflector to form reflected light. The reflected light passes through the third lens and the fourth lens in sequence to form an image on the image plane. The focal power of the first lens is φA1, the focal power of the second lens is φA2, and the focal power of the optical system is φ, wherein φA1, φA2 and φ satisfy: -0.68≤φA1 / φ≤-0.50; 0.50≤φA2 / φ≤0.68; The combined focal power of the reflector group is φB, and the focal power of the optical system is φ, where φB and φ satisfy: 1.05≤φB / φ≤1.

25.

2. The unobstructed long focal length star sensor optical system according to claim 1, characterized in that: The combined focal power of the rear lens group is φC, and the focal power of the optical system is φ, where φC and φ satisfy: -6.85≤φC / φ≤-5.

85.

3. The unobstructed long focal length star sensor optical system according to claim 1, characterized in that: The distance from the aperture stop to the image plane is the total length L of the optical system, and the focal length of the optical system is f, then L and f satisfy: L / f≤0.

34.

4. The unobstructed long focal length star sensor optical system according to claim 1, characterized in that: The first lens, the second lens, the third lens and the fourth lens are all made of the same crown glass.

5. The unobstructed long focal length star sensor optical system according to claim 1, characterized in that: The primary reflector and the secondary reflector both adopt spherical surface type.

6. The unobstructed long focal length star sensor optical system according to claim 1, characterized in that: The first lens has a front surface curvature radius of -311.8mm, a rear surface curvature radius of -561.2mm, a center thickness of 8mm, and a lens aperture of φ75mm; the second lens has a front surface curvature radius of 2911.5mm, a rear surface curvature radius of -958.6mm, a center thickness of 17mm, and a lens aperture of φ75mm; the main reflector is a concave reflector with a curvature radius of -598.6mm and a light aperture of φ8 4mm; the secondary reflector is a convex reflector with a curvature radius of -273.8mm and a light-clearing aperture of φ24mm; the front surface curvature radius of the third lens is -47.1mm, the rear surface curvature radius is -66.5mm, the center thickness is 8mm, and the light-clearing aperture is φ21mm; the front surface curvature radius of the fourth lens is 32.6mm, the rear surface curvature radius is 21.2mm, the center thickness is 15mm, and the light-clearing aperture is φ16.2mm.

7. The unobstructed long focal length star sensor optical system according to claim 1, characterized in that: The distance between the rear surface of the first lens and the front surface of the second lens is 0.1mm; the distance between the rear surface of the second lens and the front surface of the primary reflector is 215mm, and the distance between the front surface of the primary reflector and the rear surface of the secondary reflector is 215mm; the distance between the front surface of the third lens and the rear surface of the secondary reflector is 178.5mm; the distance between the rear surface of the third lens and the front surface of the fourth lens is 0.1mm, and the distance from the rear surface of the fourth lens to the image plane is 18.3mm.

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