An improved optical system for testing large-aperture concave aspheric mirrors using the Offner method

Abstract

The present invention discloses an improved Offnal optical system for testing ultra-large aperture concave aspheric mirrors. Light emitted by a testing device is transmitted through a first compensation lens and a second compensation lens, and then returns after self-alignment to the concave aspheric mirror to be tested. The optical system uses a large incident light aperture angle, and the first compensation lens bears a large aspheric normal aberration, which greatly improves the compensation ability and achieves the balance of aspheric spherical aberration under a small aperture ratio; the ratio of the compensator aperture to the aspheric aperture to be tested is very small, only 0.025; a two-piece compensator structure is adopted, the number of compensation lenses is small, and a full-spherical design is used, which breaks the design limitation of making the compensation system aspheric in order to test large-aperture aspheric surfaces, the scheme is simple, and the processing cycle is shorter; the image quality of the test optical path is excellent, and the wave aberration reaches a PV value better than 0.1λ, which is suitable for high-precision aspheric surface processing. The improved Offnal test can realize the test of aspheric mirrors with an ultra-large aperture of 14m and an ultra-large relative aperture of 1:1.43.

Description

Technical Field

[0001] The present invention relates to the inspection of concave aspheric mirrors with ultra-large aperture and ultra-large relative aperture, and specifically refers to an improved Offner optical system for inspecting aspheric mirrors based on zero compensation. The optical system is suitable for inspecting concave aspheric mirrors with a curvature radius of at least 40m and an aperture of 13.98m, and a relative aperture of 1 / 1.43; the light passes through the concave aspheric mirror to be inspected once. Background Art

[0002] Large-aperture optical systems have been increasingly widely used in the fields of astronomical optics, space optics, and ground-based space target detection and identification. Aspheric surfaces are a key technology in large-aperture optical systems because they have excellent properties such as expanding the field of view, reducing system complexity, improving spatial resolution, and increasing signal energy. The design of large-aperture aspheric mirrors is achieved by inspecting the aspheric mirrors through auxiliary optical systems, and the optical detection scheme restricts their widespread application in optical systems. Therefore, the research on large-aperture aspheric surface detection methods is of great significance to improving my country's astronomical observation, space remote sensing, and early warning levels.

[0003] The zero-compensated aspheric mirror test uses the spherical aberration generated by the compensating lens to compensate for the normal distance error of the concave aspheric mirror to be tested, and performs a self-collimation test. A typical representative method is the Ofner concave aspheric mirror test, which uses a compensating positive lens located behind the center of the curvature radius of the concave aspheric mirror to test the concave paraboloid. The concave paraboloid to be tested is a self-collimation surface, which belongs to the post-zero compensation aspheric mirror test.

[0004] After 1962, scientists applied the compensation method to the interference optical path, which significantly improved the value of the compensation method. According to the interference pattern, not only can the deviation be qualitatively analyzed, but also a quantitative judgment can be made with high precision. At present, many typical large-scale space telescope system primary mirrors abroad are tested using the Offnal compensation method. Among them, the typical American MMT (Multiple Mirror Telescope) primary mirror has an aperture of 6.5m and a relative aperture of 1:1.25. The primary mirror is tested using an Offnal compensator structure with two field mirrors and one compensation mirror. The test results show that the residual wave aberration RMS is 0.03λ, and the maximum aperture of the compensation mirror is 270mm. The Royal Observatory of the United Kingdom once used a three-piece Offnal compensation system to test a parabolic primary mirror with a light aperture of 2.5m and a relative aperture of 1:1.2. The residual wave aberration RMS after compensation was 0.31λ.

[0005] The large mirrors currently developed in my country, such as the 2.16m telescope primary mirror of the Nanjing Tianyi Center, have a relative aperture of 1:3, and the inspection method uses the Offner compensator and knife-edge instrument to achieve qualitative measurement. The relative aperture of the 1.56m telescope primary mirror of the Shanghai Institute of Optics and Fine Mechanics is 1:3.3, and the inspection method uses the classical Hartmann screen and shearing interferometer. The 1.1m parabolic primary mirror developed by the Chengdu Institute of Optics and Electronics has a relative aperture of about 1:5, and the inspection method uses the plane mirror self-collimation technology and the Offner compensator combined with the knife-edge instrument to achieve qualitative measurement.

[0006] The zero-compensation Offnal aspheric mirror inspection method can realize the inspection of large-aperture concave aspheric mirrors by using a smaller compensation lens or reflector. Foreign literature shows that a three-piece compensator structure is used for a 6.5m aperture and a relative aperture of 1:1.25. At this time, the compensation mirror has a relatively large aperture of 270mm; while the domestic large-aperture concave parabolic primary mirror uses a relatively small relative aperture of about 1:3. Domestic and foreign developments show that the compensator is obviously complicated when using the zero-compensation Offnal aspheric method to inspect large-aperture aspheric surfaces, and the compensator is too large to obtain materials. Therefore, the present invention proposes an improved Offnal inspection optical system, which uses a 300mm small-aperture two-piece compensation mirror structure to realize the inspection of ultra-large aperture and ultra-large relative aperture concave aspheric mirrors with a 14m aperture and a relative aperture of 1 / 1.43.

[0007] Compared with the currently used zero compensation Offner aspheric mirror inspection method, the improved Offner inspection aspheric mirror is different in that:

[0008] 1. The ratio of the maximum compensator aperture to the asphericity to be inspected for the improved Offnal inspection is very small, only 0.025. The compensator aperture for the inspection of 14m diameter does not exceed 350mm, and the compensator lens material of this diameter is easy to obtain. However, when inspecting the primary mirror of the MMT telescope, the ratio of the compensator aperture to the mirror to be inspected is 0.042. When inspecting the 14m diameter aspheric mirror, a 588mm diameter compensator lens is required. The compensator aperture is huge and it is difficult to obtain materials.

[0009] 2. The number of compensators for the improved Offnal test aspheric mirror is only two, while the compensators currently used for large apertures are mostly three or four. Among them, MMT adopts a three-piece structure. The fewer the number of compensators, the higher the processing accuracy.

[0010] 3. The relative aperture of the improved Offnal test large-diameter aspheric mirror is as high as 1:1.43, while the commonly used relative aperture of large-diameter mirrors in China is about 1:3. Generally speaking, the larger the relative aperture, the higher the aspheric surface steepness, and the more difficult it is to design the test optical path.

[0011] 4. The optical system magnification of the improved Ofnal test large-aperture aspheric mirror (i.e., the ratio of the aperture angle of the incident light of the first compensating mirror to the aperture angle of the light emitting from the second compensating mirror) β is designed to be -1.7, breaking the usage limitation of the commonly used optical design -1<β≤0, finding the initial solution, and obtaining the optical parameters under the conditions of a small-aperture compensator.

[0012] 5. The image quality of the improved Offnal inspection optical system is excellent, the system wave aberration reaches a PV value better than 0.1λ (λ=633nm), and the RMS wave aberration is better than 0.025λ (λ=633nm). The design results are better than the current foreign level.

[0013] Therefore, the improved Offnal test can realize the inspection of aspheric mirrors with large diameter and large relative aperture, and even the inspection of aspheric mirrors with ultra-large diameter and ultra-large relative aperture. Summary of the invention

[0014] The improved Offnal test is suitable for the inspection of concave aspheric mirrors, and provides an optical design model for the study of large-caliber, large relative aperture, and even ultra-large-caliber and ultra-large relative aperture aspheric mirrors. An optical system for the improved Offnal test of ultra-large-caliber concave aspheric mirrors, comprising a concave aspheric mirror 1 to be inspected, a first compensation lens 2 and a second compensation lens 3, characterized in that the light emitted by the inspection device is transmitted through the first compensation lens 2 and the second compensation lens 3, and then self-reflected by the concave aspheric mirror 1 to be inspected, and then transmitted through the second compensation lens 3 and the first compensation lens 2 back to the inspection device along the original path.

[0015] The ratio of the incident light aperture angle to the outgoing light aperture angle of the first compensating lens 2 of the present invention is β1=-2.16, the ratio of the incident light aperture angle to the outgoing light aperture angle of the second compensating lens 3 is β2=0.79, and the ratio of the total incident light aperture angle to the outgoing light aperture angle of the compensating lenses is β=-1.71; the aperture ratio of the first compensating lens 2 to the concave aspheric mirror 1 to be tested is α1=-0.025, and the aperture ratio of the second compensating lens 3 to the concave aspheric mirror 1 to be tested is α2=-0.0075; the optical path can test the concave aspheric mirror 1 to be tested with a curvature radius of at least 40m and an aperture of 13.98m, and the relative aperture reaches 1 / 1.43; the optical system light bar is located on the concave aspheric mirror 1 to be tested; and the light passes through the concave aspheric mirror 1 to be tested once.

[0016] The first compensation lens 2 and the second compensation lens 3 described in the present invention are made of glass materials such as K9 and quartz; the concave aspheric mirror 1 to be inspected described in the present invention is made of a metal mirror or a glass mirror, and the surface shape is a secondary aspheric surface or a high-order aspheric surface.

[0017] From the analysis of the optical system principle and design results of the improved Offnal test of ultra-large aperture concave aspheric mirrors, the improved Offnal test capability is superior to that of the conventional zero compensation test. The improved Offnal inspection optical system uses a large incident light aperture angle, allowing the first compensating mirror to bear a larger compensation ratio of the aspheric normal aberration, thereby improving the compensation capability and achieving a balance of aspheric spherical aberration under a small aperture ratio. The ratio of the maximum compensator aperture of the improved Offnal inspection aspheric mirror to the aperture of the inspected aspheric surface is very small, only 0.025, and the material of the small-aperture compensator is easy to obtain, which is convenient for parts processing. The two-piece compensator structure is adopted, the number of compensating lenses is small, and the full-spherical design breaks the design limitation of the spherical compensation system for inspecting large-aperture aspheric surfaces. The scheme is simple, the inspection accuracy is higher, and the processing cycle is shorter. The inspection optical path has excellent image quality, the system wave aberration reaches a PV value better than 0.1λ (λ=633nm), and the RMS wave aberration is better than 0.025λ (λ=633nm), which is suitable for high-precision aspheric surface processing. When the improved Offnal is used to inspect concave aspheric mirrors, the aperture of the aspheric surface can reach 14m at a curvature radius of 40m, and the relative aperture reaches 1:1.43, which can realize the inspection of aspheric mirrors with ultra-large aperture and ultra-large relative aperture. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The optical system optical path of the improved Offner test for ultra-large aperture concave aspheric mirror, 1 is the concave aspheric mirror to be tested, 2 is the first compensation lens, and 3 is the second compensation lens. DETAILED DESCRIPTION

[0019] The present invention proposes an improved optical system for Offner testing ultra-large aperture concave aspheric mirrors, and its main technical indicators are as follows:

[0020] 1. Aperture ratio between the first compensator and the concave aspherical mirror to be tested: 0.025;

[0021] 2. The ratio of the total incident light aperture angle to the outgoing light aperture angle of the compensation lens β = -1.71, the ratio of the incident light aperture angle to the outgoing light aperture angle of the first compensation lens β1 = -2.16, and the ratio of the incident light aperture angle to the outgoing light aperture angle of the second compensation lens β2 = 0.79;

[0022] 3. The aspheric surface diameter is 13.98m, the radius of curvature is 40m, and the relative aperture is 1 / 1.43;

[0023] 4. The PV value of the optical path wave aberration is better than 0.1λ (λ=633nm), and the RMS wave aberration is better than 0.025λ (λ=633nm).

[0024] The optical system design parameters are shown in Table 1:

[0025] Table 1 Design parameters of the improved Offner test optical system

[0026]

Claims

1. An improved Offner optical system for testing ultra-large aperture concave aspheric mirrors, comprising a concave aspheric mirror to be tested (1), a first compensation lens (2) and a second compensation lens (3), characterized in that: The light barrier of the optical system is located on the concave aspheric mirror (1) to be inspected; the light emitted by the inspection device is transmitted through the first compensation lens (2) and the second compensation lens (3), then self-reflected by the concave aspheric mirror (1) to be inspected, and then transmitted through the second compensation lens (3) and the first compensation lens (2) back to the inspection device along the original path; The ratio of the incident light aperture angle to the outgoing light aperture angle of the first compensation lens (2) is b1=-2.16, and the aperture ratio of the first compensation lens (2) to the concave aspheric mirror (1) to be inspected is a1=-0.025; The ratio of the incident light aperture angle to the outgoing light aperture angle of the second compensation lens (3) is b2=0.79, and the aperture ratio of the second compensation lens (3) to the concave aspherical mirror (1) to be inspected is a2=-0.0075; The ratio of the total incident light aperture angle to the outgoing light aperture angle of the compensation lens is b=-1.71; The first compensation lens (2) and the second compensation lens (3) are made of K9 or quartz glass; the material of the concave aspheric mirror (1) to be inspected is a metal mirror or a glass mirror, and the surface shape is a secondary aspheric surface or a high-order aspheric surface.

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