Large Aperture Mirror Surface Test

The large aperture mirror surface test is the basis of optical processing and alignment, and is also the key to the development of remote sensing device. The simulation results show that the RMS values of 1.07 m primary mirror with multi-point support and sling support are 1.86 nm and 3.28 nm respec-tively. Using 36 point unloading device, sponge 36 point free support and sling support to test the mirror surface, the results are basically consistent, RMS is better than 0.02λ (λ = 632.8 nm).


Introduction
With the rapid development of the field of ground observation, deep space exploration and scientific experiment, the resolution requirements of space remote sensing cameras are also higher and higher. Because the angular resolution of optical system is inversely proportional to the aperture of the through-beam, increasing the aperture of space remote sensor is the most direct and effective means to improve the resolution. As the largest mirror in optical system, how to keep the surface stability is the primary engineering problem in the development of remote sensing camera. The primary mirror surface will deviate from the real surface due to the influence of gravity deformation and support deformation during the ground detection. How to remove the influence of gravity and support is the key to surface test [1] [2].

Principle Analysis
There are two common methods in the measurement of large aperture mirror surface, one is multi-point unloading support and the other is sling support. The

Multi Point Unloading Support
Because the mirror cannot be an absolute rigid body, according to the elastic deformation principle, with the increase of the mirror aperture, the structural stiffness of the mirror itself will gradually decrease, and the number of supporting points must be increased to ensure the accuracy of the mirror surface. The elastic deformation analysis of the surface under the equal spacing support is shown in Figure 1. G is the acceleration of gravity, and D is the spacing of support points. It can be seen that the surface of the mirror is proportional to the Fourth Square of the distance D of the support point, and is inversely proportional to Young's modulus and moment of inertia. Therefore, in order to reduce the distortion of mirror, only one way to reduce the support distance is to increase the number of support points when the mirror structure and material are determined.
When the optical axis is vertically supported and unloaded, the force of the mirror shall be balanced, so as to prevent the movement in the space of the mirror, namely, it meets the following requirements: The mirror must meet the moment balance, so as to prevent the rotation in the mirror space, namely, it meets the following requirements: When the optical axis is vertical (pointing to the roof), 36 back supporting points are designed on the back of the mirror, and the supporting diameter is taken Φ 25 mm ( Figure 2).
When the optical axis is vertical, the mirror is affected by its own weight, and the surface is shown in Figure 3, RMS is 1.86 nm.

Aberrations Expression of Detection System
The optical component of the detection system is mainly introduced into primary aberrations, and no new aberrations will be generated. The Zernike scalar expression of the primary aberrations of the system is as follows [3] [4]: i Z ρ φ is Zernike polynomial and j i C is the polynomial field fitting coefficient. Table 1 gives the expressions of the astigmatism, coma and spherical aberration of polynominal expressions, θ is the polar angle, ρ is the radius.

Unloading Device 36 Point Support
The self collimation optical path as shown in Figure 6 Figure 6. It can be seen that when the primary mirror is at zero, 120˚ and 240˚, the test results are consistent.

Sponge 36 Point Support
As is shown in Figure

Sling Support
As shown in Figure 8, the primary mirror is tested by sling, and Fv shall pass the center of gravity of the mirror. After measuring the current position surface, the primary mirror rotates 120˚ and 240˚ around the optical axis respectively. It can be seen that the test results are consistent with the optical axis vertical unloading support results.

Conclusion
Based on the analysis of the horizontal and vertical stress states of the optical axis of the large caliber primary mirror, the simulation results of the surface of the primary mirror under different supporting conditions are given. The primary mirror surface is tested by using multi-point support and sling support, and the test results are basically the same. The results of multi-point unloading support can be used as the basis for optical processing of mirrors, and the results of sling support can be used as the basis for structural design and optical alignment.