Abstract
An aspheric mirror can improve the quality of an optical system by reducing the complexity of the optical system. With the development of optical manufacturing and optical testing, the aspheric mirror is used commonly in optical systems. The aperture of the primary mirror increases to a few meters, or even dozens of meters. The aperture of the secondary mirror approaches one meter[
Optical testing is a precondition of optical manufacturing. High-precision manufacturing of a convex aspheric mirror with a large aperture needs high-precision testing. The method used to test aspheric mirrors includes contour detection, non-aberration test, null lens test, stitching test, and so on[
Since the optical mirror is fabricated based on the results of the test, the null lens defines the shape of the final optics. There is always a possibility that the null lens could be flawed, resulting in the final shape of the optics being incorrect[
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In this Letter, we focus on testing an aspheric null lens in testing a large convex aspheric mirror. The large convex aspheric mirror is tested by an aspheric null lens. According to the parameter of the aspheric mirror, we design and manufacture an aspheric null lens that translates a flat wavefront to an aspheric wavefront to match the aspheric mirror and get the wavefront surface of the aspheric mirror by another traditional null lens. To calibrate the aspheric null lens, we design a computer-generated hologram (CGH) to test the aspheric null lens. The result of the aspheric null lens is
Large aperture convex aspheric mirrors are always tested by a null lens. The principle of the null lens is that it translates a flat wavefront or spherical wavefront from the interferometry to an aspheric wavefront that matches the tested aspheric mirror. The standard lens for the interferometry can be spherical or flat. But a standard sphere cannot be aligned to the right position relative to the null lens. A standard flat interferometric lens is usually used to test an aspheric mirror with a null lens.
For a
Figure 1.Deviate wavefront from the aspheric wavefront to its best fit sphere wavefront.
For a large convex aspheric mirror, the density of the fringe exceeds the resolving power of the interferometer. A digital mask cannot be used to test the mirror. The mirror is not a conic mirror and cannot be tested by a non-aberration test. An aspheric null lens is designed and manufactured to test a large convex aspheric mirror. The optical layout and result of the design are illustrated in Fig.
Figure 2.Test layout and design results of the null lens.
The parameter of the convex aspheric surface in the aspheric null lens about the radius of curvature is 535.439 mm, k equals
Figure 3.Deviate wavefront of the aspheric convex surface in the aspheric null lens.
Figure 4.Test layout and design results of the null lens.
After manufacturing the aspheric null lens, the result of the aspheric null lens is tested by the traditional null lens is shown in Fig.
Figure 5.Final result of the aspheric null lens tested by the traditional null lens.
The simulated result of the null lens that uses the actual radius of curvature and thickness is shown in Fig.
Figure 6.Final result of the null lens for the convex aspheric mirror.
To calibrate the aspheric null lens, a CGH is designed. The optical layout of the calibration is shown in Fig.
Figure 7.Optical layout for testing the aspheric null lens by CGH.
The precision of the CGH exceeds
Figure 8.Test result of the null lens.
The null lens is used to test the
Figure 9.Optical layout of the test for the convex aspheric mirror.
Figure 10.Test result of the convex aspheric mirror.
This Letter provides a method to test an aspheric null lens, which is used to test a large convex aspheric mirror. A
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