Interferometric optical testing using computer-generated holograms(CGHs) has proven to supply a very good and
accurate measurements method of the aspheric surfaces. However, the CGHs are diffractive optical elements which use
diffraction to create wavefronts of light with desired amplitudes and phases. The different diffraction order of the light
would be make some ghost image to the fringe pattern. It would introduce some spinal error to the measurement results.
This error would not be avoided after the CGH designed and manufactured. In this work, we take two measurement steps
to reduce the spinal error. The first step, the apheric mirror was tested with the CGHs. The second step, the aspheric
mirror was tested with transmission sphere directly. Then the subaperture theory was used to obtain the final
measurement results of the aspheric mirror surface. The experimental demonstrations were provided by testing an
aspheric mirror. The results are shown that this method could reduce the spinal error.
Radius of curvature R and conic constant k are important parameters of aspheres.Null testing or CGH are usually used to evaluate the processing quality of aspheric mirrors in fabricating process . When the null compensator emerges a problem, additional method to ensure the accuracy of paraxial radius of curvature and conic constant is required. Based on the equation of conic aspheric, the computing model from which the paraxial radius of curvature R and conic constant k can be obtained was established, and a set of solving algorithm using singular value decomposition (SVD) method was derived. The simulating result of a 1800mm aspheric mirror is presented and the solving precision reaches R=6120±0.026mm, k=-1.0194±0.0008, thus the supplement to null testing of aspheric mirror is achieved effectively .
It is a difficult work to measure the large convex surfaces. However, as the secondary mirrors, the convex aspheric surfaces have been to several meters in diameter to meet the giant telescope. In this paper, the method for testing the large convex hyperbolic mirror with Hindle and stitching methods were introduced. Two experimental demonstrations were provided by testing a convex hyperbolic. The first experiment for testing the convex hyperbolic mirror is 8 subapertures testing in one ring. In the secondary experiment, the convex hyperbolic mirror was divided into two rings to be tested. By comparing the results of this testing method with the QED measurement results, it is shown that the both of the stitching results are in good agreement with the QED measurement results.
We introduced a high accurate subaperture testing method. This stitching method could reduce the interferometers deviation to the stitching results. This high accurate subaperture testing is determined by the data processing technique based on multiangle averaging method and Zernike polynomial fitting method. This technique does not require any assumptions about the surfaces under test. The experiment results shows that this high accurate subaperture testing method not only get the full absolute figure of the large mirrors but also can get the reference mirrors figure and calibrate it and the root mean square (rms) of residual figures between the two methods are ~0.80nm and ~0.87nm.
Ronchi grating test has been used widely to test optical surfaces in a qualitative way since it was contrived, while rarely
to test the parabolic surface in a quantitative way. This paper discusses the application of Ronchi grating test to optical
aspheric surfaces in a quantitative way on the base of self-made software which includes Ronchi null grating design,
collection of Ronchi graph, data procession and so on. The whole system has been used to test a concave parabolic
mirror with diameter 140mm and F number 2, and the result is approximately the same as that of the outcome of
interferometer. The analysis software and test method establish a good foundation for the coming of quantitative
measurement of big error of large-aperture aspheric surfaces.
Annular subaperture interferometric method has provided an alternative solution to testing rotationally symmetric
aspheric surfaces with low cost and flexibility. However, some new challenges, particularly in the motion and algorithm
components, appear when applied to large aspheric surfaces with large departure in the practical engineering. Based on
our previously reported annular subaperture reconstruction algorithm with Zernike annular polynomials and matrix
method, and the experimental results for an approximate 130-mm diameter and f/2 parabolic mirror, an experimental
investigation by testing an approximate 302-mm diameter and f/1.7 parabolic mirror with the complementary annular
subaperture interferometric method is presented. We have focused on full-aperture reconstruction accuracy, and discuss
some error effects and limitations of testing larger aspheric surfaces with the annular subaperture method. Some
considerations about testing sector segment with complementary sector subapertures are provided.