Fengtao Yan

Influence study of solving correction forces caused by fitting errors for thin meniscus mirror

The actuator influence functions of a thin meniscus mirror can be expanded in Zernike polynomials. And the correctness of influence functions has a great effect on solving the correction forces. The Zernike coefficients are applied as parameters in the all-floating support system. We analyze the main interferential factors when different deformation modes are corrected. The influence caused by fitting errors is studied in this paper. A preferable result can be obtained after eliminating the interferential factors. The method can obtain useful correction forces. Comparing the peak-to-valley and root mean square values among the results calculated by different accuracy influence functions, we find that there is a limited convergence property when the accuracy increases.

A new method on measuring radius of curvature of a conic aspherical mirror

We present a method to measure the radius of curvature of a concave conic asphere. By analysis the central area of the asphere, we can measure the radius of an arbitrary point in the central area instead of the vertex of asphere. In the procedure, we firstly adjust the interferometer until the interferogram of the central area approach nulls, then put the laser tracker ball at the beam focus of the interferometer and move the tracker ball to touch the central area of the aspherical surface to get the two positions. With these measurement data, we can calculate the radius of curvature of the aspherical vertex and its uncertainty.

Absolute subaperture testing by multiangle averaging and Zernike polynomial fitting method

A method to obtain the absolute shape of a large plane or sphere surface in the subaperture testing with multiangle averaging and Zernike polynomial fitting method is proposed. The subaperture data obtained by the interferometer contain not only the test mirror deviation but also the reference surface deviation. The reference surface deviation has become the main limit of the subaperture testing accuracy. To correct the deviation, the measurements are made so that the reference deviation could be eliminated by rotational asymmetric deviation and the rotational symmetric deviation. This technique can get not only the full absolute shape of the large mirrors, but also the reference surface shape. Experimental results have been given to verify the effectiveness of this method

A novel method for calibrating the image distortion of the interferometer

Based on the distortion theory of the image system and interferometer theory, the distortion of interferometer system is analyzed, and a novel calibration and correction method for the imaging distortion is proposed. Two different subapertures are gotten by the interferometer, which have the common area of the test surface. The image distortion coefficients of the interferometer system are gained through the relationship of the two different subapertures, and we use the optical distortion coefficients to reconstruct the phase map that correspond to the real distribution.

The method to reduce the spinal error in the aspheric mirror testing with the CGH

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.

Experimental study on the measurement of convex hyperbolic mirrors with Hindle and stitching methods

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.

High accurate subaperture testing

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.