Large convex aspheres
This case shows the application of near-null subaperture stitching interferometry to a convex even asphere. We use the counter-rotating Zernike plates shown in Fig. 12 as the near-null optics. In the experimental setup, the two phase plates are mounted on two center-through rotary tables and then inserted between the interferometer and the test surface, as shown in Fig. 34. The surface of mirror 2 described in Section 4.4 is used. The single-pass wavefronts of the subapertures at the center, rings 1, 2, and 3, are shown in Fig. 35 with the CGHs counter-rotating by proper angles. It is easy to see the aberration correction effect when we compare Figs. 35(c) and 35(d) with Fig. 36, which is obtained without CGHs counter-rotating (a = 0). The interferogram of the outmost subaperture shown in Fig. 36(b) is irresolvable while the counter-rotating CGHs successfully reduce it to about eight fringes (double pass). In addition, the interferogram has good fringe contrast for the uncoated glass surface, and there is no visible ghost fringe.
Figure 36 Subaperture wavefronts without CGHs counter-rotating: (a) ring 2 (PV 21.311 X, RMS 3.669X) and (b) ring 3 (irresolvable).
Figure 37 Measurement results of the convex asphere: (a) near-null subaperture stitching, (b) front null test, and (c) back-through null test.
By employing the subaperture stitching algorithm to optimally correct the misalignment-induced aberrations, all subapertures are stitched together, and the final surface error map is shown in Fig. 37(a). For the purpose of cross-test, the front null test and the back-through null test are also developed to measure the same surface error. The measurement results are given in Figs. 37(b) and 37(c). The optical layout is shown in Fig. 38. Note the back-through null test suffers the lower dynamic range of measurement because the surface figure of the convex asphere is magnified by n times where n is the index of refraction. That explains the discontinuity of the error map, as shown in Fig. 37(c).
Figure 38 Design of the cross-test: (a) front null test and (b) back-through null test.