Fig. 1. Structural diagram of spherical step test board

Fig. 2. Optical path model of interferometer measuring spherical step test board

Fig. 3. Calibration process of ITF

Fig. 4. Sub-aperture planning diagram

Fig. 5. Spherical step test board measuring system

Fig. 6. Sub-aperture stitching testing images. (a) Central sub-apertures; (b) First ring sub-apertures; (c) Second ring sub-apertures

Fig. 7. Step contour images. (a) Step contour image obtained from multiple sampling; (b) Average step contour image

Fig. 8. ITF. (a) ITF of central sub-apertures; (b) ITF of first ring sub-apertures; (c) ITF of second ring sub-apertures