Testing the null corrector using computer-generated holograms

Abstract

Large primary mirrors for optical telescope are usually interferometrically tested using null correctors. In fabrication of primary mirror, the optical surface is polished to precisely match the wavefront generated by the null corrector. The final shape of the primary mirror will be incorrect if a flawed null corrector has been used.In this paper a new test to certify null corrector was designed and implemented that uses small computer—generated holograms (CGH) fabricated onto flat substrates. This test solves the difficult problem of verifying the accuracy of the null correctors. A new technique for hologram fabrication has been explored for this application. A laser writing machine was built for fabricating these patterns onto flat substrates. The null corrector tests the hologram exactly as if a real mirror is to be measured. The use of hologram to test the null corrector is surprisingly simple. The CGH is positioned at the paraxial focus ofthe mull corrector. Once the CGH is near the correct position, the shape of the fringes in the interferometer is used to align the hologram. Since the CGH appears to the null corrector to be a complete primary mirror with correct shape, the corrector can be tested exactly.