CGH null test design and fabrication for off-axis aspherical mirror tests

Abstract

A null-lens based on a Computer Generated Hologram (CGH) is designed to test the primary off-axis aspherical mirror of the GAIA space telescope. This custom-designed and fabricated CGH includes five zones (null CGH, alignment CGH, and beam-projection CGH) on the same substrate. The optical test configuration is simple and the designed five-zones CGHs can simultaneously provide the aberrated wavefront correction for null tests, CGH alignment with a commercial interferometer, pre-positioning of the mirror under test in a cryogenic chamber, and isolation of the diffraction orders in the test setup. Positioning of the five zones with respect to each other is extremely critical for the success of this custom-made CGH null testing. For this reason, the fringes of all five zones were manufactured on a single photolithographic mask. With INO's special micro-fabrication processes, including its photolithographic, etching, and coating technologies, this 4.5-inch in diameter CGH was successfully made. The RMS wavefront error is estimated at 7.33 nm for the fabricated null CGH.