Optical test for the primary mirror of a space telescope using a CGH null lens

Abstract

MSC (Multi-Spectral Camera) for KOMPSAT2 (Korea Multipurpose Satellite 2) is a Ritchey-Chretien type telescope. Its primary mirror (PM) is a hyperboloidal mirror with 600 mm diameter. To test the PM, we chose a CGH null lens as a null optics. The CGH null lens is much smaller than the Hindle shell, is consist of only one element, and is aligned more easily than the Offner type null lens. The CGH null lens is designed to align and test the aspheric surface at the same time. The CGH lens includes the main part to test the aspheric wavefront from the PM and also includes the alignment part to align the CGH and the PM. The alignment CGH part is consisted of three amplitude type reflection segments to align the CGH and the interferometer, and three phase type transmission segments to align the CGH and the PM. The CGH null lens is made of fused silica by a laser beam writer. We measured the Engineering Model of the PM using this CGH null lens.