Certification of Null Corrector for 4 -m f/1 Primary Mirror for VISTA Telescope

Abstract

An optical test for measuring null correctors for 4-m f/1 primary mirror for VISTA telescope has been developed. Test uses a rotationally symmetric computer-generated hologram (CGH) to synthesize the wavefront that would be reflected by a perfect primary mirror. CGH with 190 mm diameter and 0.6 µm minimum features was fabricated by polar coordinate laser writing machine. Methods of CGH certification are discussed. Practical results are presented. Two null correctors (refractive and reflective types) for 4-m primary mirror were measured at the Lytkarino Optical Glass Factory using this test. Zonal aberration 0.2 λ rms caused by error in distance between lenses for first corrector and 0.11 λ rms astigmatism for second corrector were revealed and removed.