Null correctors for 6.5-m f/1.25 paraboloidal mirrors

The instruments used to interferometrically measure the optical surfaces of the 6.5-m f/1.25 primary mirrors for the University of Arizona telescope projects must compensate over 800 μm surface departure from the best fitting sphere. The errors in the optical test must not contribute more than 0.04 arc seconds FWHM to the final image and the conic constant must be held to 0.01%. This paper presents an overview of the instruments used to measure these giant mirrors to such high accuracy.