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The instruments used to interferometrically measure the optical surfaces of the 6.5-m /1.25 primary mirrors for the MMT conversion and the Magellan 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 that were built to measure these giant mirrors to such high accuracy.