Figure testing of 300 mm Zerodur mirrors at cryogenic temperatures

Deep Impact is a NASA Discovery Mission to impact and observe the nucleus of Comet Temple 1. The instrumentation includes a 300 mm aperture telescope that will operate at 130K once in deep space. It is critical that the telescope mirrors maintain their figure at the operational temperature. We report on measurements of the surface figure changes of three Zerodur primary mirrors from room temperature to 130K, using a PhaseCam(R) interferometer from 4D Vision Technologies, Inc. Although the mirror substrates were taken from the same melt and annealing, they did not perform equally, with differential surface figures ranging from 0.014 waves RMS at 633 nm to 0.082 waves.