Determination of absolute change in optical power of a reference mirror at cryogenic temperature

Abstract

We have recently demonstrated the ability to measure the absolute change in optical power (focus) of a 152 mm diameter flat mirror in vacuum between room and cryogenic temperatures (133K) with a peak-to-valley measurement error of only 22nm. Such a measurement would be crucial to the verification of the focus of a cryogenic instrument during ground testing. The testing utilized a vibration-insensitive interferometer and a reference mirror maintained at room temperature located within the thermal vacuum chamber. Special considerations were taken to ensure that the reference mirror experienced low axial thermal gradients, since structural modeling indicated that axial thermal gradients and axial variation of substrate coefficient of thermal expansion are critical in maintaining flatness under cryogenic test conditions. This paper will discuss the testing equipment and methodology and the corresponding analysis and results.