The SIRTF optics are intended for operation around 10°K. Construction and evaluation of the telescope assembly at room temperature would be most convenient, but require that optical performance changes (other than focal shifts) on cooling remain within the assigned system tolerances. "Diffraction limited" performance for 2-μm radiation is a desirable goal for SIRTF. With support from the Space Technology Branch, Space Science Division, NASA Ames Research Center, Itek Optical Systems has completed measurements of the optical figure of a 0.65-meter, lightweight, fused silica mirror at temperatures in the 13 to 17°K range. Test requirements and preparations were reviewed at the 26th Symposium. Test results, and our evaluation of their significance are presented here.
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