The Infrared Multi-Object Spectrograph (IRMOS) is a facility instrument for the Kitt Peak National Observatory Mayall Telescope (3.8 meter). IRMOS is a low-to mid-resolution (R X/AX 300-3000), near-JR (0.8-2.5 rim) spectrograph that produces simultaneous spectra of -100 objects in its 2.8 x 2.0 arcmin field of view using a commercial MEMS multi-mirror array device. The instrument operating temperature is '-8O K and the design is athermal. The optical bench and minors are machined from aluminum 6061-T65 1.In spite of its baseline mechanical stress relief, aluminum 606 1-T65 1 harbors some residual stress, which, unless relieved during fabrication, may distort mirror figure to unacceptable levels at the operating temperature (8O K). Other cryogenic instruments using aluminum minors for both ground-based and space JR astronomy have employed a variety of heat treatment formulae, with mixed results.We present the results of a test program designed to empirically determine the best stress relief procedure for the IRMOS mirrors. Identical test minors with spherical and flat optical prescriptions are processed with five different stress relief formulae from the literature and institutional heritage and compared to samples without any additional processing. After figuring via diamond turning, the mirrors are tested for figure error and radius of curvature at room temperature and at -8O K for three thermal cycles. The heat treatment procedure for the mirrors that yielded the least and most repeatable change in figure error is applied to the IRMOS mirror blanks. We correlate the results of our optical testing with heat treatment and metallographic data.

We present the results of an on-going test program designed to empirically determine the effects of different stress relief procedures for aluminum mirrors used in cryogenic applications. Earlier test results identified a preferred heat treatment for flat and spherical mirrors diamond turned from blanks cut out of Al 6061-T651 plate stock 1 . Further tests were performed on mirrors from forged stock to measure the effect of this variable on cryogenic performance. The mirrors are tested for figure error and radius of curvature at room temperature and at 80 K for at least three thermal cycles. We correlate the results of our optical testing with heat treatment and metallographic data.