Cryogenic 1-5 micron atmospheric dispersion corrector for astronomical adaptive optics

A cryogenic mechanism has been built to correct for the blurring effects of atmospheric dispersion in adaptive-opticsimages from large aperture telescopes. Steward Observatory's 6.5m telescope features a deformable Cassegrainsecondary mirror compensating for atmospheric turbulence at wavelengths from I -5 microns. At 1 micron, smearingcaused by atmospheric dispersion equals the diffraction-limited image width (0.04 arcseconds FWHM). In order tocorrect for this effect and to maintain the low thermal background provided by the adaptive secondary, we have designedand built a cryogenic (77 K) atmospheric dispersion corrector. Operating over a spectral range of 1 -5 microns, two pairsof counter-rotating calcium/lithium-fluoride prisms provide diffraction-limited imaging over a field of 1 .7 arcminutes ata zenith angle up to 45 degrees.Keywords: atmospheric dispersion, MMT, ARIES, infrared, cryogenic prisms