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Window Cooling Improves Optical Performance of the Sac Peak VTT (1Mar96)
(from NSO, NOAO Newsletter No. 45, March 1996)

Over the past two years a series of experiments has demonstrated
conclusively that temperature variations across the entrance window of
the VTT seriously degrade its imaging performance. These optical
aberrations arise from temperature-induced changes in the index of
refraction in the fused quartz window, and are particularly severe
around its edge. During the past several months a new system for
controlling these temperature variations has been installed and tested.
This system includes five temperature sensors, a high-capacity cooling
loop, and a sophisticated control concept, all of which represent a
significant advance over previous attempts to control the optical
figure of the window. A series of interferograms recorded recently over
several days and a variety of conditions show that the uniformity of
the temperature across the window is now maintained to a few tenths of
a degree, whereas the variation in the uncontrolled window can amount
to several degrees. A preliminary examination of these interferograms
shows that the residual wavefront errors are now a fraction of a wave,
compared to several waves in the uncontrolled window.

Dick Dunn, Richard Radick, Thomas Rimmele

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