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Modification to Window of SP/VTT (1Sep94)
(from NSO, NOAO Newsletter No. 39, 1 September 1994)

During the day, the edge of the window on the SP/VTT has always heated up
relative to the center of the window because the temperature of the turret,
which has the cell for the window built into it, tracks the diurnal change in
temperature more rapidly than the window itself. This introduces a "turned up
edge" of several waves in the transmitted wavefront. Many experiments have
been made over the years in an attempt to reduce this effect. The most
successful was actively cooling the edge of the window, the cell and bezel
with liquid coolant. This worked in the past with a 10-cm thick glass window
that was slightly smaller than the cell, allowing coolant to be circulated
around the edge of the window. Unfortunately, the glass window had
considerable polarization, and it was replaced with a borrowed 4-cm thick
fused-silica window of better quality. The replacement was larger than the
cell, requiring a complete rebuild of the mount, which included a scheme for
applying a mount to the edge to reduce the tensile load caused by the vacuum
pressure by about 10%. Unfortunately the edge of the window could no longer
be liquid cooled effectively, leading to another series of unsuccessful
experiments to try to insulate the edge and lower the thermal input.

Recently we had a second fused-silica window edged from 90-cm to 86-cm, once
again allowing use of the original cell and allowing the coolant to be in
very good thermal contact with the edge.

The window and coolant system have been reinstalled. The temperature sensors,
which will complete the system, will be installed during an August
maintenance period. This project should substantially improve the window
performance during the day.

Richard B. Dunn

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