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A Better Mc (1Jun93)
(from NSO, NOAO Newsletter No. 34, 1 June 1993)

As a first step in the proposed 4-m upgrade of the McMath-Pierce telescope, a
"proof of concept" development project is under consideration. The present
1.6-m quartz No. 2 (concave) mirror would be replaced by a same-diameter thin
aluminum mirror, cooled and "actively" supported. According to a preliminary
design by Larry Barr, this mirror would be both axially supported and
temperature controlled by air jets. Lateral support would be a simple band.
Mirror figure would be adjusted by a symmetric array of slowly driven
mechanical couples.

To understand the constraints imposed by seeing, a free-atmosphere image
monitor would be installed on top of the telescope structure. Its output would
be a "site-seeing" parameter independent of the influence of Kitt Peak terrain
and the telescope building. As for internal seeing, the conclusion of earlier
studies by Coulman indicates that the principal cause of internal deterioration
is air turbulence at the tunnel-heliostat interface. This area might be sealed
off, and remedy effected, by some kind of web-like cloth paneling. The success
of the existing wind-fence indicates the potential for control of air flows by
such means. Perhaps a tubular suspension of similar material could be extended
from the heliostat yoke inward for some distance.

Laser beam and internal micro-thermal sensors, already in place, would be
activated and begin the daily recording of the telescope environment. Finally,
image measurements at 2 um would be undertaken to evaluate the requirements for
an IR adaptive optical system to fully correct images in "good" seeing.

Larry Barr, Fred Forbes, Bill Livingston

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