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Blow Those Seeing Blues Away (1Jun95)
(from CTIO, NOAO Newsletter No. 42, June 1995)

As of April this year, air extraction fans have been implemented at the bottom
of the 0.9-m telescope tube. Six 20-watt DC fans were mounted on the outer
shell, under the supervision of G. Pe‚rez and J. Briones. D. Rojas and the
mountain electronic crew wired in the power supplies. The fans are controlled
via a Variac in the observer console.  Initial reports are very encouraging and
over the past two weeks images at the 1" level have been seen repeatedly. The
improvement in seeing appears to be 0.2"-0.3" almost immediately upon
starting the fans.

Temperature measurements made by G. Brehmer on several nights during the
past semester indicated gradients of several degrees between the primary or
"bottom air" and the top of the tube air or mid-high dome ambient. By
conventional transformations, the elimination or reduction of these thermal
gradients should yield improvements of approximately 1", which would be
removed in quadrature from the 1.6" median seeing previously seen.

We are still gaining experience with the running, use, and limits of this
system. Comments and reports by visitors and the nighttime staff should allow
the seeing of this telescope to approach the optical limitations of its design.

I thank the night assistants G. Martin and M. Fernandez for following through
on this project and providing me with feedback.  I also thank Frank Pe‚rez
(OCIW) and our colleagues at Las Campanas for providing information on the
fans used on their telescopes.

Bob Schommer (rschommer@noao.edu)

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