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A Big Filter Wheel and Shutter for the Schmidt (1Mar94)
(from KPNO, NOAO Newsletter No. 37, 1 March 1994)

Since its inception, the Schmidt CCD system has used a sliding filter
bolt containing 2-inch x 2-inch slots and a small shutter, all recycled
from the original 4-m prime focus CCD system. The corners of the 2048 x
2048 CCD (S2KA), the workhorse detector at the Schmidt, were vignetted
by the filter and shutter assemblies.  In addition, because there were
no mechanical stops (i.e. detents), filter positioning was not as
repeatable as desired, which occasionally resulted in low-level flat
fielding problems.  To overcome these difficulties, a new 4-inch x
4-inch filter wheel and shutter have been built for the Schmidt CCD
system.  These are slight modifications of the filter wheel and shutter
in use at the 4-m and 0.9-m telescopes.

An added benefit of this system upgrade is that the filter position
will be readable and controllable from the CCD software (ICE).  Schmidt
observers will thus find that their headers actually contain the
correct filter information, which makes processing the images much
easier.

A dedicated set of 4-inch UBVRI filters has been fabricated for the
Schmidt, as well as 2-inch inserts for filters not available in the
4-inch size.  The collection of 4-inch narrow-band filters procured for
use at the 4-m and 0.9-m will be available for use at the Schmidt.  In
addition, a set of 4-inch Stromgren uvby filters has recently been
purchased and is resident at the Schmidt (although these are, of
course, also available at the 4-m and 0.9-m). All users of 4-inch
filters (other than UBVRI) should be aware that there is now increased
demand for these filters; therefore specifying accurately which filters
you plan to use on your ORPF ("pink sheet") has become increasingly
important.

We would like to acknowledge the many individuals who have contributed
to the implementation of the new filter wheel and shutter, particularly
Tony Abraham, Scott Bulau, Lee Groves, Bill Schoening and Rob Seaman.

Finally, an X-Terminal has been installed for the Schmidt to allow
users to work on reductions, etc.  in a warm, well illuminated area.

Taft Armandroff, Phil Massey

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