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A New Focal Ratio for the 4-m (1Sep92)
(from KPNO, NOAO Newsletter No. 31, 1 September 1992)

In the past, infrared instruments at the 4-m utilized either the f/30
chopping secondary (photometry, CRSP), or the f/8 R-C secondary (IRIM).
Use of either IRIM or CRSP at the 4-m entailed modifications to the
internal optics to accommodate the change from f/15 operation at the
1.3-m and 2.1-m telescopes. During the course of designing the present
generation of IR array instrumentation (SQIID, COB, PHOENIX), we
decided that conversion of the 4-m to f/15 would be more cost effective
and yield better instrumental performance than providing focal ratio
conversion optics in each instrument.  The original design of the 4-m
included an f/12 Cass focus which was never implemented, although the
mirror blank and mechanical fixtures were fabricated and placed in
storage on the mountain.  These have been modified and recycled into an
f/15 configuration which is being installed on the telescope during the
summer shutdown. Starting this fall semester, expect to use this new
focal ratio for all infrared instruments at the 4-m.

Dick Joyce

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