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Update on 4-m Image Quality (1Jun94)
(from CTIO, NOAO Newsletter No. 38, 1 June 1994)

The last edition of this Newsletter reported that the f/8 secondary had
been re-installed on the 4-m telescope, but that there were a variety
of image problems including structure in the image on the subarcsec
scale.  We're pleased to report that at least the most obvious of those
problems appears to have been solved.  We were able to take the
secondary mirror back off the telescope for several weeks starting in
late February and make some further modifications to its cell.  When
the secondary was re-installed the two- and three-pointed structure we
had been seeing in the images were gone, and instead we saw round
images with FWHM = 0.7 arcsec.  Seeing measurements taken at the zenith
during the following several nights of f/8 observing frequently gave
FWHM values in the vicinity of 0.8 arcsec.

We believe that all three of the 4-m foci (prime, f/8 and f/30) are now
routinely capable of giving 0.8 arcsec images at the zenith, seeing
permitting.  But there is still an important problem affecting all
three foci.  The primary mirror becomes very astigmatic when the
telescope is pointed to largezenith distances.  This is particularly
obvious for positions north of the equator, where slightly out-
of-focus images often appear double.  An important point here is that
the focus of the telescope does change as a function of zenith
distance, and with careful refocussing it is possible to obtain
round(ish) images with 1.5 arcsec or better FWHM even as far north as
+30 degrees.

This astigmatism problem has been there since we started making
quantitative measurements with image analyzers back in 1990, but it has
been slowly getting worse.  During the past several months we have
repeatedly checked all of the parts of the primary mirror support
system that we can get to while the mirror is in the telescope, and we
cannot find anything wrong.  We suspect this is a problem in the radial
support system.  We hope to be able to solve it while the mirror is out
of the telescope during the August shutdown (see accompanying story),
either by finding and fixing the actual problem, or by compensating for
it with the new active axial support system.

Jack Baldwin, Brooke Gregory

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