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New and Improving Images on the 0.9-m (1Dec94)
(from CTIO, NOAO Newsletter No. 40, 1 December 1994)

Over the past several years we have been evaluating the performance of the
0.9-m telescope. Since this instrument is used exclusively for CCD direct
imaging (and now almost totally with a Tek 2048), image quality becomes a
scientific concern for many programs. As the CCD size and field have
grown, the limitations of the existing system have become obvious: these
include spherical aberration, coma, and astigmatism, with typical seeing of
1.5"-1.6" (V bandpass). The causes for this rather inferior performance
include faulty mirror supports, inherent optical limitations, poor guiding
implementation, and inadequate thermal control over the telescope, dome,
and building.

We have begun a program to upgrade this and other small telescopes on
CTIO. With resources made available by the MACHO agreement (see
accompanying article) it appears that we will be able to find ways to
implement some of the obvious improvements over the next year or two.

To talk about something which has been done, a shortened guider box was
installed in July on the 0.9-m. Tests over the past 1.5 years with a Hartmann
screen had indicated that significant spherical aberration existed, and that the
minimum spherical focal plane was 96 mm above the nominal detector
position. Although this is a slow f/13.5 telescope, this despace error
contributes approximately 0.6" to the image spread. When removed (in
quadrature), typical seeing therefore might approach 1.2"-1.3", and the best
nights should produce subarcsecond images (if other effects do not mask
this).

Jorge Briones and Oscar Saa fabricated a new guider box and TV guider
mount. Tests show that the system seems rigid and reliable, so it is in
routine use. Image quality at the guide TV immediately seems improved,
and reports of 1.2" seeing seem more common, although the database is still
quite limited. One observer has reported a guided image with FWHM =
0.95".

We are currently aluminizing the 0.9-m mirror and taking the opportunity to
inspect the primary support system (which has several flaws). We also will
be installing exhaust fans in the (closed) telescope tube over the next month,
which should improve some local seeing and entrapped air issues (gradients
of 1^oC have been measured along the tube).

This is, of course, one of the first scientific telescopes put into service on
Tololo. In fact, this telescope was one of the first KPNO telescopes, and was
subsequently shipped to Chile when the KPNO #1 0.9-m became a R-C
telescope with a flip-top secondary. I would dearly love to improve it to the
point where it routinely produces the subarcsecond images that we know the
site delivers. Many of the scientific programs could derive significant
benefits from such an improved performance.

Bob Schommer

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