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Improvements at the 0.9-m (1Jun95)
(from KPNO, NOAO Newsletter No. 42, June 1995)

Recent improvements to the 0.9-m telescope now provide observers with better
data than ever before. As noted in  NOAO Newsletter No. 36, KPNO has
adopted the strategy of using the 0.9-m as an inexpensive proving ground for
image quality upgrades where they can be implemented and evaluated quickly.
At large telescopes, similar goals are costly and incur long feedback delays.

Improvements fall into two primary categories:

1)  Optical components:

     o  A two-element corrector was installed in the fall of 1993 (NOAO
        Newsletter No. 36, December 1993). With the corrector in the beam,
        the point-spread functions are nearly constant across the full 23'
        square field of the Tektronix 2048 X� 2048 CCD, thereby simplifying and
        improving the photometry from the telescope. The corrector also
        dramatically reduces the need to refocus the off-axis guide camera
        frequently, thereby raising observing efficiency.

     o  The f/7.5 secondary mount was replaced in the fall of 1994. Changes
        in collimation and secondary mirror shape with ambient temperature
        are now eliminated. The telescope was collimated after the new mount
        was installed and has not needed any adjustments during the four
        months of operation thus far.

2)  Thermal environment:

     o  During the summer of 1994, the electronics rack on the observing floor
        was moved into the computer room.

                             [Figure not included]

     o  Also at that time, eleven ventilation louvers were placed in the dome
        skin (see picture above). These can be individually or collectively
        opened to flush the dome quickly and frequently of any heat buildup,
        while providing sufficient control to avoid wind shake when the air
        flow becomes vigorous (e.g., wind speeds >  7 m/s).

     o  In the spring of 1995, the telescope tube was ventilated to pull ambient
        air down the tube and across the mirror. This will prevent a
        temperature stratification in the tube and reduce mirror seeing.

Recent tests demonstrate that these steps have had a positive impact. Users
report that the images are limited by the 0.68" pixel sampling nearly half the
time. Tests with a small pixel CCD (15 um = 0.43") showed that the telescope
is capable of producing images smaller than 0.85", these being limited by the
inherent astigmatism of the primary.

Clearly, better pixel sampling is needed to take full advantage of the better
delivered image quality and wide field available at the 0.9-m. We are planning
to upgrade the f/13.5 secondary mount by January 1996 for those programs
which can benefit from the improved images. This will provide a scale of
0.38"/pixel and a field of view of 13' square.

Tests to evaluate the state of the primary will take place in the fall. Should
we find that the images frequently are limited by the primary mirror
astigmatism, we will consider alternatives to improve performance.

George Jacoby, Phil Massey, Taft Armandroff,
Bill Schoening, Todd Boroson, Ron Probst

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