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The Incredible Shrinking 0.9-m Mirror (1Jun92)
(from CTIO, NOAO Newsletter No. 30, 1 June 1992)

As part of our program to improve the imaging performance of all our
telescopes, on 17 March we installed a mask on the outer edge of the
0.9-m primary. Tests of the image quality over the past year have shown
a variety of problems, including an obvious outer "hairy edge" on
extra-focal images. We suspected this to be an effect of a turned down
edge on the primary mirror, which is now confirmed by the improved
appearance of the images. The extra-focal images are now more regular
and symmetrical, and the radial profiles of in-focus stars appear
smoother.

For reference, the clear aperture of the telescope is now 34.5 inches,
or 0.88 meter. The resulting loss in throughput is 0.085 magnitudes.
This should be more than compensated for by the improved concentration
of light in the image.

Astigmatism is still evident when focusing this telescope, and we
suspect problems with one of the mirror support mechanisms, so further
improvements are being planned. Since this is a classical Cassegrain
telescope, coma is also evident away from the optical axis. We have
adjusted the collimation of the primary to place the optical axis as
close as possible to the center of the chip, although we are still
about 1.5 arcmin away from perfect centering (the optical axis is
displaced to the SE). Because of this coma, images at the corners of
the chips are noticeably degraded. This becomes especially evident with
the Tek 2048 CCD (which has a 13 arcmin field of view), and observers
attempting PSF fitting should be aware of this problem.  A recent run
with this detector showed a FWHM degraded at the corners by 0.2-0.3
pixels (0.08-0.12 arcsec), in moderate seeing (about 1.4 arcsec FWHM).

Bob Schommer, Bill Weller, Oscar Sa 

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