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Imaging on the 0.9-m, 1-m(?), and 1.5-m...(1Mar93) Telescopes: An Update (from CTIO, NOAO Newsletter No. 33, 1 March 1993) Over the past semester, routine collimations have been performed on all these telescopes, after aluminizations. The good news is that the 1.5-m at f/13.5 has produced sub-arcsecond images in routine use. The focus frames taken on the engineering nights (Sept 10-12) show 0.75 arcsec FWHM (optical, unfiltered). This is the best performance we have seen in the optical on any of our telescopes in recent history. This telescope might be a good choice for proposals needing high resolution imaging on objects bright enough that the 4-m telescope's aperture is not required. Note, however, that the 1.5-m is almost as heavily oversubscribed as the 4-m. The bad news was that on a flip to the f/7.5 secondary during that night, a FWHM of 1.1 arcsec was measured. There was some time variability on this night, but other evidence, including laboratory testing when the secondary was refigured four years ago, indicates that the optics may only deliver 0.9 arcsec FWHM at this focus. While the telescope appears well collimated at both focal ratios, the f/7.5 wide field also definitely shows variations (primarily coma) over the sky. The secondary support system currently does not maintain high precision over the sky. Fortunately the tests seem to indicate the "best focus" value does not change (i.e., there is negligible piston), but at high airmasses (particularly north of +10 deg. declination) you will not be happy with the image profiles across the field. On the 0.9-m, coma from the Cassegrain configuration is easily noticeable when the Tek 2048 is used. Typical FWHM values vary from 70-110 um (about 3-4.5 pixels, unbinned Tek 2048) over the field on a decent night (1.3-1.8 arcsec). This definitely affects PSF photometry at the 0.1 mag level, as shown by tests done by Alistair Walker. Linear and quadratically varying PSFs help, but a cubic spatial variation is seen. Peter Stetson is currently working on such PSF implementation, and has helped us analyze some of these images on his recent visit here. It is possible that such variable PSF reductions will make stellar photometry acceptable on this telescope, but we also have a preliminary design for a doublet corrector to improve the image quality over the 2048 and future mini-mosaic fields. Since the coma goes as field angle squared, these effects are negligible on our small CCDs, and just noticeable on the Tek 1024 images. One other comment is that many filters, particularly interference filters (including several of our R band filters), produce astigmatic images. Since the 0.9-m telescope also has spherical aberration, astigmatism, focus and pointing glitches, we are going to test the image quality on the 1-m during some free time later this semester. This is an f/10 wide-field imaging telescope, which has been used for photometry and spectroscopy during the past five years. It has had historical focus uniformity and stability problems, and some wind- shake, but it should show a flat field and is actually on one of the superior sites of the mountain. Bob Schommer
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