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Better Images Coming Your Way at the 0.9-m (1Jun93) (from KPNO, NOAO Newsletter No. 34, 1 June 1993) The field accessible with Tektronix 2048 x 2048 CCDs at the 0.9-m f/7.5 focus is an impressive square 23 arcmin on a side. The Mini-Mosaic, currently under development, will have an even larger field with the added bonus of smaller pixels to better sample the observed point spread function (PSF). However, observers who have been blessed with good seeing and who have carefully examined their 2048 images may have noticed that stars become elongated near the corners of the current field of view. Such a variation is inherent to the Ritchey-Chretien design of the 0.9-m telescope at f/7.5. In essence, there are two optical problems: (1) astigmatism, and (2) focal plane curvature. (Physical CCD non-flatness does not significantly contribute to the observed PSF variations, at least with our current Tektronix chips.) The observed variations primarily compromise photometry that attempts to fit a constant PSF to stars across the entire field. In order to overcome this problem, Liang Ming has designed a two-element corrector for the 0.9-m f/7.5 focus. His modelling of the uncorrected optical system accurately reproduces the spatially variant PSF currently seen. This PSF variability disappears when the new corrector is included in the model, and the theoretical images are well behaved over the entire CCD and guider fields of view. The fused silica for the corrector has been procured, and we are currently obtaining bids for fabrication and anti-reflection coating. The coating material will be MgFl. The corrector will be mounted in the guider on a sliding assembly so that it can be inserted/removed from the beam as desired. All f/7.5 observers interested in a stable point spread function will want to use the corrector. The corrector is not designed to be used with the f/13.5 secondary and will be removed from the beam when that secondary is used. It may also prove desirable to remove the corrector for f/7.5 projects where minimizing scattered light is more important than PSF constancy. In parallel with the implementation of the corrector, we are working on improving the 0.9-m "dome seeing." Recent changes at the 0.9-m include: (1) turning off the selsyns, (2) turning off the heater and disconnecting the hot water heater in the bathroom, and (3) trying to alert observers of the "right way" of keeping doors open and closed in order to best ventilate the dome with the dome fans activated. An effort is also being made to keep the optics well collimated and aligned. We are in the process of finalizing a schedule for bringing the corrector into use. It is our goal to make the corrector available by mid-October. We will provide updates in future issues of the Newsletter. Taft Armandroff, Dave Silva, Phil Massey
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