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NOAO Newsletter - KPNO Operations - March 2000 - Number 61


Improving Imaging Performance at WIYN

D. Sawyer, C. Corson, A. Saha

A combination of recent improvement projects and focused maintenance have more than quadrupled the likelihood of obtaining 0.6" or better images at the WIYN telescope. And the potential exists for yet further improvement. These efforts, coupled with the implementation of the MiniMosaic imager, which offers improved image sampling (0.14"/pixel), are allowing observers to routinely obtain images with what was once remarkable resolution.


A section from a recent 20 minute MiniMosaic exposure by D.J. Pisano and E. Wilcots (Wisconsin) with 0.4" stellar images shows the improved imaging performance at WIYN.

The improvement in delivered image quality (DIQ), as demonstrated in statistics based on 10-sec R-band exposures at the beginning of each night, are largely due to efforts that included balancing the primary mirror thermal control, refining the wavefront analysis routines and methods of tuning, improving the secondary mirror active control, and improved pointing and tracking (NOAO Newsletter, January 1999).

Delivered image quality in February-May 1999 at WIYN included a number of nights when 0.4" images were obtained, imaging performance not seen in the 1995 time period.

Since May 1999, the DIQ performance has further improved due to two major improvement projects. A 24-Hz resonance in the top end of the telescope, which was degrading DIQ by as much as 0.15" in certain conditions, was successfully eliminated. In addition, a mechanical problem with one of the primary mirror lateral supports, which was contributing to unstable performance of the active support system, was discovered and corrected.

Our current imaging performance is the best it has ever been, with 0.5" or better images obtained on 9% of the nights and 0.4" seen on 2% of nights during the February-May 1999 interval. Wavefront aberrations are more stable night to night, with errors amounting to less than 200 nm RMS open-loop. Quantitative comparison of the current DIQ will be available when the wavefront camera is back on-line after upgrades that will allow the telescope operators to tune the optics on short enough timescales that wavefront measurements, and perhaps corrections, may be accomplished several times during the night without impacting observing efficiency.

An auto-focus sensor (AFS) that uses a Shack-Hartmann test has been successful in tracking focus changes, even during exposures. The AFS is currently in the final tuning stages, and we expect to have it used routinely during science operations within the next month or two.


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