WIYN Delivered Image Quality (DIQ)

Since mid-1994, the WIYN Observatory staff has been making delivered image quality (DIQ) measurements every usable night. Most of these measurements were made by our Observing Technicians, Alex Macdonald, Bridget Watts, Dave Summers, and Craig Mackey. Dave Sawyer, the WIYN Site Manager, and Nick Roddier, the NOAO engineer with primary responsibility for the WIYN active optics control, also significantly contributed to this effort in the early days. A histogram of these measurements is presented above.

All measurements were made with a cooled science-grade CCD with 0.2 arcsec pixels in the (Harris) R-band. The fields imaged were typically at airmasses less than 1.05 (i.e., at zenith distances less than 25 degrees). For ease of use, image sizes were measured using the IRAF task IMEXAMINE. The measured FWHM were rounded up to the nearest tenth of an arcsec before being included in the Figure.

The WIYN DIQ was measured usually immediately after making a delivered wavefront measurement and tuning the WIYN active optics system, if the measured wavefront indicated that the latter was necessary.

These measurements were not guided. In fact, many of the longer exposure images show evidence for slight tracking errors and small deviations from ideal focus.

Qualitatively, the typical WIYN stellar image does not look purely Gaussian. Rather, it has a tighter core sitting on top of slightly broader wings, reminiscent of a Moffat function.

This raises the interesting question: how does one quantify DIQ? Different measurement techniques give different answers. For example, the FWHM calculated by the IRAF task PSFMEASURE, now in wide use by KPNO staff and visiting observers, is typically 0.1 arcsec larger than the FWHM calculated by IMEXAMINE. On the other hand, a fitted Moffat function can have a FWHM which is 0.1 arcsec smaller than the IMEXAMINE FWHM. This is demonstrated by the image published in the September 1994 NOAO Newsletter (p. 3). A Moffat function kindly fitted by Peter Stetson (DAO) had a mean FWHM of 0.45 arcsec, which is the image size reported in the Newsletter. The PSF FWHM measured by IMEXAMINE, however, was 0.6 arcsec. The WIYN Observatory will continue to use IMEXAMINE since it is easy to use and appears to deliver a conservative DIQ estimate.

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Last Updated: 8/2/99, wiynq@noao.edu