The IRAF tasks PSFMEASURE and STARFOCUS measure the point-spread function (PSF) of stellar images and provide sophisticated graphics to analyze the measurements. The functionality and capabilities of these tasks are described in a separate paper. This paper describes the algorithms used to measure the PSF. The PSF measurements include an enclosed flux profile, a flux per pixel radial profile, a radius or full width at half maximum, and an ellipticity and position angle. When there are multiple measurements at different focus values an estimate is made of the best focus. The critical algorithm is that for measuring the enclosed flux profile. The algorithm provides accurate measurements for point spread functions with 50 percent enclosed flux radii down to half a pixel. Test results from simulations with very narrow PSFs are presented. A demonstration of the software is available NOAO/IRAF poster.
Astronomical Data Analysis Software and Systems III
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