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For FWHM to be useful as a quality measure within a given system, it does not matter what the units are. However, if you use it to compare with the results others get - which was implicit in the OP question - then the system FWHM in pixels must be converted to arc secs via the plate scale.
I just did a comparo to see how much measures varied with software - Pixinsight, Iris and Nebulosity all reported ~ the same results (well within 10%) for a few chosen stars (which must be unsaturated of course) in a recent image. Pixinsight gave almost identical FWHM to Iris when forced to use Gaussian rather than Moffat profiles. Neb uses HFR (half flux radius) which is similar to FWHM, but being a radius, needs to be doubled to a diameter to be comparable - results were very similar to the other two when this was done.
FWHM (or HFD) in arc seconds is a useful absolute measure of seeing for any scope larger than about 100mm, provided the pixel sampling is reasonably close to the Nyquist optimum.
Last edited by Shiraz; 01-10-2013 at 12:12 AM.
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