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I am using noao/digi/daophot to do aperture photometry.In photpars I set the aperture to a given radius.I do phot on an image, and in the output photometry file there is an entry for "AREA". In general, it is quite close to PI x Radius^2, but not exactly the same. There are variations of up to 3-4% in the equivalent radius (defined as sqrt(AREA/PI)). Why is this so?Many Thanks!

The AREA is not computed directly as PI*R^2, it is an approximation based on the sum of the fractional areas. If you want to see the actual code that does this, see apphot$phot/apmeasure.x

OK, thanks, that makes sense.Now, when doing aperture corrections to PSF magnitudes as a function of aperture, how is this changing area incorporated in the correction?Thanks again.

I'm afraid I'm not familiar enough with the photometry code to answer your question. If you phrased it in terms of "How/Does task XYZ use the AREA value in a .mag file" I'd be happy to look through the code to see if I can figure it out. Otherwise, have a look at the 'Docs' link above at some of the photometry cookbooks to see if it is explained there (in particular the DAOPHOT reference manual seems to mention aperture corrections).