UVOT Photometry:

The Swift UVOT Users's guide http://swift.gsfc.nasa.gov/analysis/UVOT_swguide_v2_2.pdf, P. 76 says, “ look at http://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/swift/docs/uvot/” for integrated PSFs.

UVOT Users guide p. 152 says to use radius=3“ apertures: “In general faint point sources should use circular apertures with a radius that maximizes the signal-to-noise ratio in the aperture. This is typically about 3 arcsec.”

UVOT W band PSF is FWHM = 2.37 ” http://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/swift/docs/uvot/uvot_caldb_psf_02.pdf THERE IS AN IMPORTANT TRICK FOR CALDB: the curve-of-growth is normalized at 5“; ALL FLUX is really about 115% for W, not 100%.