Stored as psfMag. For isolated stars, which are well-described by the point spread function
(PSF), the optimal
measure of the total flux is determined by fitting a PSF model to the
object. In practice, we do this by sync-shifting the image of a star
so that it is exactly centered on a pixel, and then fitting a Gaussian
model of the PSF to it. This fit is carried out on the local PSF KL
model at each position as well; the difference
between the two is then a local aperture correction, which gives a
corrected PSF magnitude. Finally, we use bright stars to determine a
further aperture correction to a radius of 7.4" as a function of
seeing, and apply this to each frame based on its seeing. This involved
procedure is necessary to take into account the full variation of the
PSF across the field,
including the low signal-to-noise ratio wings. Empirically, this
reduces the seeing-dependence of the photometry to below 0.02 mag for
seeing as poor as 2". The resulting magnitude is stored in the
quantity psfMag. The flag PSF_FLUX_INTERP warns that the PSF photometry might be suspect.
The flag BAD_COUNTS_ERROR warns that because of interpolated
pixels, the error may be under-estimated.
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