Thanks to hard work by Chengze, the new point source
aperture corrected magnitudes are now available for the entire g002
reduction of the NGVS using the NGVS catalog query page.

These magnitudes are standard SExtractor aperture magnitudes,
corrected to infinite aperture for a point source. The aperture
correction is done in two steps: 1) Using bright sources in all the
images, a mean aperture correction is derived from an N pixel diameter
aperture (where N = 3,4,5,6,7,8) to a 16 pixel aperture, as a function
of position in the field. This creates one mean "N->16 pixel aperture
correction surface" across the field for each filter. The zeropoint
for this surface is determined on a field-to-field basis by the mean
seeing. 2) The 16 pixel aperture magnitudes are then corrected to
infinity using bright stars with SDSS magnitudes. Chengze can correct
me if I've missed something.

This is an improvement on previous catalogs because we now have
homogeneous, calibrated point source photometry across the entire
survey. This catalog also takes into account the small but measurable
spatial variations in the PSF, which should make bright star
color-color and color-mag diagrams tighter.

These magnitudes can also be used for star-GC-galaxy separation. Like
Alan's APS, you can take the difference between two apertures (with
APS, it's 4-8) to make a concentration index and plot versus
magnitude. The nice thing about these new magnitudes is that the
difference between any two corrected aperture magnitudes is always
equal to zero for a point source, for every field. In principle, this
is unaffected by the presence of GCs, since the stars used to derive
the aperture corrections are all brighter than GCs. I suspect that a
combination of this with APS (which has the advantage of actually
quantifying the thickness of the stellar locus as a function of
magnitude in every field), will be useful for target selection.

The new fields in the catalog are as follows for g band (and are
similarly named for all other bands):

G_MAG_APCOR3 - 3 pixel diameter aperture magnitude, corrected to 16
pixels using a spatially varying point source correction, then to
infinity using SDSS
G_MAG_APCOR4 - 4 pixel diameter aperture magnitude, ...
G_MAG_APCOR5
G_MAG_APCOR6
G_MAG_APCOR7
G_MAG_APCOR8
G_MAG_APCOR16 - simply the 16 pixel aperture magnitude corrected to
infinity (SDSS)

For a point source, all of the above magnitudes should be identical
within the errors.  The errors themselves are the same as for the
uncorrected aperture magnitudes.