The DAOPHOT aperture photometry of the M92 fields was carried out
using an 8
diameter aperture and 5
wide sky annulus
for each source.
This is a relatively large aperture and is thus influenced by more noise
in the background sky than is desirable. If the image quality
is good enough, better signal-to-noise measurements can be obtained
using a smaller aperture and an aperture correction term that corrects
to an effectively infinite sized aperture. This correction factor is
traditionally measured from the photometric curve-of-growth taken from
images of relatively bright, high-signal-to-noise points stars in uncrowded
regions. The use of small apertures plus correction terms also makes it
possible to use aperture photometry in more crowded regions.