[vsnet-chat 1374] comparison of GSPC and USNO-based V mags

[aah@nofs.navy.mil]

Because of the special analytical psf that Dave used, the bright, saturated stars should
have magnitudes that follow their linear size.  This is similar to what most previous
attempts at Schmidt photometry used, and so the magnitudes should be relatively consistent.
Once the star becomes unsaturated, it is much more like standard psf photometry.  What
happens is that there are two regimes (bright and faint), and they require different
calibration.  Dave used Tycho for the bright regime, and as many *deep* sequences as
he felt comfortable with for the faint regime.  Brian's sequence compilation is good,
but doesn't cover every POSS field, nor does it go deep enough in most cases to calibrate
these plates at 20th magnitude.  Instead, you must use a plate overlap adjustment to
calibrate plates without photometry, and errors creep in.  So if I were a gambling man,
I'd say the bright magnitudes should be at least as good as the faint ones, and that
the faint regime is more likely to have cases of large discrepancy (especially in the
south where fewer sequences were used).

A further note: the GSPC-I was not designed to have high precision, especially at the
faint end, so using it for plate calibration isn't necessarily the best.  Besides, it
only goes to V~15, not faint enough to calibrate the deep POSS plates.  It also only
calibrates the center of each POSS-I field and so does not supply vignetting information.
GSPC-II will be better if and when it is completed.

Bottom line:  as Brian has stated many times, to properly use any catalog,
you need to understand its limitations.

I still have a week's worth of work reprocessing some old CCD frames, but then will start
making a new version of my mag17 catalog with the new USNOA2.0, and will do some checking
of the photometry at that time.

Arne

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