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Using a large set of V,I photometry near the south galactic pole,
Chris Watson in San Diego showed that UCAC1 had a serious scale error
at the faint end. Fainter than about 14.5 the data are no good, but
was just fine (with consistent zero-point offset in the mean compared
to V). It was clear that a region-by-region test/correction of zero-point
might work very well for the parts of UCAC observed from CTIO, where
the percentage of photometric weather is high. The zones north of about
+20 (but ragged) is being done from USNO-Flagstaff, so the percentage
of data taken under non-photometric conditions will be much worse---
and thus local corrections could be much more problematic. I don't know
what the Flagstaff observing protocol is, so it could be that only a
small portion has been done under unstable sky (usually it's either
clear or cloudy enough here that the limiting magnitude would be affected
badly enough to just stop observing).
\Brian
Return to the Powerful Daisaku Nogami
vsnet-adm@kusastro.kyoto-u.ac.jp
