[vsnet-chat 2055] Re: Supernovae estimates using USNO SA2.0

[bas@lowell.Lowell.Edu (Brian Skiff)]

     Fraser wasn't quite right about the photometry for USNO-A1 and A2
being the same.  The calibrations were quite a bit different.  Just as a
recent fer instance, in the NGC 6822 field, the A1.0 red magnitudes are
slightly changed, but the blue magnitudes are a full magnitude brighter.
A1.0 was based broadly on the GSC photometry (!), while A2.0 includes a lot
of "real" photometry in the mix, but again with a global smoothing applied.
In the south, the faint stars have a quite serious scale error such that
stars with red magnitudes of about 18.0 or fainter are nearly always given
as 18.0 or thereabouts, even if the stars are 21st magnitude.
     Also, be careful to note that the star magnitudes in the ACT _are_not_
standard magnitudes but instead the uncorrected Tycho "VT" and "BT", which
have a color term.
     Finally, is it not faster for folks in Australia and New Zealand to use
the Japan mirror of the Strasbourg VizieR to do look-ups in A2.0 (rather
than USNO or ESO or Lowell)?  Give it a try at:

http://z13.mtk.nao.ac.jp/viz-bin/VizieR?-source=USNO-A2.0&-file=-c

\Brian

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