[vsnet-chat 168] Magnitude scale of Atlas of Selected Areas

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

     Sr. Pereira asked about the magnitude system of the Brun-Vehrenberg
charts showing faint stars in the Kapteyn Selected Areas.
     These are all nominally close to the old "International" photographic-
blue system of magnitudes, which corresponds fairly closely to the current
standard B passband.  There is plenty of scatter in the results compared
to careful photoelectric observations of the same stars (such as those
done by Landolt in the equatorial Selected Areas).  In addition, the faint
stars in the Mount Wilson subset (the faintest stars) have scale error,
in the sense that stars fainter than about mag. 16 are recorded as being
too bright.  This error increases toward fainter magnitudes, such that at
mag. 21, the stars are too bright by about 1.5 mag. (i.e. the survey reached
to much fainter stars than was thought originally).
     The charts published by Ed Everhart contain photo-visual magnitudes
(roughly standard V) determined by Chiu.  These data are more reliable than
the older Harvard/Mt. Wilson stuff, but I'm pretty sure than follow-up work
by Steve Majewski, David Koo, Rich Kron, and others in these same fields
indicates the fainter stars again have a scale error or color term problem
(perhaps both).  Chiu's work was done on photographic plates with an iris
photometer, and was very good for it's day (non-digitized plates in the
pre-CCD era), but the sequences have been superceded by results from the
other authors just mentioned.

\Brian Skiff  (bas@lowell.edu)

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