[vsnet-chat 1354] Re: Magnitude estimates from unfiltered CCD images

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

     It is first important to know that the GSC magnitudes _are_not_ V
magnitudes!  Those reporting V magnitudes based on the GSC are doing so
in error.  The blue and red magnitudes given in USNO-A1.0 are only very
approximately so, completely inadequate for any real estimates of variable-
star brightness.
     For asteroids, since they have nearly the same colors for the most part,
it is permissible to observe in some non-standard passband, such as unfiltered
CCD, and make only a zero-point correction transform to V or R for example.
     For variable stars, because of their wide range in color, one must know
the colors of both the reference stars and the variable in question in order
to transform one's "instrumental system" (even with a filter) to the 
standard one.  EVen from poor climates, one can often do reasonably good
calibration to the standard system simply with two stars of known magnitude
and color as reference.  One makes an observation by first taking a frame or
two of a standard star (or small field with several of them), then taking
frames of the variable, followed as soon as possible by another set of images
of either the same reference stars or others nearby.  Use the image-processing
software you probably already have to get the brightnesses of the relevant
stars on all the frames and determine the relative brightness of the variable
from these observations.  DO NOT use the GSC or A1.0 stars for any of this!
Instead use stars from Landolt's equatorial standard list, stars in the
southern E-regions, or stars selected from the very long list I keep at:

http://ftp.lowell.edu/pub/bas/starcats/loneos.stds

About half the stars in this list are brighter than mag. 13, and so are
suitable for small telescopes with CCDs and fairly short exposure times.
There are always stars within a few degrees of any target to be found in this
list.

\Brian

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