[vsnet-chat 1358] Re: Magnitude estimates from unfiltered CCD images

[Taichi Kato <tkato>]

Re: Magnitude estimates from unfiltered CCD images

   Some practical notes.  I know several unfiltered CCD observers who have
reported in different ways.  They can be categorized into:

  1) Zero-point adjusted using a selected bright comparison star within the
same field of the variable (consistent within an observer) either using
GSC/USNO magnitudes.

  2) Same as in 1), using V manitudes from several sort of chart materials.

  3) Zero-point adjusted using the whole set of detected stars in the same
field, either using GSC/USNO.  In this case, the number of comparison stars
may be variable between exposures.  Several automated softwares and surveys
use this policy.

  Both "C" (unfiltered CCD) or "R" bands have been referred to in reporting
these magnitudes.

  We, when reporting unfiltered CCD magnitudes, use the policy 1) or 2),
selecting the bluest brightest star in the same field (this can be easily
examined by comparing GSC/USNO magnitudes and obtained instrumental
magnitudes), and report them in the "C" system.

  The problems of this strategy are as already mentioned, but we consider
it better than to report nothing, esp. in dwarf novae observing.  Even
observations with an error of 1 mag are crucially important in many cases.

Regards,
Taichi Kato

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