[vsnet-chat 1359] Magnitude estimates from unfiltered CCD images

[aah@nofs.navy.mil]

Taichi Kato wrote about the methods their group uses for reporting unfiltered CCD magnitudes.
Some comments:
  (1) picking the brightest, bluest star in the field as the primary comparison is
      reasonable, but with caveats.  First, if you do have color information for the
      variable and the comparison stars, you should pick a comparison star that is
      close in color to the variable.  For CVs, that would be a blue star, but a
      yellow star might be better for cepheids, etc.  Second, the comparison should
      be close in brightness to the variable so that you don't push the dynamic
      range of your CCD.  If the only comparison star with a known magnitude (such as
      a Tycho star) is much brighter than the variable, then it is better to bootstrap
      to a fainter comparison star and use that fainter star(s) as your new primary
      comparison.
      
  (2) You are right in calling your unfiltered observations something other than R
      (since R implies that you have done the transformation into the Johnson/Cousins
      system).  "C" might be a reasonable choice for nomenclature, but there are other
      filter systems that also have a "C" filter.  I would propose using more than
      one character for your filter field, and perhaps calling your measures "Ru" for
      R-unfiltered, as an example.  Also, remember that unfiltered bandpasses are
      quite different between different CCD/telescope systems.  For example, while
      a front-illuminated chip like the TC245 has a wide R response, a back illuminated
      chip is much bluer, and refractors have different response than reflectors.
      One should not assume that "C" magnitudes are on some common system.
      
  (3) I agree that observers *should* report their unfiltered measures even if they
      are not on a standard system.  Kato gave dwarf novae observing as an example,
      but unfiltered measures as useful for just about any variable star observing
      in defining the period and epoch of variation or even the classifying the
      variable.  Unfiltered measures will allow the observer to go fainter since
      you are using the entire bandpass of the CCD.  With that said, I still strongly
      recommend that serious CCD observers use standard filters (but won't go into
      the numerous reasons in this short response).
      
Arne

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