[vsnet-chat 5929] Re: [AAVSO-DIS] Reporting observations: Multi-lettercodes

[Arne Henden <aah@nofs.navy.mil>]

I have been pushing the adoption of the terms "CR" and "CV"
for use with unfiltered CCD systems that are R-like and V-like
respectively, and which then use Rc and V magnitudes for the
comparison stars/zeropoint.  The logical extensions would be
"CI" and "CB", though as Kato-san mentions, there would be
very few occasions where those terms would be useful.  One
such case is similar to Berto's example:  if you were observing
a Mira unfiltered, and had a very red comparison star as well,
then the effective wavelength of your unfiltered system moves
towards the red and using the Ic magnitude of the comp star
and reporting "CI" might make sense.  Another might be where
you use a "blue blocking" filter or an "IR blocking" filter
for some reason, still giving a much wider bandpass for your
system than even a Johnson/Cousins filter, but one that has
been moved redwards or bluewards respectively.
   The problem, as has been mentioned over and over again, is
that unfiltered magnitudes from different CCDs/telescopes/locations
are really hard to combine except under special circumstances.
Working with CVs, where they usually have nearly zero color and
don't change color with time, is one such case, which is why
CBA does so well.  If you are observing by yourself, and creating
light curves exclusively from one CCD system, you can often work
with other classes of stars for getting periods and basic light
curve shapes.  However, filtered observations are definitely preferred
whenever possible.
   Other than this, I won't get pulled into yet another discussion
of the relative merits of filtered vs. unfiltered photometry.
You know my opinion.  However, if you *are* doing unfiltered
photometry, you need to pass on as much information about how
the observation was "calibrated" as possible, which is why
terms like CR and CV have relevance.
Arne

Taichi Kato wrote:
> Re: [AAVSO-DIS] Reporting observations: Multi-letter codes
> 
> 
>>For blue stars, incl CVs, the effective wavelength of this observing
>>window of most CCD cameras is near that of the R band. Therefore R
>>magnitudes of reference stars (with non-red colours) in the field can be
>>used. Such observations are usually reported as CR.
>>If this is applied to yellow-orange stars (G-K) one will end up with a
>>waveband close to I. I magnitudes of similarly coloured reference stars
>>could be used for them. Observations are reported as CI.
> 
> 
>    Please don't be confused.  For most of stars with usual colors, the
> effective wavelengths of the most CCDs have close to that of Rc.
> Effective wavelengths are integrated quntities, and little vary around
> the mean wavelengths of the original CCD/filter passbands.
> Your explanation only applies to extremely red objects which virtually
> emit no light shorter than 650 nm.
> 
> Regards,
> Taichi Kato
> 
> 
>

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