[vsnet-chat 1363] Re: Magnitude estimates from unfiltered CCD images

[Taichi Kato <tkato>]

Re: Magnitude estimates from unfiltered CCD images

Arne Henden wrote:

> 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.

    Aside from the fact that most of our targets are CVs, the suggested choice
for stars of similar colors to variables is neither always possible nor
always consistent.  For, as an extreme example, carbon stars, it is usually
impossible to adopt a constant star of the similar color in the same field.
The knowledge in colors of variable stars depends on observers: some may or
may not know its their true colors.  This means the result may be largely
affected by the knowledge of each observer.  Instead, we suggest to use
comparison stars with B-V=0 (or V-I=0), which may be expected to have similar
magnitudes in any passbands.  The bluest field stars are relatively aceptable
approximations when we are observing in the usual CCD response domain.

> Second, the comparison should be close in brightness to the variable so that
> you don't push the dynamic range of your CCD.

    That's true.  One should always avoid saturation and non-linearlity.
This is particularly true for ABG-equipped CCDs, which show a large degree
of non-linearity.  One may not worry much when the linearity of the response
is experimentally assured.

> 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.

    In the history of VSNET and pre-VSNET era, I remember the "C" system name
was first introduced in archiving SN 1993J observations.  The "C" system
then meant unspecified unfiltered CCD observations, just as in unspecified
unfiltered phographic estimates.  The VSNET data archiving system allows a
combination up to three alphabets, some of which have been used to represent
more specific systems (e.g. TxP for Tri-X + PO0 filter).

    Regarding this point, several observers have shown interest what system
names can be most appropriate in reporting several types of non-standard
filtered CCD observations (e.g. IR-blocking filter).  We'd appreciate
suggestions in these cases, and like to know what variations of filters
are actually used in CCD observations.

Regards,
Taichi Kato

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