[Aavso-photometry] Transformation coefficients question

[gianlucaros at gmail.com]

Arne,
what if the filters have different thickness? I've heard that the
Shuler's are 4mm in thickness while the Custom Scientific's 5mm. Does
it affect precision in the coefficients or not?
Thnak you
Gianluca (RGN)


Date: Tue, 1 Jan 2008 15:05:52 -0500
From: "Arne Henden" <arne at aavso.org>
Subject: Re: [Aavso-photometry] Transformation coefficients question
To: gianlucaros at gmail.com
Cc: aavso-photometry at mira.aavso.org
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       <9c3d14ae0801011205u6f89652cl2dafafc16b375c8e at mail.gmail.com>
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On Jan 1, 2008 2:40 PM,  <gianlucaros at gmail.com> wrote:
> For CCD photometry I currently use a V filter made by Custom
> scientific and would like to buy a B filter in order to transform
> observations. I was looking at the market and found a B Schuler filter
> at substantial reduced price. My questions are:
>
> - is it possible to use the B and V filter effectively given they come
> from different manufacturers?
>
yes.  It is the bandpass that is important, not the vendor.  Most
manufacturers use the Bessell prescription for the kinds of glass
and their thickness.  So most are nearly identical anyway.  Even
if the prescription is the same, the glass used can often have
slightly different characteristics, so even filters from the same
vendor will have different transformation coefficients.  That is
why you have to determine the coefficients for your individual
system.

> - is it possible to use tranformation coefficients using B and V
> filters for differential photometry when there is no reference on
> color index of the comparisons stars (I think of the many Mira and
> semiregular stars in the AAVSO charts)?
>
Most of the comparison stars available through VSP now have
multicolor photometry.  In addition, we have an ongoing program
at Sonoita Research Observatory (SRO) to calibrate *all* Mira
fields on the AAVSO program; we've done over 200 and have
about 140 left to go.  Many of these BVRI calibrations will
be available in the next compstar data release (mid-January).
However, to answer your question: you need the color index
for the comparison star in order to calculate the color index of
the variable star, and to transform the data properly.  You can
do a rough transformation if you have the untransformed B,V
magnitudes for the variable and you know the transformation
coefficients and the average (B-V) color index for the variable,
but it is always better if you also know these things for the
comparison star.
Arne