[Aavso-photometry] Transformation coefficients question

[Steven Orlando]

Hello and Happy New Year to All!

Arne,

You wrote:

"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."

My question is, if you know the CIs of the comp (from the charts) and the 
variable (from charts or SIMBAD), what is the formula used to calculate the 
mag of the variable using a check star and transformation coefficients?

Steve
----- Original Message ----- 
From: "Arne Henden" <arne at aavso.org>
To: <gianlucaros at gmail.com>
Cc: <aavso-photometry at mira.aavso.org>
Sent: Tuesday, January 01, 2008 3:05 PM
Subject: Re: [Aavso-photometry] Transformation coefficients question


> 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
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