[Aavso-photometry] Transforming

[Barbara G. Harris]

Here are the TCs that I obtained about 1 month ago.  I re-imaged M67 a few
nights ago to see how repeatable my numbers were but I have not performed
the calculations yet for those images:
Tbv:  1.268
Tvr:  0.981
Tri:  0.868
Tv:  -0.189
Tr:  -0.173

What do the negative numbers for Tv and Tr signify?  How do I know if a
particular star that I am observing has a constant color?

Barbara

-----Original Message-----
From: arne [mailto:arne at aavso.org] 
Sent: Sunday, March 29, 2009 9:03 AM
To: Barbara G. Harris
Cc: AAVSO Photometry
Subject: Re: [Aavso-photometry] Transforming

Barbara G. Harris wrote:
> I have been doing CCD photometry of variable stars for about 2-3 years now
> primarily with the V filter.  I do have a complete set of photometric
> filters.  One of my New Year’s resolutions was to obtain my transformation
> coefficients.  I am sort of learning as I go.  I found Lou Cohen’s paper
on
> how to calculate transformation coefficients and so far I have imaged M67
in
> B,V,R, and I.   I have followed his instructions and calculated my
> transformation coefficients.
> 
>  
> 
> My next step is to apply these coefficients to my observations.  I am
still
> a little lost in this area.  My question is if I image a star through a V
> filter, how many other colors (at a minimum) do I need of that star to
> transform the V estimate?  Also can someone point me to additional reading
> on applying transformation coefficients?
> 
>  
Hi Barbara,
The answer is always "it depends."  If your target star has a fairly
constant color (doesn't vary dramatically through pulsational cycle,
eclipse, etc.), then you can often transform using just a single filter
if you know the mean color of the target star and of the comparison star.
If you don't, then at minimum you need observations in two filters.
Those two filters create an instrumental color index, which can then
be transformed into a standardized color index.  You then use that
standardized color index to transform the V magnitude.  There is a good
description of the process in our "Astronomical Photometry" book
for V and (B-V), but you can extend the same process to any magnitude
and color index combination.

What are the values of transformation coefficients that you have
determined?
Arne

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