[Aavso-photometry] Multi-color photometry

[Jeff Hopkins]

I think there may be some gross misunderstanding here.

Color transformation coefficients (epsilon and mu for V and B bands) 
are determined by observing standard stars.  You DO NOT use program 
stars (variable stars) to determine the coefficients. You MUST use 
standard stars. These transformation coefficients are a function of 
your equipment. These usually do not change unless the equipment 
changes.

After you have calibrated your equipment an determined the epsilon 
and mu you then make measurements of the comparison and program stars 
in say the blue and visual bandpasses using appropriate filters for 
each. This gives you the raw B and V magnitudes [M= - 2.5 * log 
(analog number representing brightness of star)] which then needs to 
be transformed using the coefficients. If you are doing differential 
photometry you need not be too concerned about zero points or 
extinction as they will cancel.

Now it is true if you know the V magnitude and (B-V)  magnitude you 
can simply find B by just adding the V magnitude to the (B-V). If you 
only measured the V magnitude there is absolutely no way you can 
determine the (B-V) or B magnitude. Consider a binary pair a red and 
blue star. Out of eclipse the V magnitude is such and B magnitude is 
such. When the blue star passes behind the red star the V magnitude 
changes very slightly where as the B magnitude dips considerably.

Consider zeta Aurigae
see:  http://www.hposoft.com/Astro/PEP/ZetaAurigaeData.html
or
31 Cyg
see http://www.hposoft.com/Astro/PEP/31Cygni.html

Transformation coefficients are applied to both program and 
comparisons stars. They transform the result to the standard system.

Now when using a comparison star it is assumed you know the V and 
(B-V) accurately from somewhere. You can easily determine the B 
magnitude of the comparison star as indicated above. You then use 
these magnitudes along with the differential magnitudes to determine 
the program star's magnitudes. This is AFTER you have done all the 
transformation calculations using epsilon and mu for the comparison 
and program stars.

Jeff

At 18:19 -0700 11/3/05, Brian C. Barnes wrote:
>Michael,
>
>     That makes sense. Which transformation coefficients to apply is
>determined in part by the quality of the COMP star data you have for each
>band.
>
>     In your case, for example, if you did use B-V, you might actually
>degrade your computed V value by introducing a noisier element into the
>equation.
>
>     When you calculate your transformation coefficients in the first place,
>do you record the S/N of the comp stars used, or the "fit" of the resultant
>slopes, for later reference when deciding which ones to apply to subsequent
>data?
>
>Brian.
>
>-----Original Message-----
>From: Michael Koppelman [mailto:lolife at bitstream.net]
>Sent: Thursday, November 03, 2005 12:56
>To: Brian C. Barnes
>Cc: 'AAVSO Photometry'
>Subject: Re: [Aavso-photometry] Multi-color photometry
>
>Yes, you can do this. Usually, though, you'll have the best S/N in 
>certain bandpasses and should just use those. For my system, for 
>example, I don't use B-V to determine V, I use V-R or V-I because my 
>B data is always worse. I could get V from both and average them.
>
>Michael

-- 
Jeff Hopkins
HPO SOFT
http://www.hposoft.com/Astro/astro.html
Hopkins Phoenix Observatory
7812 West Clayton Drive
  Phoenix, Arizona 85033-2439 U.S.A.
www.hposoft.com