[Aavso-photometry] Multi-color photometry

[Michael Koppelman]

I think Brian is calling standard stars comp stars. I suspect he  
knows that you don't create xforms from your program stars. But, yes,  
of course you are correct, you create xforms from standard stars and  
then apply them to unknown stars.

I personally have never used xforms along with comp stars. I've done  
it all-sky or not at all. The AAVSO guide does have instructions for  
using xforms when you have comp stars with colors.

Brian, I don't choose which xforms I use based on comp stars or  
program stars, I just use the ones where I had the best data when  
doing my xforms. You can use various measures, such as the S/N or the  
error in the fit. If you go so far as to calculate the uncertainties  
in all your coefficients, this would tell you which your best ones are.

I suspect in this day and age most people are better off calculating  
V as a function of V-R or V-I rather than B-V.

Michael


On Nov 3, 2005, at 8:27 PM, Jeff Hopkins wrote:

> 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
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