[Aavso-photometry] Transforming

[Tom Krajci]

 >From: "Barbara G. Harris" <barbharris1 at hughes.net>
> ...so far I have imaged M67 in
> B,V,R, and I.   
>   
I'm also curious to see what values you got for transforms.
> 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?  
>   
If you follow in Arne's footsteps (i.e. cover fields/targets that he's 
already calibrated, and the variable is a well known type that can use 
an 'assumed' color index)...then you only need one filter.  Thanks to 
Arne's trailbreaking work...it's simpler.

I have done some single-filter (V) imaging of M67 and put together an 
even simpler spreadsheet that shows how I can determine the 
transformation value for this single-filter approach.  If you want, I 
can send you a copy of this spreadsheet.  (Again, this single-filter 
approach only works if you are studying fields/stars that have been 
multi-filter calibrated beforehand.)

For what it's worth, my spreadsheet shows that when I compare my 
instrumental v mag's to calibrated/catalog B-V color index values in 
M67...my slope is -0.02.  In other words, my V filter/scope/CCD is awful 
close to catalog values even with no transform applied.  (But I didn't 
know that until I did this analysis...your mileage may vary.)

Hope this helps.

-- 
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Tom Krajci
Cloudcroft, New Mexico
http://picasaweb.google.com/tom.krajci

Center for Backyard Astrophysics (CBA)
http://cbastro.org/ CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
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