[Aavso-photometry] BVRI Photometry

[Chuck Pullen]

Keith - a good place to start is Arne Henden's Book "Astronomical 
Photometry" available from Wilman Bell.  Note that the Bensen example has 
some errors in it, any might be confusing.  I'd do it exactly as Arne does 
in his book.

Another web resource in PDF is http://www.observatory.ou.edu/book2513.html 
from Bill Romanishin.  Very complementary to Arne's hard cover book.

Chuck Pullen


At 14:36 10/27/04 -0500, Keith Graham wrote:

>Hi,
>
>I have decided to take the next step up from V photometry and try BVRI. I
>have been reading the papers by Priscilla Bensen and Lou Cohen as well as 
>other sources, and I have a few questions
>
>
>1) In order to compute transformation coefficients, I first need to derive the
>instrumental magnitudes for b,v,r, and i  I use AIP4WIN, and it has the 
>ability
>to derive instrumental magnitudes. However I need to put in the zero point.
>I am stuck on just how I can determine Z. One way I attempted it was to
>click on a star of known magnitude in the image. I then adjusted the zero
>point until the instrumental magnitude matched the actual magnitude. But
>this appears to me to "fudge" the actual instrumental magnitude to give the
>real magnitude of the star. So my question here is just how do I calculate
>the instrumental magnitude for the purpose deriving transformation
>coefficients. I did find a way to calculate instrumental magnitude w/o the 
>need for a
>zero point using the formula:
>
>m1 = -2.5log(F1/F2) +m2.
>
>  But this formula requires knowing the magnitude of a second star.  Since m1
>is instrumental magnitude, I would assume that m2 would also be instrumental
>magnitude. Lou Cohen's paper says to select a star of known magnitude for
>m2, but then, again, this would appear to me to be the actual magnitude, not
>instrumental. So, is it correct that for this formula I can insert the actual
>magnitude of m2?
>
>
>
>2) I have developed a spreadsheet for BVRI photometry. It appears to me 
>that ones needs instrumental values for v, r, and i (along with comps) in 
>order to ultimately obtain trnasformed standard mags BVRI.  I found that 
>if I removed any of the star - comp  v,r, or i values, all other values 
>were affected for that given star-comp . This would stand to reason since 
>the formulas for deriving those values all depend on one another. So, how 
>would one derive transformed B,V, and R values without the I filter?
>
>
>
>3) Zero point really has me spinning. I still don't know what that is all 
>about. I think it means a "common ground" factor that standardizes the 
>instrumental mags, but just how to determine it is a mystery to me. Any clues?
>
>
>
>
>
>Thanks for any help you can offer.
>
>Keith Graham
>_______________________________________________
>Aavso-photometry mailing list
>Aavso-photometry at mira.aavso.org
>http://www.aavso.org/mailman/listinfo/aavso-photometry
>
>
>The Discussion Groups are a free service from the AAVSO. If you find it 
>useful please consider donating to the Janet A. Mattei Research Fellowship 
>Program. Information is available at: 
>https://www.variablestars.com/janetfund.shtml