[Aavso-photometry] BVRI Photometry

[Keith Graham]

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