[Aavso-photometry] B-V, etc.

[Keith Graham]

Hi All,

I have a basic question regarding color indices determination. I have been trying (without success) to match sequence B-V calculations and am wondering if I am missing something in my computations. 

If I simply use the B and V comp star mags as reference and then take B & V values of other sequence comp stars from B & V filtered images, I am pretty darn close ( to within a few hundreths). My problem lies when I attempt to get B-V without the use of comp stars. I assume I must use instrumental mags to arrive at the color index values, so I have been merely extracting the B instrumental mag from a B filtered image and a V instrumental value from a V filtered image and subtracrting the two values. I know I am missing something here because the software (AIP4WIN) does not know that these images are B & V filtered. It merely extracts what it "sees". So my question is how can I get B-V values from B and V filtered images without the use of a comp star with known B & V values? 

Cheers,

Keith Graham