[Aavso-photometry] B-V, etc.

[Keith Graham]

Arne, Greg, and Tim,

Thanks much for your input and very helpful comments.

Just to clarify, it has been too cloudy lately to do much imaging, so I have 
been playing around with CIs. I got to thinking that there had to be a way 
to determine color indices without the use of comp stars for which CIs had 
been determined. I figured I could check the accuracy of my system if I 
could derive CIs of comp stars on my BVRI images and compare them with the 
known CIs for those comps. I know I can used a comp star to determine the 
CIs of the others, but if my reasoning is correct,  this would mean the comp 
star would, in essence, be the zero point upon which the CIs of the 
remaining comps would be determined. I have tried this method and have come 
very close (within a few hundreths) of the sequence CIs.

So, just how are CIs determined in a new field where no sequences exist? If 
my thinking is correct, instrumental mags need to be determined for each 
filter. Transformation coefficients and zero points would then be applied to 
get the true mag for each filter. The CIs could then be derived using the 
formula(s) in Brian Warner's book. Since I am using CCD images, I would not 
need to be concerned with extinction coefficients. Does this sound right? I 
am unclear about 1st and 2nd order extinction and need to read up on those.

I do have both Arne's and Brian's books. Greg, your mention of Brian's 
calculations lit the fuse for me. And Arne, your comments about zeropoints 
and transformation coefficients are just what I was thinking. I did find 
sections in both Arne's book and Brian's book that are making sense to me. I 
also have Canopus, but have not delved into it in great detail. I shall look 
into its use for CI determination. More fun for cloudy nights.


Cheers,

Keith