[Aavso-photometry] B-V, etc.

[Tim Crawford]

Hi Keith,

1)  Subtracting instrumental mags in the manner of B-V
is somewhat similar to mixing apples and oranges, if
you stop and re-think about it, in as much as the
instrumental values come from TWO DIFFERENT images and
are therefore simply arbitrary numbers that are
independent of one another, UNLESS each images comp
stars were "zero pointed" prior to generation of the
instrumental values.

2)  To really insure accuracy of the B-V values,
rather than a  simple differential solution, the data
should be transformed.

I will have an interest in other responses...  just in
case I have misunderstood your queston and I might
learn something, also.

Per Ardua ad Astra

Tim Crawford, CTX
Arch Cape Observatory

--- Keith Graham <kgraham at rkymtnhi.com> wrote:

> 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  
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