[Aavso-photometry] B-V, etc.

[arne]

Keith Graham 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? 
> 
As others have said, you can't get there from here.
If you use the instrumental color index, you will have two
systematic offsets from the standard system:
(1) a zeropoint, based on the throughput of your system for each
filter.  The filters may transmit different amounts of input light;
the CCD may have different QE at each wavelength, etc.  This creates
a simple offset from the standard system, so that the instrumental
color index will be wrong, but it is the same amount of "wrongness"
for every star in your field.  Therefore, you can use the instrumental
color index much like differential photometry - look at the color index
for your target star, and pick other stars in the field with equivalent
instrumental color indexes and you will be picking stars with the
same true color, usually a good idea for comp stars.

(2) a transformation coefficient, which generally tells how your
instrumental value differs from the true value as a function of
star color.  This is usually a small correction, important if you
need high accuracy but unimportant if your target and comparison
stars are similar in color.

The extinction terms are almost always negligible in CCD photometry, since
you are imaging target and comparison stars in the same field and that
field is a few (tens) of arcmin in size.

If you need absolute values, and you don't know the standard colors for
some star in the field, then you have to do all-sky photometry and all
of the extinction, Landolt standards, etc. aspects become important.
Arne