[Aavso-photometry] Multi-color photometry

[arne]

Brian C. Barnes wrote:
> 
> 1) Assume I have B, V, and R filtered images of M67 along with B, V, and R
> images of a target. I can create transformation coefficients for B-V, and
> using those, I can calculate B and V. I can also create a transformation
> coefficient for V-R, and I can then calculate R by calculating V-R and using
> the V value from the B-V calculation, or I can calculate V-R and then V and
> R individually just like I did with B and V. This would give me one B value,
> 2 V values, and 1 R value. I can then average the 2 V values to get a final
> V value. Is there an advantage to calculating this extra V value and doing
> the averaging?
> 
There is an advantage.  If you have systematic errors in your determinations,
then the systematic errors may cancel somewhat by doing the averaging.

> 2) Extending question one somewhat, if there is an advantage to having 2 V
> values and averaging them, then it would seem that having extra B and R
> values would also be a good thing. This is easily done by creating
> transformation coefficients for B-R, and calculating separate B and R values
> from these. I would then end up with 2 B values, 2 V values, and 2 R values,
> which would each be averaged. Is this a valid thing to do? And if so, would
> it also be valid to have transformation coefficients for the other possible
> combinations of UBVRI? (i.e. U-V, U-R, U-I, B-R, B-I, and V-I)
> 
Similar advantages.  However, remember that there are diminishing returns.
Usually you assume that your transformation coefficients are good and
your photometry is good, and don't bother calculating all of these
optional fits.  If you do it blindly, you can actually introduce error
depending on the quality of the other equations.  For red stars, for
example, using (B-V) or (V-Ic) for your color index will give quite
different results due to the molecular features that show up in the Ic
bandpass.  So in general, I'd suggest sticking with one color index
for each transformation.

Arne