[Aavso-photometry] Multi-color photometry

[Brian C. Barnes]

I'm just starting to get into doing photometry with filters other than "V",
and have been reading up on calculating transformation coefficients and how
to apply them. I've hit upon a couple of questions that I have regarding the
mathematics of it all.

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?

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)

Obviously, this is a lot of number crunching, but automating it all in a
spreadsheet would be simple enough. I just don't know if I would get better
numbers, or worse numbers, doing this.

Brian C. Barnes
BBO