[Aavso-photometry] Transform coefficients and two-filter measurements

[tom_krajci at tularosa.net]

I've been on vacation and visiting mom in upstate NY (Elmira), and doing
some thinking on transform coefficients and two-filter measurements.  (My
earlier posts and thoughts have been on single-filter stuff...so far.)

I'm still trying to get some concepts and ideas straight in my head.

With single-filter measures and transforms, your plot of deviation of your
filter (instrumental mag) from the ideal/catalog values (V-band, etc.) is
a slope that is close to, but probably not zero.

In general/abstract (and mostly plain English) terms...what are you trying
to determine in your two-filter measures and deviations?...the deviation
of your instrumental B-V from catalog/ideal B-V?

And if you plot your instrumental B-V versus catalog/ideal B-V...you get a
slope that is close to, but probably not one...yes?

The deviation of your single filter measures is awful close to a straight
line.  (Your filter bandpass is somewhat redder, or bluer, than the
ideal.)

But with two-filter measures you have four possible scenarios:
- B filter bandpass is bluer than ideal
- B filter bandpass is redder than ideal
- V filter bandpass is bluer than ideal
- V filter bandpass is redder than ideal

Hmmm, how do those four possible situations plot (X-Y) for instrumental
B-V versus catalog/ideal B-V?  All they all straight lines?...or do some
situations result in a clearly curved (second order curve?) plot?

Thanks in advance,
Tom Krajci
(Elmira, NY until 24 July)