Two Color Transform mechanics

American Association of Variable Star Observers (AAVSO)
Tue, 06/14/2022 - 19:44

Can someone state the mechanics of the "Two Color Transform" in VPhot?

I have used it many times but I am interested in the internal workings.

Please assume an ensemble of comp stars and color B-V

Given that 2 images of different filters (say B and V) are selected then "Two Color Transform" is clicked, then a sequence is selected, what is done to display the "Color Photometry" report?

When are the transform coefficients (T_BV, T_V, for B-V) applied?

On p. 46 of "The AAVSO Guide to CCD Photometey" it states, "...ensemble photometry. What this does is to compare one-by-one the variable star with each comparison star you selected. Using the equations above [differential mag equations] it will then compute the standardized magnitude of the target star based on each comparison star and return⁰ the results as a weighted average of all these values"

Is this what "Two Color Transform" does? 

If so, after this, does it then transform the result?

Does there exist some documentation about this? 

My motivation is to repeat the results, but replace the instrumental mags with ones derived from PSF photometry.



-Pierre "Pete" Fleurant