[Aavso-photometry] Determination and use of BVRI transformation coefficients with Maxim DL

[Lionel Catalan]

Based on comments from Arne and Gord Sarty, I thought that the most
straightforward way of using Maxim photometry data to derive
transformation coefficients would be to convert Maxim's differential
magnitudes into instrumental magnitudes. This recognizes the fact that
transformation equations are written in terms of instrumental
magnitudes.

Because the photometry analysis tool in Maxim DL only calculates
differential magnitudes, a special procedure is required to derive
instrumental magnitudes. Differential magnitudes in Maxim DL are
calculated using a reference star with known standard magnitude as
follows:

mdiff (star) = minst (star) – minst (ref) + mstd (ref)

where mdiff, minst and mstd refer to differential, instrumental and
standard magnitudes, respectively. The instrumental magnitude of the
reference star can be calculated as follows:

minst (ref) = -2.5 log (Int(ref))

where Int(ref) represents the intensity of the reference star
calculated as the sum of all pixel counts within aperture less
background. The value of Int(ref), which is simply called "Intensity"
in Maxim DL, can be read from the information window in aperture mode
by centering the aperture on the reference star while using the
photometry analysis tool. One must be careful to ensure that the
centroid of the reference star has the same coordinates when reading
its intensity and when calculating the differential maginutes of the
other stars.

Combining the two previous equations gives:

minst (star) = mdiff (star) -2.5 log (Int(ref)) - mstd (ref)

This equation is applied to magnitudes obtained with the B, V, R, and
I filters using Excel.

Lionel