We used MONICA WST CCD images taken of the globular cluster M53 in
spring 2005, reduced and stacked by C. Gössl
well as published photometry of this cluster
by P. Stetson (private communication).
The stars used for this comparison are outlined in the below
shown pseudo-color WST CCD image of M53.
The instrumental magnitudes m(WST) were calculated with the
ZP(B) = 22.0, ZP(R) = 23.2, ZP(I) = 21.8,
following Koppenhoefer's Diploma thesis, where
m(WST) = ZP_instrument-2.5*log(cts)+2.5*log(exp_time[sec])
We then made a linear least square fit of the Stetson-magnitudes m(S)
against the m(WST) and the WST colors as:
B(S) = a + b * B(WST) + c * (B-R)(WST)
R(S) = a + b * R(WST) + c * (B-R)(WST)
I(S) = a + b * I(WST) + c * (B-I)(WST)
The additive constant reflects the fact that Stetson stars are extrapolations for AM=0, whereas our AM is of course not zero (airmass was not included in the ZP!).
The table given below shows the results indicating that the so-called color
terms of our CCD-Filter system is pretty small (for the
Cousins-Landolt standard system):