We used MONICA WST CCD images taken of the globular cluster M53 in
spring 2005, reduced and stacked by C. Gössl
with
fitstools, as
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
zero-points
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):
Filter B R_c I_c |
|
a 0.133 0.136 0.199 |
+- 0.053 0.036 0.037 |
b 0.985 0.983 0.973 |
+- 0.010 0.008 0.004 |
c not sign. -0.033 0.0027 |
+- not sign. 0.016 0.0005 |