MONICA B R I Color terms (Cousins)


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.

WST M53

We selected isolated stars to study the color terms of our system. The filters used are the broad B Filter and the Cousins R and I filters of the CCD camera MONICA. Measurements of the stars on the stacked Wendelstein images (B = 7200s, R = 4530s, I = 5949s.) were done with SExtrator of Bertin & Anouts, 1996, A&AS, 117, 393, applying the automatic magnitudes of SExtractor.

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 showss 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



Stella Seitz, Ulrich Hopp, 31.5.2005