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XMM­Newton CCF Release Note
XMM­CCF­REL­120
OM Photometry
Bing Chen
July 5, 2002
1 CCF components
Name of CCF VALDATE List of Blocks
changed
CAL VERSION XSCS flag
OM COLORTRANS 0007 2000­01­01T00:00:00 COLORMAG No
2 Changes
The zero­points and color­transformations for optical filters are set based on observations of several
hundreds standard stars. For the UV filters, the zero­points and color­transformations are still based
on the simulations because we do not have enough calibration data yet.
3 Scientific Impact of this update
Current zero­points are based on two spectrophotometric standards (LLB 227, GD153) and the
color­transformations are set by folding the Bruzual­Persson­Gunn­Stryker spectra with the in­
flight response curves of the OM. This new CCF is based on our ground­based calibration program.
Though the simulations and the observations agree well in general ( now at a level of 0.02 mag).
this new CCF will improve the accuracy of the OM photometry.
1

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Figure 1: The difference in standard magnitude between Stetson's measurement and OM measure­
ment as a function of Stetson standard magnitude.
4 Estimated Scientific Quality
The photometric accuracies are better than 3% for B, V filters, and 5% for U filter. For the UV
filters, the calibrations, which are based on simulation, are more uncertain and can have errors up
to 10%.
5 Test procedures
This new CCF has been tested using SAS public version 5.3.3 at xvsas01.vilspa.esa.es. I use a new
OM calibration observation in rev. 407 to test OM photometry. The ODF has been run through
SAS ``omichain'' task. Everything looks OK. Once ``omichain'' finished, instrument magnitudes from
each filters are written into source lists. These source lists are combined using ``omsrclistcomb'' to
get the whole­observation source list, which include the standard magnitudes and colours. Then I
compared these standard magnitudes derived from OM with existed standard photometry published
by Stetson (2002).

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Figure 2: Comparing the simulated UV color­transformation (dashed lines) with the data in the
SA95 field. The lower case letters denote the OM instrumental system, the upper case letters the
Johnson system

XMM­Newton CCF Release XMM­CCF­REL­120 Page: 4
6 Summary of the test results
In Figure 1, we plotted the difference in standard magnitude between Stetson's measurement and
OM measurement as a function of Stetson standard magnitude. From this figure, we found that
the OM standard magnitudes are in good agreement with the Stetson's measurements. Because
Stetson standards have very high photometric accuracy (¸ 0.005 mag), the standard deviation on
the residuals between the Stetson and OM measurements can be used to derive the OM photometric
accuracy. From Figure 1, we found that the OM photometric accuracies are 0.01 and 0.03 mag for
B and V filters.
In Figure 2, the simulated UV color transformations (the dashed lines) are compared with the
data in the SA95 field. We can see that the simulated UV color­transformation based on this new
CCF can represent the observations. A small offset between the observations and the simulations is
probably due to error on the zeropoint determination of UVW1 filter.
7 Expected updates
New CCF is based on all available observations. We don't see that a new update is needed soon
for optical filters unless OM response changes significantly. UV color­transformations need to be
updated when more calibration data have been taken.
8 Acknowledgements
Thanks to OM team members, especially Igor Antokhin for his contributions.