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XMM­Newton CCF Release Note
XMM­CCF­REL­20
OM Detector Response
R. Much
October 3, 2000
1 CCF components
Name of CCF VALDATE List of Blocks
changed
CAL VERSION XSCS flag
OM LARGESCALESENS 0002 2000­01­01T00:00:00 FILTER­U No
FILTER­B No
FILTER­V No
FILTER­UVW1 No
FILTER­UVM2 No
FILTER­UVW2 No
FILTER­WHITE No
FILTER­MAGNI No
FILTER­GRISM1 No
FILTER­GRISM2 No
OM DARKFRAME 0003 2000­01­01T00:00:00 DARKFRAME Yes
OM PIXTOPIXSENS 0003 2000­01­01T00:00:00 PIXTOPIXSENS No
LEDTEMPLATTE No
2 Changes
First release
The file om largescalesens describes the largescale flatfield response of the combination detector
plus filter. It was not possible to generate a sky flatfield: while the local straylight features can
be handled by omitting the relevant areas in an OM image, the global straylight feature did show
up as a large disk in the final, summed sky flat. At this stage it was decided to set the largescale
flatfield to unit, with the underlaying assumption that the initial detector response is flat. This
means that for the moment any flatfield correction will only be done relative to the early inorbit
flatfield measurements.
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The file om darkframe describes the dark count rate of the detector. The analysis is based 11
dark ENG4 dataset summing up to a total exposure time of 44 ksec excluding darks during Zeta
Pupis and Capella observations. The dark count rate is rather stable with time. The mean OM
dark count rate is 2.56e­4 c/s/subpixel. However there is a variation in dark count rate across the
detector of \Sigma 9% showing mainly up as radial dependence, being highest in an annulus of about 8'
radius and lowest in the center. Despite using a blocked filter for the OM, when the spacecraft is
pointing at very bright star, the dark rate is noticably increased (e.g., up to 65% higher for the V=0
star Capella), suggesting some straylight leakage.
The file om pixtopixsens describes the pixel to pixel response variation and the LED illumination
template. The latter is derived from 8 ksec smoothed LED 8 ENG4 flats. The pixel to pixel response
variation had to be set to unity, as the largescale flatfield was absent and because the statistics in
individual pixels was not sufficient.
All three files describe the redundant OM detector channel.
An update of the LED illumination template in the file om pixtopixsens is expected once more
ENG4 and tim16 data with LED on are analysed.
We expect an update of the file om darkframe either when a change in the dark count rate is noticed
or once significantly more ENG4 darks or dark tim16 datasets become available. The latter shall
mainly affect the uncertainties.
Updates on the file om largescalesens can be expected after careful analysis of revisited fields with
pointing offset.
3 Scientific Impact of this Update
First release
4 Estimated Scientific Quality
The dark count rates are accurate to the 10 % level.
Because of the poor statistics in the aquired data the LED illumination template is only accurate
within a factor 2.
Pixel to pixel variation and large scale flatfield response are best guess estimates.
5 Acknowledgements
Thanks to OM team members, especially Bob Shirey (UCSB) for providing the input.