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XMM-Newton Science Analysis System Page: 1
omgprep
Vladimir Yershov (MSSL, vny@msll.ucl.ac.uk)
September 26, 2008
Abstract
The task omgprep prepares the OM grism image for detection and extraction of
spectra (introduces distortion corrections, properly rotates the image, etc.)
1 Status
version (from VERSION ): 1.6.1
2 Change history
 Version 1.8 (2008-09-26) A bug causing a subscript message is xed.
 Version 1.7 (2008-09-17) Introducing two new parameters in order to include the capability
of removing scattered-light features from the rotated grism image.
 Version 1.5 (2006-11-09) A lot of small changes are introduced to get more accurate RA and
DEC coordinates of the grism image sources (SSC-SPR-3633)
 Version 1.3 (2006-07-11, VNY) The values given in the MODES table of the output le are
corrected (to correspond to the values given by DETSEC keyword in the header of the main
table (SSC-SPR-3622)
 Version 1.2 (30.06.2006) A new keyword (OMGWIN) was introduced into the header of the
output le instead of the return ag to indicate the extractability of the spectrum from the
current window (SSC-SPR-3616). If the keyword is set to zero then the window size is too
small and the spectrum cannot be extracxted. The value 1 indicate that the window is large
enough for spectra extraction. Another small correction was introduced in the header of
the output le: the reference pixel indices are set to the specic reference pixel of the grism
lter (the coordinates of the center of the image are not valid as the reference for the grism
lters).
 Version 1.1.2 (09.06.2006) A small bug is xed (SSC-SPR-3607)
 Version 1.1.1 (09.06.2006) A ag indicating the extractability or non-extractability of the
spectra from the image is introduced. In the case of a very small window, when less then a
half of the spectrum is present, the task will return a non-zero ag indicatimg that further
processing of the current exposure is useless (SSC-SPR-3606)
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 Version 1.1 (28-09-2005) The keywords such as OBSERVER, OBJECT, etc. are propagated
from the input to output (SSC-SPR-3525)
 Version 1.0.4 - (01-06-2005) The faint moire pattern that appeared due to the undistortion
algorithm is removed
 Version 1.0.3 - The WSC keywords are removed from the output le, because they were
causing the appearence of a very odd coordinate grid when working with ds9 and fv tools
(SSC-SPR-3363).
 Version 1.0.2 - the number of extra pixels added at the edges of the window is divided by
the binning factor (for the consistency reasons); SSC-SPR-3346
 Version 1.0.1 - the output image is slightly enlarged in order not to miss the pixels that go
outside of the image due to distortion correction (SSC-SPR-3318)
 Version 1.0 - passed the version number to 1.0
 Version 0.1 - (10-24-2003) Created a new SAS task to be included into the grism chain (for
processing OM images with grism lters)
3 Instruments/Modes
Instrument Mode
OM IMAGING
4 Use
pipeline processing yes
interactive analysis yes
5 Description
This tasks clones the OM Imaging Mode Data Image File with the Grism-1 or Grism-2 lter for subsequent
pipeline processing. The image le is prepared for extraction of spectra produced by grisms: rst the
distortion correction is applied, then the image is rotated by the angle corresponding to the grism lter
in order to align spectra with respect to the pixel columns (for optimisation of spectra extraction).
Values for additional FITS header keywords necessary for the creation of SSC Data products by sub-
sequent pipeline tasks are taken directly from the OM Priority Window Data Auxiliary File, the OM
Periodic and Non-Periodic Housekeeping Files and from the OM Observation Summary File through the
ODF Access Layer(OAL) and stored in the Primary Header of the output les.
The task rst reads the binning factors (BINAX1 and BINAX2) from the FITS header, along with the
BPE binning ag (BINBPE) and the x and y size of the OSW window (in centroided pixels) along with
the x and y osets of the window. The pixel coordinates of the OSW source list are then converted to
the PIXCOORD reference frame (2048 by 2048 centroided pixels).
The OM optics have a small amount of distortion, and the CAL call omDistortion is used to get the
osets of the pixel position from a linear scale. These osets are then applied to the pixel positions. A
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call to GetPlateScale (CAL) is subsequently used to convert from pixels to radians. The center of the
detector, i.e. the position of the OM boresight is pixel 1023.5, 1023.5.
The shift and add process is all performed relative to a reference frame (used to nd the tracking stars), so
a call to OAL getAttitude is used, with the time of the reference exposure, to get the spacecraft attitude.
The CAL Boresight routine is used to obtain the boresight oset matrix, which is applied to the s/c
attitude to get the OM boresight direction. The nal section of the code takes the OM OSW image
and rotates it in order to align the spectra with respect to the pixel columns. If needed, the scattered
light features can be removed from the background by using the parameter removescatteredlight=yes/no.
Please, note that using this parameter essentially increases the working time of the task (up to 10 minutes
for large images).
6 Parameters
This section documents the parameters recognized by this task (if any).
Parameter Mand Type Default Constraints
set yes string none
Input OM image lename (output from ommodmap)
ppsoswset yes string none
PPS OM OSW FITS output image lename
undistset no string none
output undistorted image lename
removescatteredlight no boolean none
switches on the possibility of removing scattered light features from the output (rotated) image
backgroundset no string none
output background image lename
7 Errors
This section documents warnings and errors generated by this task (if any). Note that warnings and
errors can also be generated in the SAS infrastructure libraries, in which case they would not be docu-
mented here. Refer to the index of all errors and warnings available in the HTML version of the SAS
documentation.
diBin (fatal)
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Dierent binning in x and y directions
RA of OM boresight (warning)
corrective action: Information about the Right Ascention of the OM boresight
DECL of OM boresight (warning)
corrective action: Information about the Declination of the OM boresight
DETSEC keyword missing (warning)
corrective action: Issue a warning about the missing keyword in the input le. DETSEC
keyword for output le is not inserted
8 Input Files
1. OM OSW FITS image (as produced by ommodmap)
9 Output Files
1. PPS OM OSW FITS image with spectra aligned with the pixel columns.
2. optional: FITS image with distortion correction only.
3. optional: FITS image with the smoothed background (without sources).
10 Algorithm
subroutine omgprep
open output file
read the OSW window parameters, binning factors from header
call CAL_GetBoresightMat
call OAL_GetAttitude (with time of reference frame)
find OM boresight direction
undistort image
rotate by the angle corresponding to the grism filter
write out to a fits file.
close OSWSL
close output file
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end subroutine omgprep
11 Comments
 none
12 Future developments
Currently the standard CAL procedure for distortion correction is implemented. On the availability of
new calibration data related to the additional distortion introduced by the grism lters the distortion
correction will be improved.
13 Developer's notes
13.1 CAL usage
 CAL GetBoresightMat
 CAL omDistortion
 CAL omGetPlatescale
References
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