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XMM-Newton Science Analysis System Page: 1
omatt
January 10, 2003
Abstract
Convert a source list from OM detector to sky coordinates, and produce a sky
coordinate OM OSW image
1 Instruments/Modes
Instrument Mode
OM FAST
OM IMAGING
2 Use
pipeline processing yes
interactive analysis yes
3 Description
This task takes an OM OSW source list, with source positions in pixels, and converts to sky coordinates.
These sky coordinates are then used to produce a sky coordinate image.
The task requires the user to enter the name of an OSW source list and its corresponding OSW image.
The name of the output sky coordinate image must also be passed to the task, along with a tolerance
parameter (see below).
The task rst reads the binning factors (BINAX1 and BINAX2) from the FITS header, along with the
BPE binning ag (TWIXEL) and the x and y size of the OSW window (in centrioded 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
CAL to GetPlateScale 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).
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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, which is then rotated by the roll angle about the
s/c boresight direction, as are the tangent plane coordinates. The tangent plane coordinates are then
converted to sky coordinates.
The source positions, in RA and DEC, are then written to the source list, along with the positional error
(the pixel coordinates are retained in the source list).
The second (optional) part of the task employs the USNO SA1 catalogue (if available) to correct the RAs
and DECs of the sources for any dierence between the nominal pointing direction of the satelliet and
the true one. The OM pointing vector and eld of view are used as parameters to a search of the USNO,
which lists all catalog stars in the eld of view. The algorithm sets up a grid of osets in the x and y
directions to be used to match up stars in the source list to those in the catalogue. For each set of osets
a maximum likelihood parameter is computed, and the osets with the maximum parameter value are
taken to be the best osets. If suфcient stars are matched then a least-squares t is performed, to yield
more accurate astrometry. The t yields more accurate values for the osets, and also any small rotation
bewteen the two coordinate systems. Error for the parameters are computed by Monte Carlo simulations,
in which new data sets are generated and the parameters computed. The mean and standard deviations
of all the tted parameters from the simulations are computed, and the standard deviations are used as
the errors. Corrected RA and DEC columns are added to the source-list le.
The nal section of the code takes the OM OSW image and rotates and rebins it into sky coordinates.
The rotated-pixel coordinates are computed through an iterative process.
4 Parameters
This section documents the parameters recognized by this task (if any).
Parameter Mand Type Default Constraints
set yes string none
OM image intermediate lename
sourcelistset yes string none
OM OSW Source List lename
ppsoswset no string none
PPS OM OSW FITS sky image lename
usecat no boolean F
Use the USNO-SA 1 catalog
rotateimage no boolean T
Create the rotated sky-image
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5 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)
Dierent binning in x and y directions
Could not allocate memory for: (fatal)
Memory allocation for the named array failed
Star catalogue not found- no astrometry correction done (warning)
Too few sources for astrometry
corrective action:
Need at least 10 stars to do a reliable correction (warning)
noCat
corrective action:
No stars in catalogue- catalogue has not been used to correct positions (warning)
Cannot do astrometry
corrective action:
No overlap between sources and catalogue stars (warning)
Too few guide stars for astrometry correction
corrective action:
Catalogue has not been used to correct positions (warning)
Too few sources matched for astrometry
corrective action:
Need at least 10 to do astrometry correction (warning)
insuData
corrective action:
Insuфcient number of data for least-squares t (warning)
singularSol
corrective action:
singular solution to astrometric t (warning)
corrective action:
6 Input Files
1. OM OSW source list (as produced by omdetect/omwavelet)
2. OM OSW FITS image (as produced by ommodmap)
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7 Output Files
1. OM OSW source list
2. PPS OM OSW FITS sky image.
3. if usecat=true a postscript le will be produced showing the goodness-of-t of the astrometry
correction.
8 Algorithm
subroutine omatt
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
rotate by roll angle
loop over individual source information
read in individual source information
extract pixel position
correct for distortion (CAL_omDistortion)
call to CAL_omPlatescale to get linear plate-scale
perform co-ordinate conversion
(including s/c boresight offset correction and roll angle)
end loop
Add columns for the RA and dec of each source to the source-list file.
if (usecat) then
read in the catalogue data.
Set up a grid of offsets deltax and deltay in the x and y directions.
Sort these offsets into increasing radius order from the origin.
For each grid shift the stars by deltax and deltay in the x
and y directions.
Try to match each star with one in the catalogue by computing the
distance between it and the selected catalogue stars and regarding
it as matched if the distance is less than the specified tolerance.
For each pair of offsets compute a maximum-likelihood parameter and take
the pair with the maximum value as the best offsets.
Use these offsets to repeat the star matching, and then perform a
least-squres fit to improve the offsets and also determine any small
rotation between the two coordinate systems.
Determine errors on the computed parameters by a Monte Carlo procedure
which generates artificial data sets from the original, computes the
parameters for each simulation, and then at the end computes the mean and
standard deviations of all the parameters.
Correct the RAs and Dec of the sources using the computed paramters.
Added corrected RA and Dec columns to the source-list file, and also
information about the fit.
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end if
Rebin, rotate and correct for distortion the image, and write out to a fits file.
close OSWSL
close output file
end subroutine omatt
9 Comments
 none
10 Future developments
References
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