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XMM-Newton Science Analysis System Page: 1
backscale
Richard Saxton (Vega /XMM SOC, rds@star.le.ac.uk)
17 Aug 2007
Abstract
A tool for calculating and writing the BACKSCAL keyword in EPIC spectra.
1 Status
version (from VERSION ): 1.3.3
2 Change history
1.4 RDS Support for MASK regions
1.3 RDS Addition of the ignoreoutoov parameter
0.3 RDS Clarication of detector pixels and algorithm added
0.2 RDS Extra parameter useodfatt added
0.1 RDS Original version
3 Instruments/Modes
Instrument Mode
EPIC SPECTROSCOPY
4 Use
pipeline processing yes
interactive analysis yes
5 Description
This task calculates the area of a source region used to make a spectral le. The area is written into the
header of the SPECTRUM table of the le in the keyword BACKSCAL.
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The nal value is:
area = geometric_area - ccd_gaps - bad_pixels
In normal use only pixels which lie within the CCD boundaries and not on bad pixels contribute to the
total area. The units of area are detector pixels which are square pixels of side 0.05 arcseconds.
If the parameter withbadpixcorr is set false then the pure geometric area will be calculated regardless
of where the source region lies. If withbadpixcorr is true then pixels lying o the edges of all the CCDs
will be subtracted from the total area. If withbadpixcorr is true and badpixlocation is set to a le
containing bad pixel extensions (typically the input event le) then bad pixels lying within the source
region area are also subtracted from the total area. By default, area outside the eld of view is not
included in the backscale calculation. This can be overridden by setting ignoreoutoffov false on the
command line.
5.1 Spatial masks
When a region has been constructed from a spatial mask, the task needs to have an image of the region
supplied in detector coordinates to be able to correctly calculate the enclosed area. This may be achieved
by the command:
backscale spectrumset=spectrum.ds detmaptype=dataset detmaparray=imagedetxy.ds
6 Parameters
This section documents the parameters recognized by this task (if any).
Parameter Mand Type Default Constraints
spectrumset yes string spectrum.ds
Name of the input le
badpixlocation no string notSpecied
Name of the le containing the bad pixels, initially this is the event le.
withbadpixcorr no boolean yes
Whether to use bad pixels and chip gaps in the calculation.
useodfatt no boolean no
Whether to use the ODF attitude le to construct position info.
ignoreoutoov no boolean yes
Whether area outside the eld of view should be included in the backscale calculation.
detmaptype no choice at dataset at
This is the detector map type. It should be left as the default ' at' in all cases except where the source
region contains a spatial mask. In this case it should be set to 'dataset' and an image of the extraction
region given in detmaparray.
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detmaparray no array detmaple.ds: none
Name of detector map dataset and array in the DAL compound notation. Only used if detmaptype is
set to dataset.
7 Errors
There are no errors specic to backscale. See the arfgen documentation for a list of its internal errors.
NB: A lot of warnings may be generated by arfgen when being run in this backscale calu-
lation mode. These may look worrying but are ALL irrelevant and should be ignored !
8 Input Files
 an EPIC spectrum le containing a datasubspace denition
 an optional second le containing the bad pixel extensions
9 Output Files
 The input spectrum is modied
10 Algorithm
Create a grid which encompasses the source region
if (source region half-width < 2 arcmins) {
grid_element_Width = 10
}
else {
grid_element_Width = 20
}
area=0
Loop over each grid element
{
if (element lies within a CCD) {
area = area + grid_element_Width * grid_element_Width
}
}
Loop over each bad pixel
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{
if (bad pixel lies within source region) {
area = area - bad_pixel_area_in_detector_pixels
}
}
11 Comments
To improve the execution time of the task the source region is divided into a grid which is used to check
whether pixels lie on a CCD. The width of each grid element is set to be 10 detector pixels ( 0:5
arcseconds) for small regions (region half-width less than 2 arcminutes) and 20 detector pixels ( 1
arcsecond) for larger regions. It has been chosen like this to give good coverage of CCD gaps where they
make a signicant contribution to the total area while keeping reasonable execution times for larger areas.
This results in an execution time of 1 minute for a circular source region of 1 arcminute radius and 5
minutes for a circle of radius 5 arcminutes.
12 Developer's notes
The code uses the spans technique, currently implemented in rgslib, to resolve the source region into a
set of pixels.
12.1 CAL usage
The metatask doesn't use the CAL directly but calls arfgen which uses the routine CAL onCcd to
determine whether a pixel lies on a CCD.
13 Future developments
There is scope for reorganising arfgen to make the BACKSCAL calculation a bit faster.
References
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