Документ взят из кэша поисковой машины. Адрес оригинального документа : http://xmm.vilspa.esa.es/sas/6.1.0/doc/backscale.ps.gz Дата изменения: Tue Nov 23 00:48:12 2004 Дата индексирования: Tue Oct 2 06:47:18 2012 Кодировка: Поисковые слова: ultraviolet

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backscale
November 22, 2004
Abstract
A tool for calculating and writing the BACKSCAL keyword in EPIC spectra.
1 Instruments/Modes
Instrument Mode
EPIC SPECTROSCOPY
2 Use
pipeline processing yes
interactive analysis yes
3 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.
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.
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4 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.
5 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 !
6 Input Files
 an EPIC spectrum le containing a datasubspace denition
 an optional second le containing the bad pixel extensions
7 Output Files
 The input spectrum is modied
8 Algorithm
Create a grid which encompasses the source region
if (source region half-width < 2 arcmins) {
grid_element_Width = 10
}
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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
{
if (bad pixel lies within source region) {
area = area - bad_pixel_area_in_detector_pixels
}
}
9 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.
References
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