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XMM-Newton Science Analysis System Page: 1
imsample
August 16, 2005
Abstract
This task takes as inputs a ts image and a list (ts table) of positions in the
image pixel coordinates. The task samples the image at each of the positions and
writes the result to a specied column in the ts table.
1 Instruments/Modes
The task is not XMM-specic.
2 Description
Given an image covering some part of the sky, and a list of sky positions, it can be useful to know the
value of the image at each of the positions in the list. The image might be a sensitivity or an exposure
map for example: one might want to translate this into sensitivity or exposure at each of the positions.
This is what imsample does.
The datatype of the output column is 32-bit real. If any position falls outside the image boundary, a null
is written to the corresponding value in the list.
At present, the list of positions (which must be a binary table extension in a FITS le) must contain
columns which give the position in image pixel coordinates. This has the advantage that image WCS
keywords are not required, but it must be admitted that RA and dec values are a more universal currency.
In future I'll probably modify the task to allow this option (in which event the image would of course
have to contain WCS keywords).
3 Parameters
This section documents the parameters recognized by this task (if any).
Parameter Mand Type Default Constraints
imageset yes dataset dummy default
The dataset containing the image which is to be sampled at each of the input positions.
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XMM-Newton Science Analysis System Page: 2
srclisttab yes table dummy default
The dataset+binary table which contains the list of positions.
xcol no string X IMA
The name of the column in srclisttab which contains the x coordinate of the position (in image pixels).
ycol no string Y IMA
The name of the column in srclisttab which contains the y coordinate of the position (in image pixels).
outcol no string IMG
The name of the column in srclisttab in which to store the sampled image value.
4 Input Files
1. A FITS dataset containing a 2-dimensional image array in the primary extension. The
datatype of the image may be 8-bit integer, 16-bit integer, 32-bit integer, 32-bit real or
64-bit real.
2. A FITS dataset containing a list of image positions in the form of a binary table extension.
The position values must be in image pixel coordinates but may be real-valued: in this case
the pixel indices are generated by calculating the nearest integer values. The names of the
columns with, respectively, x and y coordinates must be those specied via the xcol and
ycol parameters.
5 Output Files
1. A column of datatype 32-bit real and name specied via the parameter outcol is created
(or overwritten, if already existing) in the input position table.
6 Algorithm
References
xmmsas 20050815 1803-6.5.0