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Дата изменения: Thu Mar 18 23:09:39 2004
Дата индексирования: Sat Dec 22 10:14:07 2007
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XMM-Newton Science Analysis System


echip2sky (echip2sky-0.2.2) [xmmsas_20040318_1831-6.0.0]

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Description

In making a background map or exposure map in sky coordinates it is useful to be able to transform a CCD image of a particular quantity into these sky coordinates. This is the function of the present task. The task takes in a list of images in the CHIPCOORD coordinate system (which is described in the cal library documentation) and replots them to sky coordinates.

The template for the output sky coordinate image is supplied via the parameter templateset. This should contain WCS keywords to define the sky projection plane and the image pixel size; the image array itself supplies the numbers of pixels in the image X and Y directions. The values contained in this array are ignored: either an event image or an exposure map can for example be supplied as the template image. The data type of the template image array is therefore irrelevant. Also, the template image need not be from the same instrument as the chip images, or even an XMM image at all. Note however that the output image contains a copy of all keywords in the template image dataset and primary array headers. It is up to the user to ensure that these are appropriate to the output image.

In order to perform the correct coordinate transform, the task needs to know, for each input CCD image, which CCD of which instrument the image corresponds to. echip2sky determines the instrument from the INSTRUME keyword in each chip image dataset header. (Note that the chip images may come from more than one instrument.) The user has the choice of supplying a list of CCD numbers (in the standard XMM EPIC numbering scheme) via the parameters withccds and ccds. If the user sets withccds=`no' (the current default), the task looks for a CCDNR keyword in the header of each chip image dataset.

The algorithm used to rebin the CCD pixels onto a sky-coordinate pixel grid is slow but exact: the corners of each CCD pixel are first transformed to the sky frame; then the value of the pixel is divided up among the sky-image pixels according to the proportion of the resulting quadrilateral which overlaps a given sky-image pixel.

NOTE! At present the only WCS projection supported by echip2sky is RA/dec tangent.