Calibration Information
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To perform image analysis requires the following calibration items to be
known:
- the vignetting function - the radial dependence of the telescope effective
area;
- the point spread function (PSF) - the blurring of the image caused by its
intrinsic angular resolution;
- the exposure map - the number of seconds each pixel in the image was
exposed; and
- a background map - the particle background counting rate in each pixel.
The first two are critical for XIMAGE to produce any useful results. The
vignetting can be provided as an image map, or as a function. In the latter
case the XIMAGE code must be relinked. The PSF can be derived from within
XIMAGE, and read in as a file. The exposure map is optional, and if it is not
available then it is assumed the image exposure is uniform. This is usually
acceptable for detectors without any obstructing structures (e.g. the EXOSAT
CMA), but will lead to errors if there are window supports (e.g. the ROSAT
PSPC). The background map is also optional, and is only necessary in the case
where there is significant variation in the particle background across the
detector.
The vignetting function and the PSF can be energy dependent in many cases
(e.g. the ROSAT PSPC), and care must be taken to ensure that they match the
energy band of the image.
Lorella Angelini
Thu 12 Oct 16:28:11 1995