Image Analysis

Images may be sorted over any energy range with xrtsort. Occasionally, observations are incorrectly processed by the SASS pre-processing software and aspect errors are introduced. This is often the result of a poor attitude solution from the star-trackers onboard ROSAT and results in photons appearing in a different part of the image from where they were recorded. A sign that this may have happened is if sources appear to be elongated along the wobble direction. If it is suspected that an observation contains aspect problems or if very accurate work, e.g. a search for source extension, is being undertaken, the problem can be reduced by subsetting the data on the aspect error (ASP_ERR) housekeeping parameter, contained in the attitude file. xrthk should be used to produce a file of times when the aspect error parameter was between 0 and 1. An image should then be produced with xrtsort between these times. At the same time, the Master Veto Rate parameter may be used to reduce the background in the image.

The image may be background subtracted using the methods outlined in the "Background subtraction" section.

xrtcorr may be used to exposure correct an image. This includes a vignetting correction, but it should be remembered that this correction when applied to an image is only approximate because vignetting is calibrated in terms of energy and not pulse height channel.

The image may be manipulated in a wide variety of ways using Isys and the command driven image processing, within ASTERIX. See "asthelp image" for details.