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The software described here follows the methods outlined in the paper
by Snowden et al. (1994) (which is reasonably worthwhile to read before
using the procedures listed below) for the analysis of extended objects and the
diffuse background using ROSAT PSPC pointed observations. That paper
in turn was based in part on the PSPC particle background calibration
papers of Snowden et al. (1992) and Plucinsky et al. (1993) and the paper on
the scattered solar X-ray background by Snowden and Freyberg (1993). One
example of the use of this analysis software, applied to the observation of
shadowing by a high-latitude molecular cloud, can be found in Snowden,
McCammon, and Verter (1993). The results of the routines to mosaic multiple
pointings were demonstrated in Snowden and Petre (1994), where 
independent pointings at the LMC were combined into one field.
The primary purpose of these routines is to allow a more realistic and useful reduction of PSPC data when objects greater than a few arc minutes in extent are of interest. They are particularly vital when the object covers a significant fraction of the PSPC field of view or is at a large off-axis angle. They are required for studies of the diffuse X-ray background. As they stand, they provide the best available modelling and subtraction of various non-cosmic background components and corrections for exposure, vignetting, and variations of detector quantum efficiency.
The software for the HRI is more limited than that for the PSPC, but does significantly improve on the flat-fielding of observations.
The programs detailed here have grown out of several years of effort and have evolved on an ``as needed'' basis. Unfortunately, this means that they are not quite as well pipelined as they could be and are essentially stand-alone programs (they are being improved as time goes by). While considerable effort has been made to ensure that the code is bug free, it would be remiss to make any blanket guarantees. However, the code has been used for the reduction of a large number of observations with good results. Please report any apparent errors or discrepancies in the software.
Caveat Emptor