The driving idea behind the development of EXSAS [Zimmermann et al.1992] was the desire to have an extensive system for the interactive analysis of ROSAT X-ray and XUV data providing a high degree of flexibility and allowing easy implementation of individual user software. Additionally, to facilitate its widespread usage among the astronomical community, there was the decision to build EXSAS into the framework of an already existing - and accepted - astronomical data analysissystem. Since ESO-MIDAS, the Munich Image Data Analysis System developed by the European Southern Observatory (ESO), [Crane et al.1985] is now generally considered to be the current system for data analysis in optical astronomy within Europe it was decided to adopt the MIDAS environment for EXSAS. General data handling activities (I/O, image display, plotting) as well as data manipulation not specific to X-ray astronomy (merging of images, smoothing, etc.) could then be treated by genuine MIDAS commands (see section 2.2). For tasks relevant to the analysis of X-ray data in general, dedicated software was developed and organized in four packages:
The character of the Data Preparation package differs from that of the remaining three in that it is not directly used to extract scientific information from the data as in the spatial, spectral and timing analysis packages. As the name indicates it has the purpose to prepare (filter, format, correct) data for further processing in EXSAS. These data come either from the original ROSAT Observation Data Set (ROD) or are the products of previous EXSAS sessions. In any case they conform to the standard MIDAS table and image concept. Data preparation in terms of EXSAS means selection or binning of photon event data and calculating the correction of resulting spectra or light curves for instrumental and observational effects. The resulting data products possess fixed formats (file types), which completely correspond to the requirements of MIDAS. Such products are then the input to the scientific packages of spatial, spectral, and timing analysis.
From a software point of view the development of EXSAS offers several advantages. As stated above, EXSAS relies on a small number of fixed standard file formats with in addition standard file headers. Default file types strongly facilitate the lucidity of files generated in the course of an EXSAS session. With EXSAS being embedded in MIDAS its software development is totally based on the MIDAS standards thus guaranteeing portability. EXSAS runs both on VMS and different UNIX machines. The source code of all packages has been written in standard FORTRAN (F77) allowing only the same few exceptions as MIDAS. Within an EXSAS session different types of command interfaces are applicable. At the lowest level individual commands call the various functions of the EXSAS packages interactively. In the scientific analysis these commands partly have access to parameter files - ASCII files for easy editing by the user - containing a set of parameters necessary to control the specific tasks. Several commands can be combined to form a command file so that more extensive EXSAS procedures are able to run in a pre-defined quasi-automatic way.