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# 6 Appendix: EXSAS aids for the ROSAT PSFs

The EXSAS (extended scientific analysis system), see Zimmermann (1998), assists in analyzing ROSAT data sets. It is distributed to several dozens of institutes. Some commands closely related to point spread functions topics shall be explained here.

The EXSAS command

 CALCULATE/PSF (37)

evaluates for the selected ROSAT instrument the density , the cumulative PSF , the FWHM , and the median radius r1/2. In another evaluation mode, the inverse of the cumulative distribution, i.e. the quantile radius as introduced in Sect. 3, is returned. The median can be calculated using the EXSAS command (37).

Other point spread function related EXSAS commands are (38)

The first command in (38) returns the PSF in form of an image. The second one calculates tables with PSF density or cumulative values. Those tables can be plotted with the help of the third command. The figures of this paper were made with the last two commands. The EXSAS command (39)

is a further PSF related command. Assume a given pair of point sources which are close together. Then choose two disk with two radii having the point sources as centres. In one operation mode, the command returns the fraction of photons falling into its surrounding disk and, additionally, the fraction of photons from each source falling into the intersection region of the two disk. The intersection region may be void.

The PSPC point spread function density is held in table form in (40)

where stand for the two digits of the off-axis angle in arcmin. Each of the tables (40) has 21 columns. Besides the column for r=1(1)3600 arcsec, labelled ANGLE, are a further 20 columns for the energies , 2.0 keV, labelled RCTS0D1 to RCTS2D0. The first table in (40) is the on-axis table, .

Analogously, tables for the HRI point spread function are held in (41)

The table format is the same as the one for the PSPC. The last group of tables in (41) tabulates a Gauss approximation to the full PSF model.

The internet access to the most important ROSAT documents and to the HRI calibration report is possible via the two addresses (42)

Presently, it cannot be said how long the post mortem public access (42) will be maintained.  Up: The ROSAT point spread

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