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Spatial Models

Spatial models are useful when using programs which are performing intensive access of the psf system. The psf system intercepts an application's psf request and selects that model psf in which the requested position lies. The effect is to access a psf in the model bin at most once, which has enormous savings in cpu time. Model grid positions which are never accessed do not cause psf's to be calculated, saving more time.

The psf model grid may be specified in two ways - a POLAR grid and a RECTangular grid. The syntax for each specification is shown in the subtopics below.

The selection of an appropriate grid inevitably requires some knowledge of the instrument being modelled. The spatially varying psfs is the system are,

PWFC: The WFC spatial response is effectively constant within a degree off-axis, degrading by a factor of 2 at 1.6 degrees and by 3 at the edge of the field (~ 2.5 degrees). The psf also varies azimuthally. A suitable model might be, POLAR(PWFC,30:60:70:80:100:120:140:160,8) giving better sampling where the psf deteriorates most rapidly.
XRT_PSPC: The model used is radially dependent only. The specification POLAR(XRT_PSPC,0.02) gives radial bins about an arcminute wide. The inaccuracies introduced by this approximation are much smaller than the errors due simply to uncertainties in the analytic form of the psf.

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