EXAMPLES OF HESSI MAPPING OF POINT- AND EXTENDED SOURCES

The following images show the effects of including or excluding various HESSI collimators on mapping small and large sources. The motivation for this is to illustrate the tradeoffs required at high energies where only two grids will be thick enough to modulate photons.

We restrict ourselves to the 4 coarsest grids, which have resolutions (FWHM) of 36" to 186". The Point Spread Functions for each of these, alone and in combination, have been convolved with a model map. Examination of the resulting "Dirty Maps" shows what is lost or gained by using any given collimator. (Image reconstruction methods will improve such maps, but in general, if a source is not seen in the dirty map, it will be poorly mapped in the reconstruction.)

• The first image shows a model with two sources of equal flux, one with FWHM=12", and the other with FWHM=60". Source Model With Point and Extended Source

• The Point Spread Functions are shown here for Collimators 6, 7, 8, and 9. These are the sums of fully-sampled modulation patterns for a 2-second interval: PSFs for 6, 7, 8 and 9.

• Each of the above PSFs was convolved with the source model to produce a "Dirty Map". This illustrates the effects of over- and under-resolution. Collimators 6 and 7 over resolve the 60"-wide extended source, and collimator 8 distinguishes both the sources, but collimator 9 confuses the two sources due to its great FWHM: Single-Collimator Dirty Maps

• When all 4 collimators are used together with equal weights (no "taper"), a sharply-peaked response with low sidelobes results. Four-Collimator PSF

• The 4-collimators together, and combinations of two and three, produce quite different responses when convolved with the model. Compare the (6 9) response with the (6 7 8 9) response. The two-collimator dirty map shows a 15"-radius ring around the point source, and spurious sources caused by over resolution. The (6 7 9) response is very similar to (6 7 8 9), but the (6 8 9) response includes the same 15"-radius ring. Thus, for some spacings and sizes of sources, sidelobe suppression requires different combinations of collimators. 4, 2 and 3-collimator dirty maps

• Compare further the Dirty Maps using Collimator 9 alone, 8 and 9 together, 7, 8, 9 and all four: Dirty Maps Including Collimator 9 with others. For these particular sources, the (7 8 9) combination gives the "best" dirty map. This shows that tapering will be necessary to map flares which have widely differing spatial scales.

• The following maps show different perspectives of the PSFs for the (6 9), (6 8 9) and (6 7 9) combinations.