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.
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