Here is a simple simulation of the effectiveness of HESSI in imaging the Crab Nebula. The energy band is 15 - 100 keV, and the accumulation time is 12 hours. In this energy range, we expect to see 9.4 photons/sec per detector, or a total of 2.9e6 for 9 detectors over a 12 hour period. The angle from the rotation axis is 1/2 degrees, this cuts down the actual number of photons detected, due to shadowing of the grids. We expect a total of 1.4e6 background counts in the same time range. Since the average number of counts that pass through the collimators is 0.25, the expected signal-to-noise ratio is approximately 0.5. This image only includes data from collimators 4, 5, 6, 7, 8, and 9. Since the source is far from the rotation axis, the modulation happens very fast in time. As can be seen in the plot below, which plots the modulation for each detector, for one spin period. The blue line is the modulation as it would be without Poisson fluctuations. The red line includes the fluctuations.
This is the model image,
And this is the image that the Max. Entropy Method returns using the above data. Not too bad.
15-Apr-1998, J.McTiernan