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Box 5 Explanation
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Box 5 shows how the image can be reconstructed from the probability distributions shown in Box 1 and the counting rate in the detector shown in Boxes 3 and 4. The reconstruction process is simply to add together all the probability distributions shown sequentially in Box 1 but weighted according to the detector counting rates shown in Boxes 3 and 4. As you will see, the image in Box 5 is initially the same as the image in Box 1 but then more images from Box 1 are added on top of the original image. Finally, at the end of the movie, the reconstructed image is shown and this is duplicated in Box 6. The point source can be clearly seen in the bottom left corner, exactly where we had assumed it to be for the purposes of this demonstration. A second "ghost" source appears in the upper right corner and faint rings, referred to as "side-lobes" appear around both "source" locations. These artifacts can be removed by more sophisticated techniques than can be shown in this demonstration. Using techniques named Clean, Maximum Entropy, Pixons, and Fourier Transform analysis, scientist have been able to reconstruct images that compare in resolution with optical images.
The main limitations of this type of imaging is to be able to see sources that are very extended rather than being point-like and to "see" a weak source in the presence of much stronger sources. Extended sources can be imaged using, surprisingly, pairs of coarser grids in combination with the finer grids that give the finest angular resolution. By using multiple detectors behind grid pairs with a range of slit widths and spacings, images can be reconstructed showing the location and extent of the different sources in the field of view. There is always some maximum source extent that cannot be imaged, however.
The ability to image weak sources in the presence of stronger sources is difficult even at optical wavelengths because of the problem of scattered light. In our case, the difficulty arises because of the need to remove the ghost sources and side-lobes from the strong source before the weaker sources can be seen. This problem has generally limited the useful dynamic range of this technique to less than about ten. HESSI, with its multiple detectors and precisely aligned grids, should achieve a factor of up to ten better.
