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Ghost Images


[Nousek and Lesser1993]

As this effect is related to the softest X-ray photons detectable by the PSPC, very soft X-ray sources were used to quantify the effect.  HZ 43 was observed at various positions with no wobble, over ROR sequences 110014-110066, in the same way as the PSF observations were carried out (above). The primary use of these sequences is to illustrate the severe ghost imaging effect.

The electronics processing the signals from the PSPC cathode wires have a lower level discriminator setting    which allows distinction between electronic noise and soft X-ray events. For soft X-rays the number of photoelectrons and ions created is small. This means that there is a substantial probability that the sum of the cathode induced signal plus electronic noise may never produce a trigger large enough to pass the lower level discriminator. If all cathode signals in one direction except one are lost, the processing unit assumes that the event occurred directly over the remaining cathode wire (in that direction). If the other direction has a good (more than one contributing signal) position, this results in the formation of a line of events not at the true X-ray location. If both directions only have one cathode contributing to the position determination, the distribution of events will be at four places corresponding to the projection of the source position to the nearest crossing of the cathode wires.

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