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ROSAT Guest Observer Facility

What are the "ghost images" I've heard about?


The "ghost images" occur because the electronic pulse for low-energy events is sometimes too small to exceed the lower-level discriminator threshold, so an incorrect position of the photon is assigned. Position information comes from the crossed cathode wires (energy information comes from the anode wires), with an interpolation done for the nearest wires. At low pulse height, only one voltage reading is available, hence the incorrect position. The PSF becomes non-uniform, and non-monotonic at the lowest pulse heights. Images containing electronic ghosts will have a boxy appearance, with the real source surrounded by a box of satellite sources, one satellite at each remaining position of a 3x3 square. The image size, including satellites, is about 5' on a side. If a circle of radius 2'.25 is centered on the source, about 50 percent of the flux lies outside it for channels 1 and 2 (34-channel matrix), about 15 percent lies outside for channels 3 and 4, and about 0 percent lies outside the circle beyond channel 5.

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