RXTE's HEXTE Instrument

HEXTE has eight sodium-iodide crystal detectors that are gathered into two clusters, each containing 4 scintillation detectors that measure X-rays in the 10-200 keV range. While one cluster is pointed directly at the source, the other cluster is slightly offset, looking at blank sky. Each cluster is on a mechanism that "rocks" (by rotating on its axis) once every 32 seconds. So a given cluster will be pointing at the source for 32 seconds, then shifts to an off-source position and stays there for 32 seconds, then shifts back to the on-source position. (The other cluster is doing the opposite: off-source, on-source, off-source.) The data from one particular cluster are analyzed by subtracting the off-source position data from the on-source position data. The background subtraction helps reduce the "noise".

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