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RXTE's All-Sky Monitor Instrument
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The ASM has three cameras that are mounted on a boom off the end of the spacecraft. Inside each camera is a proportional camera sensitive to energies in the 2-10 keV range. Each camera is able to look at a different region of the sky; two of the cameras have an overlapping field of view. The entire boom that the cameras are mounted on rotates by 6 degrees once every 90 seconds. Through this sequence of rotating and then staying fixed for 90 seconds, etc., the ASM covers 80% of the sky in 90 minutes.