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The technique of chopping involves rapid sampling of the sky by moving the secondary in phase with the read out of the detector. In what follows, the ON beam refers to the positive image, the OFF beam refers to the negative image. In addition to chopping the telescope nods so the ON beam at position A overlaps with the OFF beam of position B. The typical nodding sequence is ABBAABBA ... etc. See the ISAAC user manual for more details.

Christopher Lidman