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How is OBJTIME (integration time on source) computed and why doesn it agree with my intended value?

April 26, 2017agree computed Integration source time value

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How is OBJTIME (integration time on source) computed and why doesn it agree with my intended value?

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The OBJTIME keyword value in LWS FITS headers is the correct value for on-source integration time. OBJTIME can be a source of confusion since you don’t always get what you ask for. The reason it doesn’t agree with the intended value (the value entered in XPOSE-LWS ) is that LWS software computes a “ceiling” value based on other parameters. The determination of OBJTIME in chop-nod mode is based on the number of telescope nod cycles. Since the minimum number of nods is 1, the minimum OBJECT time is based on a 30 second (*2) nod dwell. When you enter an OBJTIME in the exposure tool, the system calculates the nearest ceiling value to make sure you get at least that much time on your object. The default 30 second nod dwell time is based on telescope performance and is tailored for longer integrations.