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Spending all time on source and avoiding the relatively long blanking times due to moving telescope components, frequency switching is the most efficient spectroscopic observing mode. A FSw scan consisting of 2 subscans of 150 sec each lasts about 320 seconds (>90% efficiency) if the 100 kHz filter and the autocorrelator spectrometers are connected. This efficiency is more than a factor of 2 higher than that available when the main telescope or the subreflector has to be moved. Most of the overhead with FSw is due to the settling time of the ref. 2 synthesizer (blanked at 10 msec per phase) and to data transfer between backends and antenna computer.
In view of this enormous gain in observing efficiency we propose that the 30m users take frequency switching into serious consideration for their observations.
Wolfgang WILD and Clemens THUM