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Overhead for the Nod-and-Shuffle Mode

Currently, the overhead on the nod+shuffle mode is large: the overhead on each nod (defined as a exposure on target, offset to sky, an exposure on sky, and an offset back to the target position) is 30sec. That is, the total overhead added by each offset is 15 sec. As mentioned above, for the best sky subtraction, the sky must be sampled on time scales of 60 sec. Hence, a 30 min exposure on target, broken up into 60s integrations, takes 75 min, not including the 2 min readout time of the CCD. For shorter exposures and more nods, the overhead is proportionately increased.



Jim DeVeny 2002-05-20