The restore task must be used to remove the dual beam response whilst chopping in the scan direction. This must be run before rebinning. Since the chop direction rotates slowly on the sky (since it is dependent on the scan direction, which is in Nasmyth coordinates, and not the sky orientation) tasks that try to map the chopped data onto a sky plane (despike, calcsky, rebin) can not be used before the dual beam has been removed (despike is useless after restoration since spikes propagate through the entire scan and show up as sine waves after restore).
SURF -- SCUBA User Reduction Facility