The flatfield observation consists of a series of measurements taken at various pixel heater settings. One standard SCUBA-2 raw data file is stored for each measurement.
An optimum pixel heater setting is chosen at the time of observation. The procedure is to record measurements at heater settings around this optimum value, continually returning to the optimum, which is used as a reference to subtract pixel zero-point drifts.
SMURF -- the Sub-Millimetre User Reduction Facility