First, let it be said that "overscanning" or equivalently, use of the "extended range of the LWS grating" is a routine mode of instrument operation, but the determination of when it is used is completely out of the hands of the observer: overscanning is completely controlled by the software, and is used to optimize the performance of the instrument.

The nominal angle of the grating is in the range +-3.5 degrees. But, in order to provide some redundancy in the case one of the detectors should fail, the grating was designed to be steered over an extended range of +-7.0 degrees, at the expense of a small increase in the power dissipated into the focal plane. With the larger angles, a detector's range can be covered in part by one of its neighbors. This redundancy is in fact currently utilized. Detector LW4 proved to have excess noise and its wavelength range is currently covered by its neighbors.

The use of overscanning will be reviewed during the performance verification phase to make sure that all observations will be conducted at the grating angle producing optimal sensitivity. Since a particular wavelength may potentially be observed at one of two different grating angles, it is impossible to predetermine a unique angle for a line measurement and likewise impossible to predict what wavelength illumination will fall simultaneously on the other 9 detectors during a line or range grating AOT. Also, despite the fact that the long wavelength detector LW4 is not currently selected to cover any wavelengths specified in the the AOTs, its output will still be recorded and is expected to provide useful ancillary data (along with the 8 other "non-selected" detectors) for all LWS01 and LWS02 measurements.

See also, the FAQ relating to "9 Detectors Free."