Due to the NICMOS vulnerability to Single Event Upsets (SEUs) in the onboard opto-isolators, the NICMOS detectors are switched off when the spacecraft passes through the South Atlantic Anomaly (SAA). During this time the autoflush sequence is turned off, as are the voltages to the arrays. The cosmic ray flux is extremely high in the SAA.
Following deep SAA passages, residual images of CR hits incurred while in the SAA are still visible in long exposures following the passage. The spatially correlated nature of the decaying signal means calnica cannot find and reject them. The random distributuion of these "persistent cosmic rays" increases the noise in the image and limits faint source detections. The timescale of the decaying signal is exponential and is detectable for ~30 minutes following a passage. This is the same timescale as that of persistent images cause by extremely bright sources.
It looks like inserting multiple-readouts at the exit of the SAA may reduce this effect. Observers should make note of this so that their observations can be scheduled to avoid it, if they are sensitive to it. Note that this is a very low level effect, and is only a problem when you are looking for small, faint targets on a dark background.
Since the start of in Cycle 11, a pair of ACCUM mode NICMOS dark exposures are automatically scheduled after each SAA passage. The dark exposures will provide a map of the persistent cosmic ray (CR) afterglow at a time when it is strongest, and has just begun to decay. It is possible to scale and subtract "post-SAA" darks from subsequent science exposures taken later in the same orbit, and thus to remove a significant fraction of the CR persistence signal. All NICMOS users should view the general general post-SAA dark advisory and information. For more details, refer to NICMOS ISR 2003-010: NICMOS ISR 2003-010: Removal of Cosmic Ray Persistence from Science Data using Post-SAA Darks, E. Bergeron and M. Dickinson.