As with CCDs, cosmic ray hits will produce unwanted signal in the
output images. However, no lasting damage to the detector pixels is expected from such hits. The NICMOS arrays have been subjected to radiation doses much higher than expected in their entire lifetime in accelerator tests without sustaining any long-term damage or measurable degradation in DQE. Hence, cosmic rays should have little impact on the long-term array performance in orbit.
On-orbit measurement of the distribution of cosmic rays shows 1.2 to
1.6 events/second/Camera for 5σ
events. With a typical hit generating a 5σ
event in ~2 pixels, this corresponds to 2 to 3 pixels/second/Camera. For a 2000 second integration, about 10% of the pixels in the detector will show cosmic ray events.
The use of MULTIACCUM
mode makes it possible to filter out cosmic rays since it provides a series of intermediate non-destructive reads, as well as the final image (see Chapter 8). These intermediate reads can be used to identify cosmic ray hits. The calibration pipeline, as described in the NICMOS Data Handbook, can identify and remove cosmic ray hits from MULTIACCUM
See below for a more detailed discussion of persistence from massive
cosmic ray hits during, e.g., passages in the South Atlantic Anomaly.