17.4 Cosmic Rays

Analysis of SMOV data has shown that the rate of cosmic rays on the NICMOS detectors is about 1.2-1.6 events/Camera/sec (5 detections, see Instrument Science Report NIMOS-022). The mean size of the cosmic ray (CR) hits is 1.65 to 2 pixels. Table 17.2 below summarizes cosmic ray statistics for the three cameras, for 5 and 3 detections. The numbers for Camera 2 come from two different programs and indicate variations of about 60% in the rate of CR-affected pixels between the two observations; the variations reflect changes in the orbital position of the telescope relative to the South Atlantic Anomaly. This level of variation is what should be expected during a typical multi-orbit program.

CR Event Statistics

Camera	Threshold	Affected Pixels (#/Camera/sec)
NIC1	5	2.11
NIC1	3	2.74
NIC2	5	1.99-3.21
NIC2	3	2.43-3.96
NIC3	5	1.90
NIC3	3	2.79

For a mean CR-affected pixel rate of about 2.5 pixels/sec, about 8% of the pixels in the detector will show cosmic ray events in a 2000 second exposure.

CR hits should be detected by calnica in MULTIACCUM data, and will be flagged in the DQ extensions of the *_ima.fits files.