NIRCam Detector Readout
NIRCam detectors use MULTIACCUM readout mode
NIRCam detectors operate in MULTIACCUM mode; this means that the charge in each pixel may be read out multiple times as it accumulates during each integration. The readout process is non-destructive, leaving charge unaffected and in place—charge is not transferred between pixels as in CCDs. After each integration, the pixels are read out a final time and then reset, releasing their charge.
NIRCam repeatedly cycles through all pixels in the full frame or a selected subarray, either reading their charge or resetting them as commanded. Reading and resetting take the same amount of time: 10 µsec per pixel plus small overheads at the end of each row and frame.
Each detector readout delivers a frame of data. By selecting one of the NIRCam readout patterns, users may either save all individual frames or averages of groups of frames calculated onboard to reduce data volumes and enable longer integrations. Fitting "up the ramp" to multiple frames in an integration decreases the effective read noise, increases the dynamic range, and allows cosmic rays to be identified and rejected.
Readout of the full NIRCam detector (2048 × 2048 pixels) is performed with four outputs simultaneously (Noutputs = 4), each delivering a stripe of data (2048 pixel rows × 512 pixel columns), and taking 10.73677 s altogether. Smaller subarrays are read out more quickly, even though most are read out through a single output (Noutputs = 1). Noutputs is pre-defined for most subarrays, but users are given a choice between Noutputs = 1 or 4 in the grism time-series observing mode.