Detectors for the James Webb Space Telescope Near‐Infrared Spectrograph. I. Readout Mode, Noise Model, and Calibration Considerations

We describe how the James Webb Space Telescope (JWST) Near‐Infrared Spectrograph’s (NIRSpec) detectors will be read out, and present a model of how noise scales with the number of multiple nondestructive reads sampling up the ramp. We believe that this noise model, which is validated using real and simulated test data, is applicable to most astronomical near‐infrared instruments. We describe some nonideal behaviors that have been observed in engineering‐grade NIRSpec detectors, and demonstrate that they are unlikely to affect NIRSpec sensitivity, operations, or calibration. These include a HAWAII‐2RG reset anomaly and random telegraph noise (RTN). Using real test data, we show that the reset anomaly is (1) very nearly noiseless and (2) can be easily calibrated out. Likewise, we show that large‐amplitude RTN affects only a small and fixed population of pixels. It can therefore be tracked using standard pixel operability maps.