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

TL;DR

This paper details the readout modes, noise modeling, and calibration strategies for JWST NIRSpec detectors, demonstrating that observed non-ideal behaviors are manageable and do not compromise instrument sensitivity.

Contribution

It introduces a validated noise model for NIRSpec detectors applicable to similar instruments and analyzes non-ideal behaviors, showing they can be effectively calibrated out.

Findings

Noise scales with multiple reads and is validated with real data

Reset anomaly is nearly noiseless and easily calibratable

RTN affects few pixels and can be tracked with standard maps

Abstract

We describe how the James Webb Space Telescope (JWST) Near-Infrared Spectrograph's (NIRSpec's) detectors will be read out, and present a model of how noise scales with the number of multiple non-destructive 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 non-ideal 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.