NIRSpec Detectors

The JWST NIRSpec HAWAII-2RG detectors support multiple readout modes, readout patterns, and subarray options, and have been optimized to balance key performance metrics.

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The JWST near-infrared instruments all use nearly identical HgCdTe Astronomical Wide Area Infrared Image 2RG (HAWAII-2RG or H2RG) detectors manufactured by Teledyne Imaging Systems. The two detectors in NIRSpec were provided by NASA, and tested at the Detector Characterization Laboratory at the Goddard Space Flight Center. Information provided here is specific to the detectors in NIRSpec, with some general detector characteristics provided for context.

The NIRSpec detector information outlined in these articles includes:

General properties of the NIRSpec detectors

See also: NIRSpec FS Wavelength Ranges and GapsNIRSpec IFU Wavelength Ranges and GapsNIRSpec MOS Wavelength Ranges and GapsNIRSpec Dithers and Nods

The NIRSpec focal plane uses 2 individual detectors, also called sensor chip assemblies (SCAs). The 2 SCAs (designated as NRS1 and NRS2) are hybrid arrays with HgCdTe used for light detection, and a silicon integrated circuit for the readout. Both SCAs are 2048 × 2048 pixels in size with an 18 μm pixel pitch and a long wavelength sensitivity cutoff of 5.3 μm. Each SCA is independently controlled by a dedicated SIDECAR ASIC (System for Image Digitization, Enhancement, Control, and Retrieval; Application-Specific Integrated Circuit). These ASICs are special-purpose electronic devices that control the detector readout, and are individually tuned and matched to their corresponding SCAs.

Each of the 4 million pixels in an SCA can be addressed individually and are read out in a non-destructive way. The outer 4 pixel columns and rows framing the SCA are insensitive to light, but can be read out like all other pixels. They are used to provide an electronic reference, in order to track variations in the output signal caused by bias drifts or small temperature changes in the electronics. The noise properties of the detectors can be improved during offline data processing by subtracting the signal of the reference pixels. In addition to the traditional readout mode (which is common with the other JWST NIR detectors), the strict detector noise performance requirements of NIRSpec require an additional readout mode called improved reference sampling and subtraction (IRS2), which samples reference pixels at a higher frequency to track and reduce electronic noise.

The 2 SCAs are placed side-by-side in the focal plane array (FPA), separated by a physical gap that is equivalent to approximately 18" on the sky. In spectroscopy mode, the detector gap will cause loss of spectral information over a range in wavelengths that depends on the observing mode, the location of the target, and the dispersive element used. The impact of the detector gap is discussed separately for spectra taken through the MSA, the IFU, and the FSs. Except for observations with the IFU in high-resolution (R = 2,700) configurations, the missing wavelengths can be recovered by dithering or offsetting the targets. In MOS observing mode with the high-resolution (R = 2,700) gratings, some spectra may additionally be cut-off at the right edge of the NRS2 detector, depending on their target shutter position in the MSA. Dithering can help to recover missing wavelengths for this effect as well.

Thermal stability is crucial for optimal performance of near-infrared detectors, so special care is taken to ensure that the NIRSpec FPA temperature is precisely controlled. To accomplish this, the FPA is mounted to a thermal strap that connects to a dedicated radiator. This allows the NIRSpec SCAs to be maintained at a stable temperature of 42.8K using heaters that are controlled by the thermal control circuit. The temperatures and overall performance of the NIRSpec detectors are monitored and trended, so that adjustments can be made if necessary.


Beletic, J.W et al. 2008 SPIE Proceedings Vol. 7021
Teledyne Imaging Sensors: Infrared imaging Technology for Astronomy and Civil Space 

Bernard J. Rauscher et al. 2014 PASP 126 739
New and Better Detectors for the JWST Near-Infrared Spectrograph

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