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 two individual detectors, also called sensor chip arrays (SCAs). The SCAs (designated as NRS1 and NRS2) are IR hybrid arrays with HgCdTe used for light detection, and a silicon integrated circuit for the readout. The two H2RG detectors 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 its own SIDECAR ASIC (System for Image Digitization, Enhancement, Control, and Retrieval; Application-Specific Integrated Circuit). The ASICs are used for detector readout and control. ASICs are special purpose electronic devices individually matched and tuned to their corresponding SCAs.

Each of the 4 million pixels in an SCA can be addressed individually and read out in a non-destructive way. The outer four pixels framing the SCA serve as an electronic reference and are insensitive to light. These reference pixels are used to track variations in the output signal, caused by bias drifts or small temperature changes in the electronics. 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 shared with the other NIR detectors in JWST, in order to meet its strict detector noise performance requirements, NIRSpec can also use 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 two SCAs are placed side-by-side in the focal plane array (FPA). Light sensitive portions (2040 × 2040 pixels) of the two SCAs are separated by a physical gap that is equivalent to approximately 18" on the sky. The detector gap region is hidden behind the metal frame between MSA quadrants. In spectroscopy mode, the detector gap will cause loss of spectral information over a range in wavelengths for the MSA that depends on the location of the target and the dispersive element used.  The gap will have the same effect on spectra taken with the IFU, and most of 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 source 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 using heaters that are controlled by the thermal control circuit. The favored operating temperature is 42.8 K, as determined in laboratory tests. Temperatures of the NIRSpec detectors are monitored and logged in telemetry.



References

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




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