The in-flight noise performance of the JWST/NIRSpec detector system

Abstract

The Near-Infrared Spectrograph (NIRSpec) is one the four focal plane instruments on the James Webb Space Telescope (JWST) which was launched on December 25, 2021. We present the in-flight status and performance of NIRSpec's detector system as derived from the instrument commissioning data as available at the time of the conference. The instrument features two 2048 × 2048 HAWAII-2RG sensor chip assemblies (SCAs) that are operated at a temperature of about 42.8 K and are read out via a pair of SIDECAR ASICs. NIRSpec supports "Improved Reference Sampling and Subtraction" (IRS2) readout mode that was designed to meet NIRSpec's stringent noise requirements and to reduce 1/f and correlated noise. In addition, NIRSpec features subarrays optimized for bright object time series observations, e.g. for the observation of exoplanet transit around bright host stars. We focus on the dark signal as well as the read and total noise performance of the detectors.

In-orbit commissioning of the near-infrared spectrograph on the James Webb Space Telescope

“…The detailed characterization of detector performance (dark current, read noise, and bad pixel maps) and functionality of the ≈ 250000 individual shutters of the MSA. The results, which are detailed in companion papers [7,8], confirmed the expected in-flight performance, and marked the start of Phase 2 of NIRSpec commissioning, i.e. the autonomous operation via the Observation Plan Executive (OPE) which obtains science data without real-time intervention.…”

Optical throughput and sensitivity of JWST NIRSpec

“…Beside the optical efficiency, the other fundamental parameter driving the photometric sensitivity of an instrument such as NIRSpec is the noise performance of the detector. NIRSpec is equipped with extremely low-noise Teledyne H2RG arrays and, as illustrated by S. Birkmann et al [6] at this conference, the behaviour of the two sensors is excellent, with noise figures that meet the anticipated in-flight performances. Hence, NIRSpec achieves the sensitivity projected before launch by Ferruit et al [9], for the FS and MOS mode, and Böker et al [8], for the IFS mode.…”

“…The exposures were processed with our NIRSpec ramp-to-slope pipeline that performs the following basic data reduction steps: bias subtraction, reference pixel subtraction, linearity correction, dark subtraction and finally count-rate estimation, including jump detection and cosmic-ray rejection -see Birkmann et al [6] for more details on this last step. From the count-rate images the wavelength calibrated spectra were obtained using the Stage 2 of the NIRSpec pipeline to perform the following operations for the FS: subtract background (combining the exposures from the two-nods); extract sub-image containing the spectral trace and assign wavelength and spatial coordinates to each pixel therein; generate a rectified spectrum re-sampled on regular 2D-grid; compute the 1D-spectrum obtained by spatial integration.…”

In-flight performance and calibration of the grating wheel assembly sensors (NIRSpec/JWST)

“…The exposures were first processed with the NIRSpec Commissioning Team ramps-to-slopes pipeline that applies the following corrections: bias subtraction, reference pixel subtraction, linearity correction, dark subtraction and finally count-rate estimation, including jump detection and cosmic-ray rejection. 13 Additionally, for the spectral data, from the count-rate images the wavelength calibrated spectra were obtained using the second part of the NIRSpec calibration pipeline 14 to perform the following operations for the fixed slit: extract sub-image containing the spectral trace and assign wavelength and spatial coordinates to each pixel therein; generate a rectified spectrum re-sampled on regular 2D-grid; compute the 1D-spectrum obtained by spatial integration. One of the corrections implemented by the pipeline is the position of the grating wheel at each configuration, precisely the parameter for which we want to improve the calibration.…”