Limiting sensitivity cases are available that describe faint target limits in which NIRSpec will be able to acquire S/N = 10 spectra in about 10 ks, for given instrument parameters in the fixed slit, IFU and multi-object spectroscopy observing modes.

The performance of NIRSpec has been measured directly from on-orbit data obtained during JWST instrument commissioning (Giardino et al. 2022). Results of the on-orbit throughput measurements have been incorporated into the JWST Exposure Time Calculator (ETC) sensitivity model, which accounts for background, photon conversion efficiency, encircled energy, and detector performance (Pontoppidan 2016). Relative to pre-launch predicted instrument performance, the NIRSpec on-orbit results show an increased sensitivity over most of the NIRSpec wavelength range, the level of which varies with observing mode. This article presents the instrument faint sensitivity for specific benchmark test cases using ETC version 4.0 involving the NIRSpec modes FS (S200A1 aperture), IFU and MOS corresponding to a point source with a continuum spectrum.

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Limiting sensitivity in benchmark test cases

Limiting sensitivities for a point source benchmark test case for 3 NIRSpec observing modes are presented here: integral field unit (IFU), multi-object spectroscopy (MOS), and fixed slits (FS). The curves for limiting sensitivity define the flux to achieve S/N = 10 in about 10 ks at the presented wavelengths for each NIRSpec filter-grating configuration. The benchmark test parameters for each mode calculation are defined below.

Target calculation parameters (common for all modes):

• point source
• flat continuum spectral energy distribution in frequency units (fnu)
• 1.2 times minimum zodiacal background level (includes telescope background)

Detector readout parameters (common for all modes):

• NRSIRS2 readout pattern 
• Fourteen groups (FS and MOS modes) in the integration (10,358 s per integration), 17 groups (IFU mode) in the integration (10,037 s per integration)
• One integration
• Ten identical exposures (Total Dithers = 10) for FS and MOS mode, 4 identical exposures (Total Dithers = 4) for IFU mode
• FULL frame detector readout

MOS parameters:

• Three shutter slitlet (using 5 shutters, 3 open (-2, 0, 2))
• default background subtraction (one on-target, 2 off-target)
• Single spectral pixel

IFU parameters:

• 2-point nod (ETC JWST ETC IFU Strategies strategy)
• 3 × 3 spatial element extraction box
• single spectral pixel

FS parameters:

• slit S200A1
• separate background exposure
• single spectral pixel

Figures 1, 2, and 3 present the limiting sensitivity curves for the NIRSpec IFU, MOS and FS (S200A1) benchmark ETC cases.  Small gaps within the high-resolution (H) grating and medium-resolution (M, for MOS mode) grating data curves represent the wavelength gap between the 2 detectors. The gaps may be compensated by the use of multiple exposures using different subarrays.

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References

Giardino, G., Bhatawdekar, R., Birkmann, S. M. et al. 2022, arXiv:2208.04876
Optical throughput and sensitivity of the JWST NIRSpec

Pontoppidan, K. 2016, Proc of SPIE, 9910, 16
Pandeia: a multi-mission exposure time calculator for JWST and WFIRST 

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