NIRSpec Bright Source Limits

Sensitivity cases are available from on-orbit data that describe the brightest limits that JWST NIRSpec are able to observe for the given instrument parameters in the fixed slit, IFU and multi-object spectroscopy observing modes.

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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 on-orbit results for NIRSpec show an increased sensitivity over most of the wavelength range, the level of which varies with observing mode. This article presents the instrument bright source limits for specific benchmark test cases involving the NIRSpec modes FS (S200A1 aperture), IFU and MOS corresponding to a point source with a continuum spectrum.

Users should ultimately use the Exposure Time Calculator (ETC) for all sensitivity calculations. Information presented here is for graphical example.

NIRSpec bright source observing limits (saturation levels) 

The bright end sensitivities are presented here for a test case of 3 NIRSpec observing modes: integral field unit (IFU), multi-object spectroscopy (MOS), and fixed slits (FS). The curves for bright limits define the flux that is just below the detector saturation limit at the presented wavelengths for each NIRSpec filter-grating configuration. The parameters used for the test cases for each mode are defined below.

Target calculation parameters (common for all modes):

  • point source
  • flat continuum spectral energy distribution
  • 1.2 times minimum zodiacal background

Detector readout parameters (common for all modes):

  • NRSRAPID readout pattern
  • Two groups in the exposure 
  • One integration
  • One exposure
  • Sensitivity in this calculation is defined by the flux measured in the exposure group #2 minus flux in group #1; this is known as a "correlated double sample" (CDS) subtracted frame.

MOS parameters:

  • Three shutter slitlet
  • default background subtraction (one on-target shutter, two off-target shutters)
  • Four pixel extraction box

IFU parameters:

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

FS parameters:

  • Slit S200A1
  • FULL frame detector readout (no detector subarray used)
  • separate background exposure
  • Four pixel extraction box

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

Figure 1. NIRSpec fixed slits (S200A1) bright limits

The bright limit vs. wavelength from on-orbit data for the NIRSpec FS (S200A1) using the multiple spectral configurations available in NIRSpec.

Figure 2. NIRSpec MOS bright limits

The bright limit vs. wavelength from on-orbit data for the NIRSpec MOS using the multiple spectral configurations available in NIRSpec.

Figure 3. NIRSpec IFU bright limits

The bright limit vs. wavelength from on-orbit data for the NIRSpec IFU using the multiple spectral configurations available in NIRSpec.


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 Reference papers and reports

Latest updates
    FS, MOS, and IFU bright limits figures updated for ETC changes

    Figures updated to present instrument on-orbit performance
Originally published