NIRSpec Bright Source Limits

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

JWST sensitivity update

Preliminary results from observatory commissioning indicate that system throughput is higher than pre-mission expectations, by amounts that vary with wavelength. Until detailed results are available and updated in Pandeia, results calculated by the ETC and JIST tools will underestimate S/N. An announcement will be made when the tools are updated.

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The performance of JWST has been measured directly at both the component and system level using the fully assembled NIRSpec flight model. The results of the JWST throughput measurements have been integrated into a sensitivity model that includes the following components: (1) background, (2) photon conversion efficiency, (3) encircled energy, and (4) detector performance (Pontoppidan 2016). The components of an astronomical scene are taken into account to perform accurate measurements using the JWST Exposure Time Calculator (ETC). The method for the calculation is described in the article NIRSpec Performance. Information presented below shows bright source observing limits for the NIRSpec science modes using specific input calculation parameters.

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, two off-target)
  • Four pixel extraction box

IFU parameters:

Words in bold italics are also buttons 
or parameters in GUI tools. Bold 
style represents GUI menus/
panels & data software packages.

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.  

Figure 1. NIRSpec fixed slits bright limits

The bright limit vs. wavelength (calculated as described above) for the NIRSpec FS (S200A1) using the multiple spectral configurations available in NIRSpec. Note: this calculation is carried out in full frame detector readout mode, brighter sources can be observed if detector subarrays are utilized. See the JWST ETC for more information.
Figure 2. NIRSpec MOS bright limits

The bright limit vs. wavelength (calculated as described above) for the NIRSpec MOS using the multiple spectral configurations available in NIRSpec.
Figure 3. NIRSpec IFU bright limits

The bright limit vs. wavelength (calculated as described above) for the NIRSpec IFU using the multiple spectral configurations available in NIRSpec.


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

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