NIRSpec Sensitivity

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.

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 NIRSpec has been measured directly at both the component and system level using the fully assembled flight model. 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). Equations used for ETC sensitivity calculations are described in NIRSpec Performance.  Information presented in this article shows the comparison of the limiting instrument faint sensitivity limit in a specific benchmark test case corresponding to a point source with a continuum spectrum.

Users should ultimately use the Exposure Time Calculator (ETC)  for all signal-to-noise calculations. Information presented here is for notional example. 


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 (9,629 sec) at the presented wavelengths for each NIRSpec filter-grating configuration. These "benchmark" cases are used to compare the JWST ETC to the sensitivity calculation tools made by the ESA NIRSpec instrument development team (IDT). The benchmark test parameters for each mode calculation are defined below.

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

Target calculation parameters (common for all modes):

  • NRSIRS2 readout pattern
  • point source
  • flat continuum spectral energy distribution
  • 1.2 times minimum zodiacal background level (includes telescope background)

Detector readout parameters (common for all modes):

MOS parameters:

  • Three shutter slitlet
  • default background subtraction (one on-target, 2 off-target)
  • Four spatial pixel extraction box, single spectral pixel

IFU parameters:

FS parameters:

  • slit S200A1
  • separate background exposure
  • Four spatial pixel extraction box, single spectral pixel

Figures 1, 2, and 3 present the limiting sensitivity curves for the NIRSpec IFU, MOS and FS (S200A1) benchmark ETC cases.  

Figure 1. Limiting sensitivity IFU

The limiting sensitivity vs. wavelength (calculated as described above) for the NIRSpec IFU using the multiple spectral configurations available in NIRSpec.
Figure 2. Limiting sensitivity MOS

The limiting sensitivity vs. wavelength (calculated as described above) for the NIRSpec MOS using the multiple spectral configurations available in NIRSpec.
 Figure 3. Limiting sensitivity FS (S200A1 slit)

The limiting sensitivity vs. wavelength (calculated as described above) for the NIRSpec FS (S200A1) using the multiple spectral configurations available in NIRSpec.

References

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



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Originally published