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.

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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 NIRSpec predicted performance. Information presented in this article 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 three 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 as:

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):

MOS parameters:

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

IFU parameters:

FS parameters:

  • Slit S200A11
  • 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.
1  Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.



References

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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