The overall calibration status and estimated accuracy of NIRSpec multi-object spectroscopic observations are described in this article; please also see the article on known issues affecting NIRSpec MOS data.

Photometric calibration

Date: 19 Sep 2023 

There is an approximately 15% absolute flux accuracy, while field-dependent variations may be as large as 10%.   

It has become apparent that the original observations in the JWST commissioning program (PID 1128) of the standard star 2MASS J18083474+6927286, which were used to define the F-Flats for the NIRSpec grating settings at the Q4 field point, were not properly centered in the aperture. As a result, existing F-Flats appear to give fluxes for the NIRSpec gratings approximately 10% too high relative to both the proper absolute calibration and MOS PRISM observations. These F-Flats will be reworked soon to approximately correct for this mis-centering, and in Cycle 3 these calibration observations will be repeated with another target to verify and improve the revised F-Flats.

Analysis is underway to assess whether S-flats need to be updated for the MOS mode.

Caveats:

• Issues with the current pipeline treatment of the off-center nodding positions can significantly degrade this accuracy, although workarounds are available.
• Achieving the above accuracies may also require rerunning the pipeline with altered parameters, including changes to outlier correction, weights used for resampling, and the source extraction location.
• The emission line fluxes taken with PRISM match NIRCam photometry; however, the emission line fluxes in the spectra taken with M gratings is ~10% higher (Bunker et al. 2023). The NIRSpec team is investigating the issue and anticipates that the offset is likely due to the mis-centering of the target used for deriving the flux calibration of the M gratings. Improved flux calibration will be available soon.

Wavelength calibration

Date: 19 Sep 2023 

NIRSpec meets the requirement of wavelength accuracy of 1/8 of a resolution element (Böker et al., 2023). This assumes a definition of the resolution element as that measured using lamp spectra, which typically corresponds to a wavelength accuracy of about a 1/4 pixel.

This translates into typical velocity accuracies for the different dispersions of:

• High resolution gratings (R2700) ~ 15 km/s
• Medium resolution gratings (R1000) ~ 40 km/s
• PRISM  (R100) ~ 750 km/s

Caveats:

• The numbers above apply to well-centered point sources or extended sources that uniformly fill the aperture. Accuracy of wavelength calibration for off-centered sources is less well established.
• Variations over the MOS field of view have yet to be well characterized.

For more information, see NIRSpec MOS Known Issues.