NIRSpec IFU Calibration Status

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

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Photometric calibration


There is better than 5% absolute flux accuracy measured for the combined spectrum from the 4-point nodding pattern. For most settings, the IFU absolute flux accuracy averaged over multiple dither positions should be good to about 3%. However, the flux for individual observations may show additional low order ±2% variations due to variable path losses that depend on the placement of the target within an individual IFU slitlet. Currently, there is no correction applied in the pipeline for this. The G140M/F100LP setting shows additional offsets of up to 2%–4% due to the use of a different calibration star. This latter offset should be reconciled in the near future. 


  • The accuracy of delivered pipeline products may be sensitive to the exact aperture size and background subtraction strategy.
  • Variation of sensitivity with position in the IFU aperture is not as well characterized. 

Wavelength calibration


Wavelength accuracies are slightly worse for the IFU modes, although the details of the residuals may vary and are still being characterized. For the G235H and G395H, the IFU mode has modest global dispersion errors with the shortest wavelengths for these 2 gratings having offsets gratings corresponding to as much as -20 km/s relating both the long wavelength end of the same grating and to the true velocity. Variations over the IFU field of view have yet to be well characterized.

Additional information is available at NIRSpec IFU Known Issues.

Notable updates

Originally published