NIRISS Imaging Calibration Status

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

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Astrometric accuracy

Date:

Relative astrometric accuracy within the detector is about 3 mas (rms, single axis). Absolute astrometric accuracy for observations without target acquisition (TA) is typically less than 0.1 arcsec (single axis), while for observations with TA, the accuracy is about 2 mas (rms, single axis). Note that in rare cases (e.g., crowded fields, high background, etc), the telescope can inadvertently guide on a star that is offset from the commanded location, and this can lead to the world coordinate system (WCS) of the observed image being offset by 1–2 arcsec or more.


Photometric calibration

Updated:  

As of CRDS context jwst_1296.pmap, the scaling factors in the imaging photometry reference files (to convert count rates from units of ADU/S to MJy/ster) have accuracies raning from 0.06 - 0.42% (see Volk and Goudfrooij 2025).


Filter-to-filter image offsets (i.e., boresight offsets)

Date:  

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The "_rate.fits" images in different filters are subject to small "wedge" offsets between the filters. These values are given in Table 1 as the offsets from the position in the F150W filter to the position in another filter. These offsets are propagated to the world coordinate system (WCS) information in the headers of the "_cal.fits" files (which have been flat fielded, but not yet resampled to a distortion-free pixel grid) and the "_i2d.fits" files (which are distortion corrected). However, in the "_cal.fits" files, these filter-to-filter offsets are still present in the science images. To compare images in different filters on a common pixel grid, STScI's NIRISS team recommends running the resample step with a certain absolute pixel scale (i.e., the pixel_scale argument) to create "_i2d.fits" data products. 


Table 1. Filter-to-filter offsets in NIRISS "rate.fits" and "cal.fits" images

Filterx pixel offsety pixel offset
F090W-0.1529+0.7676
F115W+0.0397-0.0651
F140M+0.0173+0.0650
F150W0.00.0
F158M-0.4397-0.1929
F200W+2.1190-1.0476
F277W+0.2213-0.1309
F356W+0.0429-0.2291
F380M-0.2747+0.3512
F430M+0.0346-0.0113
F444W+0.1658-0.1387
F480M-0.2197+0.1298


References

Volk, K., and Goudfrooij, P., 2025, AJ, 170, 136
The James Webb Space Telescope Absolute Flux Calibration. IV. Near-Infrared Imager and Slitless Spectrograph Imaging



Notable updates
  •  
    Update the percent uncertainties of imaging photometric calibration values taken from the latest photom reference files

  •  
    Updated to report photometric accuracy as of jwst_1296.pmap and link to the imaging photometry technical report.
Originally published