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.

Astrometric accuracy

Date: 19 Jan 2024

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.

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Photometric calibration

Date: 19 Sep 2023 

Current uncertainties are of order 2% at short wavelengths, 5% at longer wavelengths, due to variation in measured values from one standard star to another. The direct statistical measurement uncertainties per star are less than 1% at short wavelengths and up to 2% at longer wavelengths. The actual stability appears to be better than 1% over time.

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

Date: 19 Sep 2023 

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

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