NIRISS SOSS Calibration Status

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

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Photometric calibration


There is an estimated 1% accuracy in order 1 and 2, based on cross-comparison of standard stars. The order 3 uncertainty is larger than 5% because the response is much lower than for the other orders. This is the uncertainty in the conversion from ADU/s to Jy. Due to the effect of bad pixels on the spectral extraction, the wavelength-to-wavelength photometric errors in the spectrum can be of order 5%, with values being systematically low where the bad pixels affect the extraction. In orders 2 and 3 the uncertainties are much larger in the wavelength ranges where the intrinsic signal is low: for order 2, wavelengths between 1.05 and 1.4 µm, and for order 3 wavelengths below 0.8µm.

Wavelength calibration


The estimated accuracy is 4.0 pixels in the raw pipeline results, which can be improved significantly to about 0.5 pixels by allowing for the small grism rotations and small source offsets from one observation to another using the PASTASOSS package (Baines et al. 2023). The raw accuracy varies with wavelength, being larger at the longest wavelengths because the "lever arm" distance from the rotation point is larger for that part of the spectrum. Table 1 shows the accuracy estimates for the SOSS wavelength calibration using measurements of H2 absorption features in the photometric standard BD+60 1753.

Table 1. Accuracy estimates for the SOSS wavelength calibration (Baines et al. 2023)


Unweighted RMS

Weighted RMS

Unweighted RMS

Weighted RMS

Number of Fitted H2 Absorption Features


5.72 x 10-4 ± 1.71 × 10-4

2.01 x 10-4 ± 3.70 × 10-5

0.58 ± 0.17

0.21 ± 0.04



2.42 x 10-4 ± 5.11 × 10-5

6.06 x 10-5 ± 2.07 × 10-5

0.52 ± 0.11

0.13 ± 0.04


The accuracy is better for wavelengths less than 1 µm in order 1, likely due to better S/N at these wavelengths due to the combination of the better grism response and the higher stellar flux density in the spectrum of BD+60 1753. The wavelength calibration for SOSS order 3 is not yet supported.

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Originally published