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
Updated:
The estimated photometric accuracy for order 1 is about 0.5% for the full wavelength range when the SUBSTRIP256 subarray is used. For order 2 the photometric accuracy is about 1% from 0.8 to 1.0 µm, about 1%–2% for the shorter wavelengths, and is much more uncertain at wavelengths larger than 1 µm where the signal level becomes very low. The order 2 spectrum should not be used at the longer wavelengths. The current photometric calibration of order 3 is poor because the original preliminary determination in commissioning is still being used, and it did not allow for the effect of the zodiacal background component. Hence, the flux densities are low by about 20% now that the background subtraction is done in the pipeline. Until a revision of the order 3 photometric calibration values can be done, the order 3 spectrum is inaccurate in both the continuum shape and the absolute level. When an order 1 spectrum is extracted in SUBSTRIP96, the accuracy is lower at the short and long wavelength ends because of the limited profile range for the extraction and having the trace close to the subarray edge, rather than because of the photometric calibration values themselves. Further details on the latest photometric calibration efforts can be found in Volk et al. 2026.
Wavelength calibration
Updated
As of pipeline build 11.1, the wavelength accuracy for extracted SOSS spectra is ~0.5 pixels. The wavelength accuracy depends on small GR700XD grism rotations (i.e., pupil wheel position) and small source offsets from one observation to another. The PASTASOSS method (Baines et al. 2023a, 2023b) measures these offsets and predicts the wavelength solution for a given GR700XD pupil wheel position, and is incorporated directly into the JWST pipeline.
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)
Order | Unweighted RMS | Weighted RMS | Unweighted RMS | Weighted RMS | Number of Fitted H2 Absorption Features |
|---|---|---|---|---|---|
1 | 5.72 × 10-4 ± 1.71 × 10-4 | 2.01 × 10-4 ± 3.70 × 10-5 | 0.58 ± 0.17 | 0.21 ± 0.04 | 31 |
2 | 2.42 × 10-4 ± 5.11 × 10-5 | 6.06 × 10-5 ± 2.07 × 10-5 | 0.52 ± 0.11 | 0.13 ± 0.04 | 13 |
3 | 7.36 × 10-4 ± 2.87 × 10-4 | 2.92 × 10-4 ± 1.63 x 10-4 | 2.20 ± 0.88 | 0.92 ± 0.52 | 12 |
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 order 3 has been updated and implemented in JWST pipeline under the PASTASOSS framework. For order 3, the wavelength calibration accuracy improves at wavelengths above 0.8 µm, likely due to the higher S/N and the presence of spectral features that provide stronger constraints on the wavelength solution.
References
Baines, T., Espinoza, N., Filippazzo, J., Volk, K. 2023a, arXiv:2311.07769
Characterization of the visit-to-visit Stability of the GR700XD Spectral Traces for NIRISS/SOSS Observations
Baines, T., Espinoza, N., Filippazzo, J., Volk, K. 2023b, arXiv:2311.07771
Characterization of the visit-to-visit Stability of the GR700XD Wavelength Calibration for NIRISS/SOSS Observations
Volk, K. 2026, JWST-STScI-009248, SM-12
Analysis of NIRISS SOSS Photometric Calibration Observations From Commissioning Through Cycle 3