The NIRISS team is pleased to announce the release of the STScI-supported Python package PASTASOSS (Predicting Accurate Spectral Traces for Astrophysical Single Object Slitless Spectroscopy), developed to correct visit-to-visit wavelength solution shifts in SOSS spectra. These shifts between observations, of up to a few pixels, result from discrepancies between the trace positions. The behavior is due to the pupil wheel, responsible for maneuvering the GR700XD grism into the optical path, not consistently aligning with its commanded position at the start of a SOSS observation.
Taking advantage of the predictability of this behavior, PASTASOSS forecasts the GR700XD spectral trace positions and their corresponding wavelength values for any SOSS observation, enabling sub-pixel accuracy in the wavelength solution for orders 1 and 2. For more information, please see Characterization of the visit-to-visit Stability of the GR700XD Spectral Traces for NIRISS/SOSS Observations and Characterization of the visit-to-visit Stability of the GR700XD Wavelength Calibration for NIRISS/SOSS Observations.
Implementation of PASTASOSS’s functionality into the JWST calibration pipeline is ongoing. The NIRISS SOSS team will continue to update the trace and wavelength prediction models as more data are gathered, as well as calibrate spectral order 3. Users who have questions or feedback to help shape future improvements based on their evolving needs are encouraged to reach out via the JWST Help Desk using the NIRISS card.