NIRISS WFSS Calibration Status
The overall calibration status and estimated accuracy of NIRISS wide field slitless spectroscopy are described in this article; please also see the article on known issues affecting NIRISS WFSS data.
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Photometric calibration
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STScI currently estimates a 2.5% accuracy of the photometric conversion factors, which is dominated by issues with spectral extraction and contamination. There is a separate issue with the effect of bad pixels on the extraction which produces systematically low values in the spectra at some wavelengths, so the wavelength-to-wavelength errors in the output flux density can be 10% or more depending on where in the cross-dispersion profile the bad pixel appears. This effect is also somewhat dependent on the width of the extraction aperture.
Wavelength calibration
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The wavelength calibration accuracy for NIRISS WFSS is ~0.3–0.44 pixels (~14–21 Å). These values are currently limited by the find_line_thresholds algorithm in specutils. Figure 1 shows the residuals between the observed wavelengths and model wavelengths of emission lines identified in the GR150C grism (left) and GR150R grism (right) in each filter from the Cycle 1 WFSS calibration program to observe planetary nebula SMP-LMC-58.
Click on the figure for a larger view.
Using data from the Cycle 1 WFSS wavelength calibration program to observe planetary nebula SMP-LMC-58 (Program ID 1510), the offset between the centers of the observed emission lines and theoretical line centers are plotted above for the GR150C grism (left) and GR150R grism (right) for different regions across the NIRISS detector. The RMS in the residuals, indicated by the light grey regions, are between 0.3 pixels or 14 Å (GR150R) and 0.44 pixels or 21 Å(GR150C).
Spectral trace accuracy
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The shape of the spectral trace changes as a function of detector position. This calibration has been completed for the GR150C/F200W and GR150R/F200W grism/filter combinations, and is reflected in the specwcs reference file as of CRDS context jwst_1228.pmap. The residuals between the predicted and measured trace centroids in the 1st order are -0.2 ± 0.5 pixels (GR150R) and 0.3 ± 0.3 pixels (GR150C).
For point sources in other filters, there can be offsets of 2–3 pixels in the cross-dispersion direction. Calibrating the change in trace shape as a function of detector position for the remaining filters is a work in progress. Additional trace offsets may be caused by issues in the pipeline source catalog segmentation map, especially for extended sources.