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
Date:
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.
Spectral trace accuracy
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For point sources there can be offsets of 2–3 pixels in the cross-dispersion direction between the predicted trace positions and the actual trace positions over the detector. Calibrating the change in trace shape as a function of detector position is a work in progress, which will improve spectral trace accuracy once completed. Additional trace offsets may be caused by issues in the pipeline source catalog segmentation map, especially for extended sources.