2025AJ....170..136V

The absolute flux calibration of the Near-infrared Imager and Slitless Spectrograph (NIRISS) imaging modes is derived using observations of photometric standards with the James Webb Space Telescope (JWST). The calibration converts the ramp slope to a mean surface brightness in MJy sr^–1, and is measured by combining aperture photometry corrected to infinite aperture with predicted mean flux densities of the stars from stellar models plus the solid angle per pixel. Photometric standards of three types (white dwarf stars, A-type stars, and solar analogs) were observed to identify possible systematic errors in the calibration. Variations in the calibration factors with time, count rate, peak pixel signal, and detector subarray were investigated. Average calibration factors have been calculated using an inverse-variance weighted mean, suitable if the values are subject to only statistical variations, and using an unweighted object-by-object mean, suitable if the values differ due to systematic effects between the stars. The two mean values indicate calibration uncertainties of 0.5%-0.9%. Standard LDS 749B, used for the commissioning calibration, is found to have values that are higher by 6%-9% compared to the mean values from the other standards in the F277W through F480M filters. This indicates an issue with the LDS 749B stellar model at these wavelengths. For some standards, there may be systematic offsets at the 1%-2% level in the longer-wavelength NIRISS filters. Cross-comparison with measurements from other JWST instruments is needed for verification of these offsets.