NIRCam WFSS and Grism Calibration Status

The overall calibration status and estimated accuracy of NIRCam wide field slitless spectroscopy (WFSS) and grism time-series observations are described in this article; please also see the article on known issues affecting NIRCam data.

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Photometric calibration


The absolute flux calibration uncertainties for the grisms are about 2% (all filters/detectors). The grism subarrays and full frame observations agree to within about 2% based on images in those subarrays using standard stars.

Wavelength calibration


Generally ~0.25 pixels, or 2.5 Å, over the whole field of view using the latest calibration data. The wavelength is tied to NIRSpec long slit observations of a planetary nebulae. 

Spectral trace accuracy


All positions, offsets, traces, and spectral shapes of the 1st orders are calibrated to within ~0.25 pixel over the entire field of view. That is, given a coordinate x0, y0 for a source, it can be predicted where the spectral trace will fall on the detector in the WFSS observation down to 0.25 pixel on the detector in WFSS mode. The modeling/calibration of this includes field dependence and variation of the shape of the spectral traces as a function of position. 

Target acquisition performance


For grism time series, TA uncertainty has been measured to be ±0.3 px (0.02") for the F335M and ±0.15 (~0.01") for the F405N filters, respectively.

Notable updates

Originally published