Flat-fielding strategy for the JWST/NIRSpec multi-object spectrograph

The Near-Infrared Spectrograph (NFICSpec) onboard the James Webb Space Telescope (JWST) will be the first space-borne Multi-Object Spectrograph (MOS), comprising a quarter of a million individually addressable microshutters to allow simultaneous observation of similar to 100 targets. We present the strategy for fiat-fielding the NIRSpec MOS, correcting for the combined effects of the telescope and instrument throughput as well as the detector response. With a highly configurable shutter array, a novel approach is required to ensure that flat-field reference observations do not significantly impact telescope efficiency. We envisage a two-step strategy: 1) Creation of a three-dimensional master flat-field reference (two spatial dimensions, one wavelength) from a small set of well-designed calibration data; 2) Correction of any data frame using a two-dimensional flat-field generated on-the-fly, for that specific MOS configuration, from the master.