The spectro-photometric calibration of the JWST NIRSpec instrument

NIRSpec is the main near-infrared spectrograph on board the James Webb Space Telescope, offering multi-object capabilities as well as an integral field unit and a number of fixed slits for studies of individual objects. In this paper, we describe the unique challenges in calibrating this complex instrument, and the approach taken to deal with them, both in terms of operational procedures and via automated processing of NIRSpec data. We provide a high-level description of the sequence of processing steps required for NIRSpec science data, and the necessary on-ground calibration files. We focus our discussion on the case of a typical multi-object observation with the MSA, in which adjacent micro-shutters are used to sample the science object and the sky background in an alternating way. This dithering strategy is particularly well suited for faint targets, but its guiding principles also apply to other NIRSpec modes.