Wide field slitless spectroscopy with JWST's MIRI

Massively multiplexed spectroscopy enables efficient, statistically robust surveys of large cosmic volumes. Space-based wide-field slitless spectroscopy reduces cosmic variance by covering extensive sky areas with consistent sensitivity and calibration. In Cycle 5, JWST introduced a new MIRI prime observing mode—Wide Field Slitless Spectroscopy (WFSS)—which utilizes the P750L double prism to deliver low-resolution (R ~ 100) spectroscopy from 5 to 14 μm across an unobstructed 74″ × 113″ field of view. We present the MIRI WFSS calibration plans and their expected accuracy. We utilized established slitless-spectroscopy techniques from HST and JWST, archival P750L and MIRI imaging data, and dedicated calibration observations for flux/trace and wavelength calibration, including adjustments for positional dependencies. We present the most recent calibration reference files used for flat-fielding, background subtraction, and flux and wavelength calibration, and outline improvement plans.