A new MIRI prime observing mode, Wide Field Slitless Spectroscopy (WFSS), will be available to the scientific community in Cycle 5. This mode uses the double prism  (P750L) to obtain low spectral resolving power (R ~ 100) from 5 to 14 μm, over an unobstructed 74″ × 113″ field of view (Figure 1). MIRI WFSS enables efficient spectroscopic observations of multiple sources in a single pointing, with sensitivity comparable to the current MIRI slitless LRS mode (e.g., a source with a Seyfert1 galaxy-like spectrum, at redshift 1, normalized to an integrated MIRI F560W of 50 μJy, would require ~1 hour total integration for an SNR of 15 at 10 μm.)

Support for MIRI WFSS is incorporated into the JWST’s Exposure Time Calculator (ETC, version 5.0) and the Astronomer's Proposal Tool (APT version 2025.5.2). Observations follow a defined sequence with direct images of the field taken before and after a set of dithered prism exposures. Optional additional dithers can enhance coverage for partially dispersed sources, and observations may be executed as larger mosaics. Repeating the sequence at a different telescope roll angle is one of the recommended strategies to help disentangle overlapping spectra.

Users working on Cycle 5 proposals are encouraged to contact the MIRI instrument team via the JWST Help Desk with any questions.