JWST Pipeline Build 12.1, currently available to users and planned for operations later in the year, includes improved pathloss correction reference files for the NIRSpec Multi-Object Spectroscopy (MOS) and Fixed Slit (FS) observing modes. The new pathloss corrections are based on in-flight data.

Pathloss is the flux lost outside an aperture and depends on wavelength, target type, and the position within the shutter/slit. This update assumes two target cases: compact point sources or sources uniformly extended throughout the shutter/slit.

Point Source Updates (MOS only): The relative flux calibration accuracy is improved from ~8% to ~5% for well-centered targets and from as high as ~70% to ~10-15% for targets near the MSA shutter edges. See Figure 1 Top panel. 

Uniform Source Updates: Now yield consistent background measurements to within ~5% across all MOS and FS apertures. These measurements also more closely match predictions from the JWST Background Tool (JBT) model. See Figure 1 Bottom panel.

Users with questions about the new pathloss corrections are encouraged to contact the NIRSpec team via the Help Desk.