Simulated count-rate image of one of the MOS-mode PRISM exposures. There are 214 targets covered in this MSA configuration, each one observed within a 3 × 1 shutter slitlet. The noise level corresponds to that of a single exposure of 2801s duration. The inset provides a close-up of a few representative traces. The spectra of several of the stronger sources are visible above the sky background, as is the spurious single-shutter sky spectrum from a defective FO shutter (fourth spectrum from the top in the inset).