The prime focus imaging spectrograph for the Southern African large telescope: Operational modes.

The Prime Focus Imaging Spectrograph (PFIS) will be the workhorse first-generation spectrograph on the Southern African Large Telescope (SALT). Scheduled for commissioning in late 2004, PFIS is a versatile high-throughput imaging spectrograph with a complement of 5 volume-phase holographic gratings for spectroscopic programs from 3200 Angstrom to 9000 Angstrom at resolutions of R=1500 to R=10000. A magazine of 6 longslits and 30 custom laser-milled slitmasks enables single- or multi-object spectroscopy over an 8 arc minute diameter field. With the gratings stowed, a dual-etalon Fabry-Perot subsystem enables imaging spectroscopy at R=500, R=3000, and R=12,500. The polarization subsystem, consisting of a polarizing beam-splitter used in conjunction with half- and quarter-wave plates, allows linear or circular polarimetric measurements in ANY of the spectroscopic modes. Three mosaiced rapid-readout frame-transfer CCDs provide the capability for time-resolved sampling at rates in excess of 10 Hz. Combinations of these subsystems permit novel observing modes for specialized scientific programs. Examples include high time-resolution multi-object spectral polarimetry of accreting compact objects, and Fabry-Perot polarimetry or imaging spectral polarimetry of nebulae and stellar clusters. The demands of queue-scheduled observing on a fixed-altitude telescope require that the instrument be capable of rapid reconfiguration between modes.