Instrument Characteristics

PEPSI has of 2 permanently mounted fiber fed focal stations (SX PFU and DX PFU). These units are used for target acquisition, guiding and wavefront sensing and house the mode selectors for utilizing the different PEPSI spectral resolutions as well as fiber coupling down to the spectrograph.

The SX and DX polarimeters are not permanently mounted. When mounted, they are at the direct Gregorian mount in place of the MODS instruments. The two polarimeters provide a full Stokes IQUV vector at a resolution of R=130,000.

Light from the 4 fibers (SX:target and sky and DX:target and sky) is injected into the spectrograph by image slicers: 7 for each 100 micron fiber; 5 for each 200-mic fiber; and 3 for each 300-micron fiber. About one percent of the light is diverted to a photon counter.  It is collimated and then dispersed by an Échelle grating that is used in Littrow configuration.  In each channel, the spectra are once again collimated and then cross-dispersed by one of three cross-dispersers, each consisting of a VPH grating and 2 prisms. The 6 cross-dispersers cover all 92 orders in the 383-907 nm échelle spectrum.