| Abstract |
The University of Wisconsin - Madison, together with Rutgers University
and the South African Astronomical Observatory, is designing and
building an imaging spectrograph for the Prime Focus Instrument Package
of the Southern African Large Telescope (SALT). The Prime Focus Imaging
Spectrograph (PFIS) will be a versatile instrument specializing in very
high throughput, low and medium resolution (R=500-12,500) imaging
spectroscopy, using volume phase holographic (VPH) gratings and a double
etalon Fabry-Perot interferometer, and spectropolarimetry from 320 to
900 nm. The optical design includes all transmissive optics for high
efficiency and compactness. To maintain throughput in the ultraviolet,
only fused silica, CaF2 and NaCl are used. As NaCl is very
hygroscopic, the design uses NaCl as the inner element in sealed
triplets only. For the highest possible first-order spectral resolution,
the collimated beam size is 150 mm - the maximum for practical
Fabry-Perot etalons. The F/2.2 camera can be articulated to tune the
efficiency of the VPH gratings; a complement of six gratings (5 VPH and
1 standard transmission grating) has been designed to fill the
resolution-wavelength space available to the instrument. Linear,
circular, and all-stokes spectropolarimetry will be performed through
the use of Pancharatnam superachromatic waveplates and a Wollaston
beamsplitter. |