Prime Focus Imaging Spectrograph for the Southern African Large Telescope: optical design

TLDR

PFIS is designed to deliver high‑throughput, low‑ to medium‑resolution imaging spectroscopy and spectropolarimetry from 320–900 nm using VPH gratings and a Fabry‑Perot interferometer. The optical design employs all‑transmissive optics with fused silica, CaF₂, and sealed‑triplet NaCl to preserve UV throughput, a 150 mm collimated beam and an articulating F/2.2 camera with six VPH and one transmission grating, and Pancharatnam waveplates plus a Wollaston beamsplitter for versatile spectropolarimetry.

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.