An Imaging Fabry-Perot System for the Robert Stobie Spectrograph on the Southern African Large Telescope

TL;DR

This paper describes the design, laboratory characterization, and optical effects of a Fabry-Perot system for the Robert Stobie Spectrograph, enabling spectroscopic imaging across a broad wavelength range on the Southern African Large Telescope.

Contribution

It introduces a novel Fabry-Perot system with detailed laboratory analysis and insights into coating-induced optical effects affecting performance.

Findings

Wavelength dependence of etalon gap and parallelism observed

Multi-layer dielectric coatings cause wavelength-dependent shape changes

Coating effects impact resolving power, finesse, and throughput

Abstract

We present the design of the Fabry-Perot system of the Robert Stobie Spectrograph on the 10-meter class Southern African Large Telescope and its characterization as measured in the laboratory. This system provides spectroscopic imaging at any desired wavelength spanning a bandpass 430 - 860 nm, at four different spectral resolving powers ranging from 300 to 9000. Our laboratory tests revealed a wavelength dependence of the etalon gap and parallelism with a maximum variation between 600 - 720 nm that arises because of the complex structure of the broadband multi-layer dielectric coatings. We also report an unanticipated optical effect of this multi-layer coating structure that produces a significant, and wavelength dependent, change in the apparent shape of the etalon plates. This change is caused by two effects: the physical non-uniformities or thickness variations in the coating layers, and the wavelength dependence of the phase change upon refection that can amplify these non-uniformities. We discuss the impact of these coating effects on the resolving power, finesse, and throughput of the system. This Fabry-Perot system will provide a powerful tool for imaging spectroscopy on one of the world's largest telescopes.