A prototype of a large tunable Fabry-Perot interferometer for solar spectroscopy

TL;DR

This paper presents a comprehensive characterization of a large, symmetric Fabry-Perot interferometer prototype designed for solar spectroscopy, demonstrating its stability and suitability in different orientations.

Contribution

It introduces a novel method to assess gravity effects on the cavity and confirms the interferometer's stable performance in both vertical and horizontal configurations.

Findings

Gravity effects are limited to a few nanometers over a 120 mm surface.

Tilt variations during spectral scans can be corrected with appropriate adjustments.

The system's dynamical response meets operational requirements.

Abstract

Large Fabry-Perot Interferometers are used in a variety of astronomical instrumentation, including spectro-polarimeters for 4-meter class solar telescopes. In this work we comprehensively characterize the cavity of a prototype 150 mm Fabry-Perot interferometer, sporting a novel, fully symmetric design. Of note, we define a new method to properly assess the gravity effects on the interferometer's cavity when the system is used in either the vertical or horizontal configuration, both typical of solar observations. We show that the symmetric design very effectively limits the combined effects of pre-load and gravity forces to only a few nm over a 120 mm diameter illuminated surface, with gravity contributing about 2 nm peak-to-valley (0.3 nm rms) in either configuration. We confirm a variation of the tilt between the plates of the interferometer during the spectral scan, which can be mitigated with appropriate corrections to the spacing commands. Finally, we show that the dynamical response of the new system fully satisfies typical operational scenarios. We conclude that large, fully symmetric Fabry-Perot interferometers can be safely used within solar instrumentation in both, horizontal and vertical position, with the latter better suited to limiting the overall volume occupied by such an instrument.