Asgard/NOTT: water vapor and CO$_2$ atmospheric dispersion compensation system

TL;DR

This paper introduces a novel atmospheric dispersion compensation system for high-contrast interferometry, addressing water vapor and CO2 effects to improve angular resolution in astronomical observations.

Contribution

It presents a new gas cell-based device for compensating atmospheric dispersion caused by water vapor and CO2, with a strategy for on-sky tuning.

Findings

Design of a variable-length gas cell for dispersion compensation

Effective correction of water vapor and CO2 atmospheric effects

Proposed on-sky tuning strategy for the device

Abstract

To leverage the angular resolution of interferometry at high contrast, one must employ specialized beam-combiners called interferometric nullers. Nullers discard part of the astrophysical information to optimize the recording of light present in the dark fringe of the central source. Asgard/NOTT will deploy a beam-combination scheme offering good instrumental noise rejection when phased appropriately, but for which information is degenerate on the outputs, prompting a dedicated tuning strategy using the science detector. The dispersive effect of water vapor can be corrected with prisms forming a variable thickness of glass. But observations in the L band suffer from an additional and important chromatic effect due to longitudinal atmospheric dispersion coming from a resonance of CO2 at 4.3 micron. To compensate for this effect efficiently, a novel type of compensation device will be deployed leveraging a gas cell of variable length at ambient pressure. After reviewing the impact of water vapor and CO2, we present the design of this atmospheric dispersion compensation device and describe a strategy to maintain this tuning on-sky.