A Direct Measurement of Atmospheric Dispersion in N-band Spectra: Implications for Mid-IR Systems on ELTs

TL;DR

This study confirms atmospheric dispersion as a key limiting factor in mid-IR imaging on ELTs, demonstrating the need for an atmospheric dispersion corrector to improve image quality.

Contribution

First direct measurement of atmospheric dispersion in the N-band using a mid-IR adaptive optics system on the MMT, informing ELT system design.

Findings

Atmospheric dispersion in the N-band is generally linear with some residual curvature.

An atmospheric dispersion corrector (ADC) is likely necessary for many mid-IR ELT applications.

Measurements align with theoretical predictions, aiding future ELT instrument development.

Abstract

Adaptive optics will almost completely remove the effects of atmospheric turbulence at 10 microns on the Extremely Large Telescope (ELT) generation of telescopes. In this paper, we observationally confirm that the next most important limitation to image quality is atmospheric dispersion, rather than telescope diffraction. By using the 6.5 meter MMT with its unique mid-IR adaptive optics system, we measure atmospheric dispersion in the N-band with the newly commissioned spectroscopic mode on MIRAC4-BLINC. Our results indicate that atmospheric dispersion is generally linear in the N-band, although there is some residual curvature. We compare our measurements to theory, and make predictions for ELT Strehls and image FHWM with and without an atmospheric dispersion corrector (ADC). We find that for many mid-IR applications, an ADC will be necessary on ELTs.