Atmospheric refractivity effects on mid-infrared ELT adaptive optics

TL;DR

This paper examines how atmospheric refractivity and dispersion, especially due to water vapour, can impact the performance of mid-infrared adaptive optics on extremely large telescopes, emphasizing the need for careful consideration in design.

Contribution

It highlights the significance of atmospheric refractivity effects in mid-IR ELT adaptive optics, which are often underestimated, and provides a first-order estimate of their impact based on site conditions.

Findings

Refractivity effects can cause pointing offsets over long exposures.

Infrared water vapour turbulence affects AO performance and is not seen by visible sensors.

Site selection significantly influences atmospheric dispersion impact.

Abstract

We discuss the effect of atmospheric dispersion on the performance of a mid-infrared adaptive optics assisted instrument on an extremely large telescope (ELT). Dispersion and atmospheric chromaticity is generally considered to be negligible in this wavelength regime. It is shown here, however, that with the much-reduced diffraction limit size on an ELT and the need for diffraction-limited performance, refractivity phenomena should be carefully considered in the design and operation of such an instrument. We include an overview of the theory of refractivity, and the influence of infrared resonances caused by the presence of water vapour and other constituents in the atmosphere. `Traditional' atmospheric dispersion is likely to cause a loss of Strehl only at the shortest wavelengths (L-band). A more likely source of error is the difference in wavelengths at which the wavefront is sensed and corrected, leading to pointing offsets between wavefront sensor and science instrument that evolve with time over a long exposure. Infrared radiation is also subject to additional turbulence caused by the presence of water vapour in the atmosphere not seen by visible wavefront sensors, whose effect is poorly understood. We make use of information obtained at radio wavelengths to make a first-order estimate of its effect on the performance of a mid-IR ground-based instrument. The calculations in this paper are performed using parameters from two different sites, one `standard good site' and one `high and dry site' to illustrate the importance of the choice of site for an ELT.