Photon mean free paths, scattering, and ever-increasing telescope resolution

TL;DR

This paper examines how scattering in the solar atmosphere affects the resolution of telescopic observations, especially with upcoming high-resolution telescopes, by modeling the impact of photon mean free paths and scattering on observed solar features.

Contribution

It introduces a unified model linking scattering effects to telescope resolution degradation and provides a way to quantify this impact through an effective Strehl ratio.

Findings

Scattering causes physical smearing similar to PSF degradation.

The model relates scattering properties to an effective Strehl ratio.

Observations in specific spectral lines can inform about atmospheric and instrumental effects.

Abstract

We revisit an old question: what are the effects of observing stratified atmospheres on scales below a photon mean free path? The mean free path of photons emerging from the solar photosphere and chromosphere is near 100 km. Using current 1m-class telescopes, the mean free path is on the order of the angular resolution. But the Daniel K. Inoue Solar Telescope will have a diffraction limit of 0.020'' near the atmospheric cutoff at 310nm, corresponding to 14 km at the solar surface. Even a small amount of scattering in the source function leads to physical smearing due to this solar 'fog', with effects similar to a degradation of the telescope PSF. We discuss a unified picture that depends simply on the nature and amount of scattering in the source function. Scalings are derived from which the scattering in the solar atmosphere can be transcribed into an effective Strehl ratio, a quantity useful to observers. Observations in both permitted (e.g., Fe I 630.2 nm) and forbidden (Fe I 525.0 nm) lines will shed light on both instrumental performance as well as on small scale structures in the solar atmosphere.