Photon Mean Free Paths, Scattering, and Ever-Increasing Telescope Resolution

摘要

We revisit an old question: what are the effects of observing stratified atmospheres on scales below a photon mean free path lambda? The mean free path of photons emerging from the solar photosphere and chromosphere is approximate to 10(2) km. Using current 1 m-class telescopes, lambda 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 310 nm, 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 point spread function. 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.