文摘

We revisit an old question: what are the effects of observing stratified atmospheres on scales below a photon mean free path λ? The mean free path of photons emerging from the solar photosphere and chromosphere is ≈-02?km. Using current 1?m-class telescopes, λ 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.