Photospheric fine structure: An observational challenge - An analysis of radiative transfer effects on the visibility of small-scale structures

The requirement on spatial resolution of a large solar telescope is frequen tly based on the assumption that the smallest observable features in the so lar photosphere could not be much different from a photon mean free path du e to smoothing by radiative transfer. Although hydrodynamic simulations sho w that much smaller structures should exist, a resolution of the order of 1 00 km is typically considered sufficient for observations in the photospher e. We performed 2D non-LTE radiative transfer computations for thin flux sh eets with widths ranging from 10 to 160 km in the solar photosphere. We dem onstrate that such small structures - should they exist could be observed a s small scale variations of intensity and polarization. We conclude that th e size limit below which photospheric structure cannot be observed due to s moothing: radiative transfer effects must lie well below 10 km. A spatial r esolution limit for telescopes based on photon mean free path arguments sho uld therefore be abandoned.