  Modern large ground-based solar telescopes are invariably equipped with
adaptive optics systems to enhance the high angular resolution imaging and
spectroscopic capabilities in the presence of the Earth's atmospheric
turbulence. The quality of the images obtained from these telescopes can not be
quantified with the Strehl ratio or other metrics that are used for nighttime
astronomical telescopes directly. In this paper, we propose to use the root
mean square (rms) granulation contrast as a metric to quantify the image
quality of ground-based solar telescopes. We obtain semi-logarithmic plots
indicating the correspondence between the Strehl ratio and the rms granulation
contrast for most practical values of the telescope diameters (D) and the
atmospheric coherence diameters ($ r_0$), for various levels of adaptive optics
compensation. We estimate the efficiency of a few working solar adaptive optics
systems by comparing the results of our simulations with the Strehl ratio and
rms granulation contrast published by these systems. Our results can be used in
conjunction with a plausible 50 system efficiency to predict the lower bound on
the rms granulation contrast expected from ground-based solar telescopes.
