This is the actual diameter in meters of the telescope entrance pupil. For small telescopes, it provides significantly more precision than the rounded value embedded in the telescope name. For large telescopes with segmented apertures, it will suffice to specify the equivalent diameter corresponding to the total collecting area. (This will give the right value for the photon noise, but not for the scintillation noise. Most segmented apertures are on large telescopes whose scintillation noise is negligible anyway, so this is not a significant problem.)