mathematical models can give an accurate model of the effects of astronomical seeing on images taken through ground-based telescopes. three simulated short-exposure images are shown at the right through three different telescope diameters (as negative images to highlight the fainter features more clearly—a common astronomical convention). the telescope diameters are quoted in terms of the fried parameter formula_1 (defined below). formula_1 is a commonly used measurement of the astronomical seeing at observatories. at visible wavelengths, formula_1 varies from 20 cm at the best locations to 5 cm at typical sea-level sites.