The theory of signal detectability is used to evaluate the effects of instrument aberrations on a total visual optical system with the human eye included. A quantitative measure of grating detectability is obtained using a single-stimulus, random-presentation procedure, with multiple-rating responses. The results indicate that the detectability of square-wave gratings, viewed through a telescope, decreases as aberrations are introduced in the instrument. The amount of degradation is determined for each of six aberration conditions. This degradation index can be used to obtain the degraded modulation transfer function of the total system for each aberration condition.