On the Correction of Astigmatism and Field Curvature in Telescopic Systems

The results are described of psychophysical measurements which characterize the subjective detection and resolution performance in a fixed-focus telescopic system, at various field angles. The targets used are sinusoidal gratings, a three-bar target and non-periodic structures. The telescope is treated as a system with a fixed amount of Petzval curvature, to which various amounts of primary astigmatism are added through a specially designed collimator. A ‘best compromise’ state of astigmatism correction is established for the test telescope. The results indicate that a substantial drop in subjective performance may be expected when one of the two astigmatic fields is outside the observer's accommodative range, and that significant amounts of astigmatism can be tolerated in order to bring both fields within, or close to that range. The results are compared with MTF-based predictions. The conclusion is that in some simple cases the MTF has significant predictive power, but the limitations of the MTF are also shown, arising from the target structure, the form of aberration investigated, and the influence of the resting state for accommodation.