Abstract

Short-time exposures have a higher resolution than long-time exposures. The resolution gain factor depends on the ratio of the telescope diameter D to the Fried parameter r₀which characterizes the atmospheric seeing. This gain first increases with D/ r₀ to a maximum value at D/ r₀=3.8, after which it decreases as D/ r₀ continues to increase. The probability of obtaining a diffraction-limited short-exposure is represented for D/ r₀ greater than 3.5 by the expression: Prob ~ 5.6exp [−0.1557 (D/r₀)²] ( Fried 1966; Hecquet and Coupinot 1985). For the CFHT, commonly observed value of D/ r₀ are quite far from the optimum, typically about 18. So, segmenting the pupil into elements equivalent to smaller diameter telescopes, with more adapted D/ r₀, will allow to achieve the highest possible spatial resolution gain.

© 1989 Optical Society of America