Conclusions

In this chapter I have discussed the timescales for high resolution imaging found both in experimental work by other authors and in my own simulations. The temporal properties at a point in the telescope image plane for simulated observations are compared with experimental measurements and found to agree qualitatively. For atmospheres with a small scatter in the wind velocities the coherence timescale for speckle observations is found to be proportional to the telescope diameter, as predicted by Aime et al. (1986); Roddier et al. (1982a). This coherence timescale is expected to be significantly longer than the coherence time applicable to current designs for adaptive optics systems. Measurements made at the NOT by Vernin & Muñoz-Tuñón (1994) suggest that a factor of two increase in coherence timescale would be expected for speckle imaging at this telescope. The dependence of the atmospheric coherence time on the $C_{N}^{2}$ profile and telescope aperture diameter is found to agree with the predictions of Aime et al. (1986) for the two atmospheric models tested, apart from a small difference in the constant multiplying factor for one of the atmospheric models.

A direct analogy can be drawn between the calculation of the coherence time for speckle imaging and the isoplanatic angle. The agreement between the timescales measured for simulations and previous theoretical predictions thus also indicates agreement between the isoplanatic angle measured in simulations and theoretical predictions. Measurements by Vernin & Muñoz-Tuñón (1994) suggest that the isoplanatic angle for speckle observations should be approximately $70\%$ larger than that which would be obtained for adaptive optics at the NOT site. The isoplanatic angle at I-band is predicted to be between $3$ and $5$ $as$ from the results of Vernin & Muñoz-Tuñón (1994).

The fraction of short exposures with a Strehl ratio greater than $0.37$ in numerical simulations is found to be consistent with the Monte Carlo simulations of wavefront perturbations by Fried (1978). The maximum aperture diameter for which exposures with a high Strehl ratio ($>0.3$) are frequently obtained is shown to be approximately $7r_{0}$. The probability of obtaining good exposures decreases very rapidly for larger aperture diameters.