Expected Strehl ratio as a function of redshift (top) and wavelength (bottom). In both plots, we assume α = 2/3 and N = 1.8. In the top figure we examine the change in Strehl ratio expected for a point source with varying redshift, z, for an observed wavelength of 400 nm. In the bottom plot, we specifically show the case of a z = 6.3 quasar, as examined by Tamburini et al. (2011), for an observed wavelength of 400 nm. As in Fig. 1, the dashed line refers to the use of comoving distance, dotted line refers to the naive use of luminosity distance, and dash-dot line refers to the treatment in Eq. (9).