Logarithmic stretch of a simulated PSF with (top left) no central obstruction and (top right) a relatively large central obstruction. Pupil geometries are shown in the insets. Bottom: coupling efficiency to a SMF from the PSF of a telescope as a function of the relative size of the central obstruction. The simulations were made with and without spiders. Grey vertical lines indicate the relative size of the secondary for several major observatories. We note that DKIST has a line at 0.