Complex sentence: Diffraction effectively rules this technique out for mirrors not much closer than the distance at which the mirrors Airy Disk is equal to the size of the other mirror: around 150 km for a 1m diameter mirror, scaling linearly with larger diameters.
Split into simpler sentences: Diffraction effectively rules this technique out for mirrors not much closer than the distance at which the mirrors Airy Disk is equal to the size of the other mirror. Around 150 km for a 1m diameter mirror, scaling linearly with larger diameters.