Star Test
An ideal Airy disk will be a small circular spot surrounded be a number of concentric, circular diffraction rings. A telescope that is significantly better than a telescope that only has Diffraction Limited Optics will produce an Airy disk which is indistinguishable from an ideal Airy disk.

While both an Astigmatic telescope and a telescope that is not properly collimated will produce elliptical Airy disks, the cause for elliptical Airy disks can be determined by the test. The uncollimated telescope's ellipses remain the same on both sides of the best focus. The Astigmatic telescope's Airy disks will shift their major axis of symmetry by 90° on either side of the best focus.

Note that the Airy disks formed when the star is slightly out-of-focus are dramatically larger than the Airy disk formed when the star is in focus.  We owe our knowledge of the properties of these disks to one of Britain's Royal Astronomers, Sir George Airy, who first studied them, and for whom they are named.