An Airy disk is a bright, circular spot of light formed in the centre of an image.
It happens as light passes through a circular aperture, such as found on a telescope, and is a result of the light waves being diffracted.
By aiming a telescope at a bright star, which is essentially a point source of light, centring it in the field of view and then defocusing the star image, close inspection will reveal a series of bright and dark concentric rings.
By examining the shape of these rings, astronomers are able to identify optical and collimation issues in their telescopes.
You could also do this by constructing your own artificial star.
Have you got a question for Steve? Email your queries to scopedoctor@skyatnightmagazine.com and they could be answered in the next issue of BBC Sky at Night Magazine.
