A solar focus and seeing monitor is described which utilizes the properties of an image of the telescope objective formed after putting a knife edge next to the solar limb. In a very simple way simultaneous measurements are obtained for the telescope focus and for the image motion and blurring produced by the earth’s atmosphere.
J. M. Beckers, "A Photoelectric Focus and Seeing Monitor for Solar Telescopes," Appl. Opt. 5, 301-304 (1966)