Strehl number

The Strehl number or the Strehl ratio is a measure of the optical quality of telescopes and other optical instruments. It is named after the German physicist and mathematician Karl Strehl (1864–1940).

The Strehl number is defined as the ratio of the observed maximum intensity of a point source in the image plane to the theoretical maximum intensity of a perfect ( diffraction- limited) optical system, so that a perfectly imaging system would have a Strehl ratio of 1.