I've been reading up a bit on what a Strehl ratio is and what it means. I understand any optical system with a Strehl of 0.82 or better is said to be diffraction limited. Strehl numbers for optical systems with multiple surfaces like an SCT can have lower numbers, largely due to the diffraction off the central obstruction, and the resolution suffers for it. But Strehl numbers for refractors can be quite high since there is no obstruction. In high end refractors it seems these numbers are often advertised to be above 0.95. I saw a scope on here recently advertised to have a Strehl of .993 which is the highest I've seen. My question is, can a human eye tell a difference when the Strehl is so high? Can a difference between 0.9 and 0.99 even be noticed? Can any difference in Strehl above the "diffraction limited" number of 0.82 even be noticeable? Like I said, I've been researching what this number means but I have not seen anyone mention how much it actually matters.
Edited by cst4, 15 June 2020 - 06:01 PM.