FWIW, I suggest people look at the older post about Strehl here: https://www.cloudyni...7-strehl-ratio/
In particular it is worth checking out the article by Zambuto that is referenced in the discussion, and is here: https://zambutomirro...oopticalca.html
One comment by Zambuto that stands out for me is this one:
"The Strehl ratio is a measurement of the large scale (geometric) surface. Regardless of the method of test being used, including the most advanced optical devices in existence, criteria such as medium scale surface roughness and turned edge are not taken into account." - this is when discussing a mirror that has tested at 0.99 Strehl, but has serious machine-induced ripples (see the discussion on "Criteria 4") that cause the image to be degraded quite badly when compared to a smooth figured optic.
While this article is about mirrors, the same criteria apply for figuring of refractors: and from memory, Roland Crouch had very similar things to say about optical quality, and didn't therefore originally give measured Strehl for his scopes. I believe AP and SV and others do now because some amateur astronomers now expect it.
Strehl ratio gives a useful indication of quality, but there is a lot more to it than simply saying "my scope is 0.98 and is better than your scope at 0.94"...
Like several comments above, I seriously doubt whether anyone can tell the difference (using the same diagonals and eyepieces) between 0.98 and 0.94 simply by looking through them if both scopes have smooth optics and have been tested in the same lab under the same conditions.
Edited by DeanD, 01 May 2021 - 12:06 AM.