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Bobby this is an area about as clear as mud.
Mirror quality is often expressed as 1/X wavelength, this generally refers to P-V values (ie peak-valley) not RMS values which are often smaller by a factor of 10-20.
Conversely when you see claims like 1/20 or 1/50 wave this is most likely RMS, and the P-V value could be barely ¼.
It originates from the Raleigh criterion, which proposed a wavefront error not exceeding ¼ wavelength. For a newtonian primary that translates to a surface error not exceeding 1/8 wave, although that assumes the secondary is essentially perfect.
A few companies test the complete scopes and provide the test results. For example Intes scope generally have a test certificate of the complete scope and Intes guaranteed wavefront error less that 1/6 P-V for their maks, or 1.8 for the deluxe models. These scopes however command a premium even secondhand - and for good reason - optically they are exquisite.
Another measure is Strehl, where 1.000 means perfect. 0.9 corresponds to ¼ wave P-V error, 0.95 corresponds to 1/8 wave, 0.975 corresponds to 1/16 wave.. To measure Strehl requires an optical test lab that can run a double-pass interferometry test (DPAC).
FWIW my scope is 0.965 strehl... visually on bright stars in good seeing it shows a very tight text-book perfect diffraction pattern if you crank the magnification up to 600X.
A really, really hard test is Dawes criterion - you can google that for yourself. Basically it requires magnification around 3X per mm of aperture and very few scopes can run to that, usefully.
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