I have tested three different Newtonian mirrors from 12″ to 16″ diameter over the past few years that were labeled as being by Meade. One had been refigured by someone else before I tested it.
Each of the mirrors initially looked nice and smooth, and the Ronchigrams looked pretty close to the theoretically-perfect image generated by RonWin or similar software.
However, when I did a zonal test, in every single case, the mirror turned out to be seriously under-corrected, in some cases by about 50%. In other words, if a perfect mirror should have the outer zone (say the outer inch) to “null out” at 0.236″ from the location where the central zone nulls out, then the mirrors I tested might null out at only 0.118″ instead.
I took care to repeat the measurements several times in each of these cases, and in one case the owner also took a set of readings; his and mine agreed pretty closely.
I don’t know if it’s my skills at reading Foucault/Couder shadow zones that are suspect, or if I’m correct. Anybody else have similar or opposite stories or experiences?