This is a postscript to my testing article found here: https://www.cloudyni...elescopes-r3095
A while back, I posted an article about testing the two C14 Edge scopes that I have in my shop. The first scope that I tested is the one that I most often use for imaging and it tested at a Strehl of 0.845 (0.065 waves rms.) Anything over 0.8 meets the Maréchal criteria for a diffraction limited system, so that's not a bad system. Still a number of folks were surprised that the system didn't measure better.
Since I did the original test, I've installed a carbon fiber tube on the telescope so I scheduled another session on the interferometer, which I just finished today. Along the way, I've seen a number of articles showing that it is possible to improve the wavefront quality of Celestron systems by simply rotating the corrector plate to minimize wavefront errors. That seemed like a good thing to try since astigmatism was the predominant wavefront error. I also knew that the secondary was nearly a perfect sphere (tested in another session on a WYKO 6000.) I was able to get the telescope aligned fairly quickly and positioned the field point to be within about +/-0.5 mm from center of the field. I used all of the same components as in my original report so I didn't go through all of the calibration steps that I did the first time around.
I then rotated the corrector plate assembly by~ 90 degrees and realigned the secondary. The wavefront improved dramatically, and I could see that it might do better with a bit more rotation. So I rotated the corrector assembly by another ~15 degrees. Measurements showed that the astigmatism term was reduced by more than half to about 1/20 wave. I was able to dial the coma term down to 0.010 waves and the total wavefront error decreased to 0.235 waves PVq(99%) [meaning that 99% of the data fell within a range of 0.235 waves] with an rms of only 0.043 waves and a Strehl of 0.93! The "measurement" was actually an average of 164 phase measurements to average out air turbulence. I've attached the "shop screen" display from the 4D Foresight software. This level of wavefront performance on a 14" telescope is exceptional--regardless of the manufacturer.
This result clearly implies that the clock alignment marks on the Celestron components are not very accurate. I think that this may occur either because the sensitivity of the test used to place the marks is not very sensitive, or that there is a systematic error in the procedure.
I didn't try to realign the second scope but I suspect that it might benefit from a similar "re-alignment."