I'm not an optician. Will you tell me what I am looking at?
Primarily the Strehl ration at 0.96, which is very good. It means the scope is putting almost all of the light where it's supposed to be focusing most of it into the Airy disc and some into the rings (which is unavoidable due to diffraction of the aperture and the obstruction, primarily, and some minor aberration.) A Strehl ratio of 1.0 is perfect (and a pipe dream), somewhere down toward 0.8 is acceptable. So, 0.96 is about as good as we can expect a good scope to be. No scope is perfect.The test was done in green light where the eye is most sensitive.
The PV wavefront error is Lambda/6 or 1/6 peak to valley in green light. I believe this is the peak deviation in the wavefront from perfect, not an average deviation. So, this says the maximum deviation is 1/6th PV wave, which is better than some diffraction limited criteria. The RMS is also better than other diffraction limited criteria and is used to compute the Strehl ratio. The intra and extra star test images are smooth, with the extra focal image showing a bright inner ring. This is an indication of some undercorrection where the central zones focus a little long, but that's not a big deal. The edge has more surface area and contributes more to the image than the center. The shadow of the secondary is darn close to being the same size on each side of focus. Both of these indicate a very good level of correction. Plus, the Ronchi lines are pretty straight, the straighter the better.
TG was being sarcastic, this is a very good sample C9. As was mentioned above, the C9 has a slightly slower primary mirror which makes correction a tad easier. More forgiving. So, the tendency seems to be they have a slightly better chance of being well corrected.
Above, you spoke to the 10" Dob operating well under your seeing conditions, but the C9 seems to suffer, regardless. This speaks to the C9 as having some unique problem of it's own. Thermal equilibrium comes to mind, right off, as why the difference when both scopes are operating in the same climate on the same night. The 10" Dob likely has better optics, as I gather from your post, but it is also obstructed. The differences in the obstruction are not likely causing this level of degradation in the image unless you are a connoisseur of exquisite optics and can tell the difference in your seeing conditions. But, even then and at such high standards and expectations, I cannot swear the C9 is really under performing. It may be performing as advertised, but the OP is just not happy with the image. But, if it is performing as advertised, then thermal management comes to mind, again. Even if the C9 is not as good as TG's sample above, you should still be getting nicer images than what is implied. So, unless this is another C9 sucks thread implying the scope is just incredibly poor optics near Lambda/2 (as a rule for all SCTs), I doubt the optics are the cause unless collimation is off.
SCTs are notorious for thermal problems. I have seen thermal plumes in the star test well into the evening. The problem is the interior mass of the primary and it's cell retain heat and conduct it slowly causing a slight temperature differential as the OTA begins to cool. You'll see a heat plume rising up from the center (appearing to rise near the shadow of the secondary) of the defocused star test as the massive mirror and supporting structure radiate residual heat into the interior of the OTA. This is enough to turn otherwise sharp planetary images into mush. The same thing will happen in the 10" Dob as the mirror cools. So, both scopes need to be thermally stable. The SCT needs some help getting there. My guess is, even after sitting outside for a while, the problem is likely thermal. A quick check for the tell tale thermal plume in the defocused star will nail it or eliminate it as the cause. If you haven't done so already, I'd rule that out conclusively before worrying about the optics.
It could be you have a very poor sample, but it's outside the peak of the bell curve and not indicative of all C9's out there. Not enough to say C9's are, in general, poor planetary scopes.
Edited by Asbytec, 22 October 2018 - 05:32 PM.