When the OP said this is an SCT, the comments above made sense as do the "non standard" (complex) patterns presented. The images are not showing classic primary spherical alone, as Glenn said, likely with some amount of secondary spherical. Secondary spherical with primary spherical is not as damaging to the image as the same amount of either, especially if they have the opposite sign. So, importantly, I would not fret over this being a bad optic based on appearances.
Nothing new here, just looking it over and making sense of what has already been said. Evaluating SCTs can be kind of difficult sometimes, but it's good practice to think it through. The corrector does some magic to the planar wavefront causing it to behave differently from pure primary spherical. I got a geeky chuckle as this seems to be what prompted Glenn to ask if the scope is a MCT, but an SCT makes sense in this context, too. Both "correct" the wavefront before the primary mirror. That's what we are seeing in the defocused images.
"Isn't one image supposed to be larger than the other, and CO shadow smaller than the other, if SA is present?"
Yes. With under correction, the marginal zones focus closer to the mirror. So when you're inside focus you are closer to marginal focus, the diameter of the slice will be smaller than outside focus. Outside best focus the slice is a bit further from marginal focus and the diameter of the diffraction artifact is larger. Of course, with over correction this is reversed. This is not what the OP shows, however, so this is not entirely what is happening.
"...but there is a zone error causing the CO shadow to be different."
Sure. I believe and understand the shadow size is influenced by the inner zones, so there seems to be a clue as to what the inner zones are doing. With over correction, the inner zones focus short of best focus and the shadow is smaller on the intra focal slice and more expanded on the extra focal slice. So, at least the inner zones appear to be focusing short of best focus indicating some level of over correction as was mentioned many times, already. The edge is likely doing something different causing the shadow breakout to be less reliable reading an overall correction error into it. So, in this case, I'm not sure I'd rely on the shadow breakout to indicate the level of over correction, maybe only for the central zones.
"...cause some off the visual on intrafocal looks more like under but the co sizes show over."
It's hard for me to tell what's going on with the marginal zones, but it's sometimes possible to see indications of both under and over correction at the same time. I believe, as Glenn said, there is some residual secondary spherical present and most likely due to the corrector. With defocus either side the diameter of the diffraction pattern is very close to the same size indicating approximately the same amount of divergence of the marginal zone. So, as a guess and because of the secondary spherical, it may be the marginal zones are focusing close to best focus. If so, I'd argue that's actually a good thing as the edge contributes more energy to the image than the center.
"...and possibly a TDE, based on the softer outer edge of the infra focal image."
This makes sense, too, again due to the corrector. The edge may be focusing a tiny bit long giving the appearance of a very slight turned edge. Again, it's another form of the general over correction we're seeing. It's just a matter of how much further out the marginal zone is coming to focus. I would not hazard to guess other than to say, "not far." It seems to be a normal consequence of the design in some cases, and likely in this case. I'd like to see a little more defocus, too, out to a full 10 waves defocus to see what happens. And see if a more controlled exposure can give us more information on the outer zones.
"It is an SCT, but I intended this to be an exercise in star testing, not so much a telescope critique."
Nothing wrong, to my mind, understanding your equipment. Dig in and get your feet wet.
The only reason I chimed in was to put all the above comments into context as everyone, including the OP, seems to be hitting on the "error" seen in the images and their non standard appearance. Each comment above makes sense in this context. The OP does not have pure over corrected primary spherical, but some hybrid version with secondary spherical included. That's not necessarily a bad thing.
Again, and I think importantly, it's important not to fret over the asymmetrical images above thinking it's a bad optic with asymmetrical patterns. They are normal and by design to some level of perfection. The answer is more complex because it is not testing like a parabola, rather a more complex design making it hard to read sometimes. The hybrid star test is very likely not as detrimental to the image as pure LSA alone, especially when each is of opposite sign.
As to how much of each is present, I'd not hazard a guess (or offer a PV estimate) other than to suggest in focus is where it counts. I'd bet it's actually pretty good sample despite the (seemingly ugly) asymmetry of the diffraction pattern and differing appearance of the shadows. My guess is it's just fine in focus and pretty snappy, too. Am I wrong?
Edited by Asbytec, 30 May 2019 - 03:56 PM.