I was testing the scope out tonight (it's giving fuzzy images) and noticed two things:
1. The scope is somewhat out of collimation, which is affecting the images.
2. There is a large amount of spherical aberration. The secondary looks HUGE out of focus (around twice the size it should be) and rather small when inside focus, so that would indicate overcorrection.
Here's a test report that looks similar to what you are describing. This scope looks like a, "piece of junk", but it may not be indicative of all VMCs out there.
With a regular scope, yes, but this thing has a sub-aperture corrector attached to the secondary. Wouldn't misalignment alter the performance?
Misalignment will affect performance, regardless.
On one site of focus on a star I have perfect collimation (at 128x magnifcation) but on the other side it is ever so slightly out of collimation.
No, it's not the eyepiece. I've seen this. I believe it's a mechanical alignment problem. The focusing mechanism doesn't follow the optical alignment through focus. Mechanically it deviates slightly through focus forcing the miscollimation on one side of focus (or both sides). In focus, though, it is optically collimated pretty much. But, you may see some stray light off to one side. If you can imagine the mechanical axis is "looking" at the optical axis (and the diffraction artifact) at a small angle, even in focus. When you move up the mechanical axis, the mechanical axis deviates to one side of the optical axis causing the apparent misalignment when out of focus.
I had this problem after I disassembled my MCT and put it back together incorrectly. A simple rotation of the primary mirror cell by 90 degrees put it back to it's factor setting with much better mechanical alignment. It fixed the intra and extra focus offset we see. Fixing the mechanical alignment solved the optical misalignment problem as I scroll through focus. The two axis, mechanical and optical, are more coincident with each other. But, what matters most is in focus optical collimation. Still, it bothers us to see the optical offset on either side of focus.
I am not a big fan of the sub aperture corrector. Light has to travel through them twice, once in then back through off the secondary. Lot of room for error there and compounded error at that. Star testing these things can be tricky because the wavefront is not designed to be spherical as with a parabolic mirror. It has a unique design wavefront that approximates a spherical one, so it will look differently on both sides of focus. But, the design can be good, regardless, if executed well and even with a lot of higher order spherical aberration. Notice in the star test image above, the system looks over corrected in what is likely an under corrected system with a lot of primary spherical aberration coming off the primary mirror (which is why there is a corrector installed). As I understand it, the difference is higher order spherical aberration which can be quite severe without much image degradation. The final proof is in the in focus image.
Edited by Asbytec, 22 October 2018 - 07:08 PM.