Kevin_A if it were tilt I would expect different elongations but what I am seeing is very bad long fuzzy comet coma in two kitty corners and not just on one side or one corner.
I was also getting terrible fuzzy (coma filled) stars on some corners before I started to focus [sic] on getting tilt adjusted to 0.02-ish mm.
For a quick look, take the lens out of focus so you get large Airy dics (HFD perhaps over 15). Are the corner disks all circular? If not, it could be a tilt problem. If none of the corners is circuar, it is a backfocus problem. So, at least get the backfocus adjusted first so that at least one corner has a circular disk.
If the non-circular disks turn circular when you change backfocus, it is a tilt problem. I.e., as you change backfocus, the Airy disks of different corners become circular. This is just a 1 minute exercise if you have the Askar backfocus adjuster.
BTW, the 40mm Sigma is not perfect. When tilt, backfocus and focus are adjusted properly (so that the corner stars are tack sharp), you can see a small red spot deviate (x-y) from the "white" spot. Tiny amount, perhaps less than a pixel or two, but it is visible. You can see this in some of the Takahashi spot diagrams, by the way.
When slightly defocused, this looks like coma, but it is just a short red tail. Not 100% APO from red to blue.
I have decided to just use the tilt screws to create a +/- 0.05mm offset for backfocus, by the way (use the Blue Fireball metal shims to get down to 0.1mm). I has considered using a 54mm stop plate, but am afraid the stop plate will itself introduce 0.02mm type errors.
However, I will create a larger tilt plate, so that the screws will clear the 90mm body of the camera. I.e. place the tilt screws 90/2mm + 1.5mm from the center of the tilt plate. A 95mm or 96mm plate should work. Then stick in 4 (non-ball head) long 2mm L hex wrenches into the pull screws, and strop the four to the body of the camera. The ball head hex wrenches do not provide very precise angles, clearing the camera body (by half the diameter of the wrench -- that 1.5mm) will allow the use of non-ball headed wrenches.
I may have to buy an additional tilt plate as I want to keep the 2600 as is as it is fine on all my other scopes.
If you do, get the new one, which is a pair of plates. This lets you adjust from the back of the camera (like what I am planning for my next set of tilt plates). You still end up with 5mm backfocus consumption, same as the original.
https://www.zwoastro.com/product/m54-tilter-2/
Like I said, I think they send the crap to Canada as my last Rokinon buy took 3 copies to get a decent one so I am not surprised by another sub par copy.
You may just have had 2 copies with a bad tilt. Not bad glass. If so, you should be able to correct by just correcting tilt. I am not kidding when I say that the backfocus is sensitive to even 0.025mm correction.
Now that I understand the problem better, I m going to one day play with the Sigma 135/1.4 ART again. I think now that it was just a tilt problem.
I don't think it is a Canadian problem. Heck, my Samyang and Rokinon came from some Amazon third party that sells candy, even though the web page says ships and sold by Amazon. The Sigma came from a NY electronics store that sells compuer games.
Chen
