Greetings, I'm trying to collimate an older AP oil spaced triplet (6" f/12.5). It has a collimatable cell, and I noticed that when I received it, there was a significantly larger gap on one side of the 2 piece cell than the other. Curious about this I checked the reflections in a cheshire and they were significantly off. Here's a photo captured with a ColliDream, the 2 crosses should coincide:
So I collimated it so they align and views were good. Seeing was average so I couldn't push it much past 300x. I checked out a star for seeing the in/out of focus pattern and noticed that it wasn't good at all. Quite a bit of astigmatism (elongated one direction inside of focus, the other direction outside of focus). The airy disk was not round, and the bottom half of the airy disk also flared out and was blurry. Stars in focus had noticeable flare to the bottom as well.
Here's a test I did this morning just doing a terrestrial artificial star (car parked about 100 yards away) showing the inside/outside focus. This was several turns away from focus, so the closer you get to focus the more pronounced it becomes, but this demonstrates that I'm not hallucinating:
I thought it might be temperature related so I let it cool down without the dew shield for an extra hour. Scope had been outside for about 3 hours at this point. Still the same issue. Tried different diagonal and eyepieces, no change.
I checked how close the focuser was to being centered on the lens with a laser and the dot on the objective was within a 1/2" of the center. I don't see any adjustments to perfect this so may need to put some shims somewhere on the back end, but I don't believe a VERY slight focuser tilt would cause what I'm seeing, if anything, maybe field flatness with a camera might be noticeable upon close inspection.
So I returned the collimation back to close to where it was before, and I saw the star test returned to close to circular and more even, but couldn't assess it very well, the out of focus was quite a bit blurry compared to the inside focus. I wouldn't say I'm a very experienced star tester though, usually the cheshire method gets it bang on every time.
I made sure the lens retaining ring wasn't too tight, the lens has a very slight rattle when you jiggle it in the cell, but it doesn't have much room to move, fraction of a mm I'm guessing.
Any ideas on what I should try next or what the issue could be? Also it's worth noting that views were pretty good with both collimation points. But with an AP lens like this, I would expect a close to perfect star test with negligible astigmatism visible, right?
Edited by jragsdale, 05 April 2025 - 11:17 AM.