Squashed stars

[BeardFace (Nial)]

Hey all. I've been working on collimating my RC8 using the DSI method, and although I think I'm getting close, I took a 60 second image of M22 last night and noticed the stars are slightly "squashed". Would that still be a collimation issue? Or maybe tilt or pinch? I've attached a closeup of of the stars (near the center) and the link below goes to the full image.

https://drive.google.com/file/d/1rgP...ew?usp=sharing

I'll probably buy a glatter laser soon, but just wanted to make sure I could get as far as I could without dropping more cash.

[multiweb (Marc)]

To me it looks like a combination of things.

1_ you're slightly out of focus so there's a hint of trefoil even on-axis. Maybe pinched optics.

2_ slight elongation in RA so guiding/tracking is not optimal even for 60s.

3_ a bit of field curvature so a corrector spacing needs to be tweaked.

But in all don't beat yourself up. It's still a very good field at that FL. Those are minor defects that can be fixed.
I wouldn't touch the collimation. The field is already symetrical.

[BeardFace (Nial)]

Well that's good to hear, thanks! It wasn't guided but I wouldn't be surprised if it was off balanced in the dec.

Is it possible that the pinch would come from tightening the primary down too hard?

[multiweb (Marc)]

Originally Posted by BeardFace

Well that's good to hear, thanks! It wasn't guided but I wouldn't be surprised if it was off balanced in the dec.

Is it possible that the pinch would come from tightening the primary down too hard?

When you get a trefoil, three points at 120 degrees it usually points to stress in a mirror holder. So if you have three points behind your secondary (collimation screws) or three points in your primary mirror cell then yes it could be one of these two. What happens when temperature changes glass contracts or expands. If it's already tight at 20c let's say when you're up and about and your scope goes down to 5c then the glass has nowhere to go, "no room to breathe". The pinch shows in your stars. Every scope has the same issues. Just loosen up the screws just enough and the pattern will go. Or you can use a crumple zone. A thin piece of cork will give enough way rather than glass direct on alu.