Is this colimation, tilt or something else

[mackiedlm]

I have been imaging with my LX200 GPS 8" for a couple of years using a 0.8ff/fr and a ASI2600mc. While not brilliant the stars were ok and acceptable to my eye and I produced some OK images (for that set up!)

I then swapped the meade electric microfocuser for a crayford, to allow for autofocus. This required significant movement of the primary mirror to reach focus.  Right away with this focuser my stars went crazy with bad shapes (coma?) accross the entire field. I assumed it was a tilt issue from the new focuser and worked to resolve that with no success.. I then refitted the meade focuser but the problem is still there.

Images to illustrate

These are un debayered single images of 180s with a simple stf stretch apprlied. The PI Abberation Inspector images show the problem more clearly.

 

 

 

There may be some tracking error in these images but the starshape issue is seen even in 5s images so this is not tracking. 180 s images were my normal exposure previously and did not have this issue.

I did star collimation before these images and thought I had done a reasonable job. But I would not claim a high level of competence in collimation. I know the focus looks soft but it was done with autofocus in APT (I worked out how to get that going on the meade focuser) and I think it is just the abberation that makes it look that way.

Connection to the scope is with a 2 inch nosepiece as its not possible to do a screw connection. And that nosepiece connection was working fine before I messed with the focusers.

Image 2 is after a rotation of the camera and FF/FR in the eyepeice holder and shows that the abberation remains the same relative to the target while the FOV rotates. This suggests to me that any issue is scope side of the connection between the camera and the FF/FR. I guess its possible there could be tilt between the nosepiece and the eyepiece holder but I doubt if the abberation would remain as unchanged if it was that. I did try ensuring a good alignment and again, it was not present before I removed and the replaced the meade focuser.

This is leading me to consider that the issue could be either a collimatio issue or tilt of the primary mirror, although I have read that that is highly unlikely.

I would greatly appreciate any guidance on where I should look to resolve this.

Can we stay away from backfocus, questions of which FF/FR etc because the camera, reducer, nosepiece system has not been changed or seperated from when I was getting acceptable results during the winter.

Thanks

 

David

Edited September 30, 2023 by mackiedlm

[Elp]

As it's worse on the one side I'd say its tilt. Did you take an image of your out of focus donutted stars to look at the collimation?

[CraigT82]

The last image in your post shows all the stars with coma and all pointing to the bottom left, so I’d say collimation is off and the sweet spot is off to the bottom left somewhere 

[mackiedlm]

6 minutes ago, Elp said:

As it's worse on the one side I'd say its tilt. Did you take an image of your out of focus donutted stars to look at the collimation?

thanks @Elp

 

This how it looked.

 

Which seems to suggest that theres a point in col in the lower left. and suggests, I think, its not simple tilt.

 

But if it was tilt, where in the sct should I be looking to adjust?

[mackiedlm]

6 minutes ago, CraigT82 said:

The last image in your post shows all the stars with coma and all pointing to the bottom left, so I’d say collimation is off and the sweet spot is off to the bottom left somewhere 

thanks Craig, yes thats my thinking too and I think supported by the donuts I just posted.

 

But I really am lost here so open to all suggestions.

 

Edited September 30, 2023 by mackiedlm

[Elp]

The collimation is way off, and looks to be exasperated via tilt. Regarding the tilt, I'd look at the camera image, or use an eyepiece and try to nudge the rear setup to see where the play is. Clamped fittings aren't the best when imaging. When I use the F6.3 reducer on my C6 it's all screwed together from the visual back.

[mackiedlm]

1 minute ago, Elp said:

The collimation is way off, and looks to be exasperated via tilt. Regarding the tilt, I'd look at the camera image, or use an eyepiece and try to nudge the rear setup to see where the play is. Clamped fittings aren't the best when imaging. When I use the F6.3 reducer on my C6 it's all screwed together from the visual back.

Yeah,  I did try to look for play, and with the crayford there was certainly a bit of "wiggle" which is why I moved back to the meade. With that, there is no visible movement and the abberration really does not change if I nudge things around. Even if I loosen, rotate and retighten the aberration does not change - i'd have thought if it was tilt at the nosepiece connection that sort of resetting within the focuser would have resulted in some change to the abberation? Yes I'd prefer screw connections but I cant get a way to screw it together and still have the microfocuser in place.

[Stuart1971]

If that is tilt, then it’s extremely bad, worse than I have ever seen…I would say that is collimation, .check out the thread of correcting tilt in the daytime with a simple laser jig…

https://stargazerslounge.com/topic/382323-chip-tilt-test-jig/

 

[mackiedlm]

6 minutes ago, Stuart1971 said:

If that is tilt, then it’s extremely bad, worse than I have ever seen…I would say that is collimation, .check out the thread of correcting tilt in the daytime with a simple laser jig…

https://stargazerslounge.com/topic/382323-chip-tilt-test-jig/

 

Thanks Stuart, as I mentioned, I dont think its tilt on the camera/sensor or even camera side of the focuser, I'm trying to understand if it could be tilt on, for example, the primary mirror or where else to look.

Edited September 30, 2023 by mackiedlm

[bosun21]

I reckon that when you changed over to the Crayford focuser and you said you had to turn the focuser for ages to allow focus with the Crayford you have slightly disturbed the primary mirror. Then when you reversed the procedure to return to the Meade focuser the mirror didn’t settle in its original position. I think all you need to do is to re collimate the scope properly on a bright star.

[mackiedlm]

18 minutes ago, bosun21 said:

I reckon that when you changed over to the Crayford focuser and you said you had to turn the focuser for ages to allow focus with the Crayford you have slightly disturbed the primary mirror. Then when you reversed the procedure to return to the Meade focuser the mirror didn’t settle in its original position. I think all you need to do is to re collimate the scope properly on a bright star.

Thanks for that. I think, and hope, you might be right. I think I'm going to order a set of BobsKnobs to make collimation easier.

[Peter Drew]

I would start again from scratch with collimation.  Use the Meade visual back straight through with the eyepieces and nail the collimation with the test star as central as possible and always use a counter clockwise approach to the focus.  Next add the focal reducer and check that the image still has the same collimated appearance, ccw approach as before.  Continue the process with all additional components.     🙂    

[mackiedlm]

1 hour ago, Peter Drew said:

I would start again from scratch with collimation.  Use the Meade visual back straight through with the eyepieces and nail the collimation with the test star as central as possible and always use a counter clockwise approach to the focus.  Next add the focal reducer and check that the image still has the same collimated appearance, ccw approach as before.  Continue the process with all additional components.     🙂    

Thanks for the advice Peter, sounds like a good method.