Help needed with how to solve tilt

[Michael1971]

I had some topics allready about my oblong stars problem.

Spend many hours trying to nail the problem.

After reading this interesting document from Craig (Stark Labs) http://www.cloudynights.com/item.php?item_id=2755 I think I know pretty sure what the problem is but not how to solve it.

Its probably tilt. I only have no idea how to proceed.

I checked my camera with a refractor and so the camera is probably ok. So what remains:

- coma corrector could be faulty, lenses shifted for some reason ( got a 2" that looks like a GSO type CC)

- the tilt is in my focuser, although I use a special compression ring to hold things central

- something else?

The only step I can think of is to try imaging with short exposures without the CC

Does anyone know what else I can do to eliminate this problem?

Thanks in advance

Mike

[Uranium235]

There is a tilt adapter you can get in order to adjust it out, something like this:

http://www.altairastro.com/product.php?productid=16549

[Michael1971]

The word "adapter" makes me nauseous... lol

If I would get my money back for all the adapters I have , I could buy a fine new scope..

[nytecam]

Are you sure it's not flexture of the guider against the imaging scope?

[Michael1971]

well 100% sure no, but I tightened everything as much a possible.

Further more, flexure would be seen in in the images as movent in one direction which I am not been able to determine yet.

[ollypenrice]

To eliminate guiding and flexure issues just take some short exposures in the Milky Way. Short subs won't trail due to guiding.

Adapters are expensive. The AA tilt adjuster may be just what you need. There are more expensive ones from Gerd Neumann...

Olly

[Michael1971]

Here a sub of NGC 7243 which I just took, guided with almost a flat line just 60 sec

Just horrible... beginning to think the mounts is not ok..

I Just noticed that my RA clutch was not locked.. WTH!

Will make some other subs..

[Michael1971]

Here a guided 60 sec of m39

......

[Michael1971]

And here a 60 sec guided sub of Deneb without a coma corrector

[tferrari]

My bet is that your camera's sensor is not perpendicular to the optical train. This non-orthogonality can be caused by many things, but it often caused by the focuser not being parallel to the optical axis.

Hopefully some other users can chime in on how to test to see if this is the case.

[Breakintheclouds]

I recently went through a similar issue as you - I had eggy stars which, after checking my guiding and so on, I decided were most likely caused by the camera sensor not being completely perpendicular to the optical axis of the scope.

The thing that leaped out at me as a probable problem was the two thumbscrews that hold the camera into my Skywatcher Quattro focuser tube. My camera/Baader coma corrector could not achieve focus if I pushed it all the way into the tube, and with it only part-way in, the fact the thumbscrews were both on one side of the focuser tube meant they almost certainly had to be pushing the camera off-axis.

So the challenge then became to find a way of being 100% certain the camera was completely centred in the focuser tube, as from there I would either fix my problem or at least eliminate one explanation and move on to seeing whether the focuser itself was misaligned.

After much faffing, and after buying a focuser extension tube that proved slightly too long for the job, I solved this recently by using the tiny ring that comes with the Baader coma corrector. This, when screwed in position, protrudes out from the camera/corrector assembly and provides a hard surface that I can press firmly against the end of the focuser tube before tightening the thumbscrews. This completely solved my oblong stars problem at a stroke.

So similarly, I'd suggest you find some way of being 100% certain that the camera is mated into the focuser tube with no slop - it needs somehow to be pressed RIGHT against the end of the focuser all around. Once you've accomplished this, you either solve the problem or at least know that it probably lies in the focuser itself not being mounted completely perpendicular. Does your camera go all the way into the focuser so that, with all your thumbscrews loose, you know it's definitely sitting perpendicular? If not, is there anything to hand you can use -- rings, extension tubes -- so that you can accomplish this?

[Michael1971]

Many thanks for all your comments.

@breakintheclowds:

Its annoying with these focusers and coma correctors that you can't adchieve focus while the corrector is all in the focuser tube.

Therefor I have a parfocal ring in place and I use a this https://www.teleskop...Newtonians.html

You can see in the attachment how I currently have it attached. I think you have it a bit like that to?

Still, the problem remains.

Maybe I should get myself a focuser extension tube so that I can put the CC completely in.

[Breakintheclouds]

Michael,

Yes, That's very close to what I have. Does that protruding ring butt up right against the end of your focuser? If so, you've got your camera perpendicular to the focuser and you then know with considerable certainty that your problem lies elsewhere - most likely the focuser is not itself sat quite on the optical axis

[Michael1971]

Yes, I try to get it tight against the focuser. But this ring is hold with 3 grub screws it might also be that its not 100% perpendicular on the CC

If its the focuser, do you know how to check / fix that? I know there are some screws to make adjustments, which I also did some time ago trying to align the focuser with the card method, but now I have no idea to adjust to what.

I am looking at it like a monkey at an old watch..lol

[Breakintheclouds]

But this ring is hold with 3 grub screws it might also be that its not 100% perpendicular on the CC

Bother, that complicates things a bit - I was lucky as my protruding ring is screwed on and so I know it is in the right plane. Any chance you have a length of nicely machined tubing lying around that's slightly wider than your corrector? I know I've got various extension tubes and so on. If you have something like this, that's slightly wider than your corrector and, say, 2-3 cm long, you could perhaps sandwich this between your parfocalising ring and the fat ring attached to your camera and ensure the parfocal ring is perpendicular that way? Or, if you've got a micrometer, you could check your parfocal ring is equally distant to the fat ring all around? I've never adjusted a focuser, so I hope someone else can help with that, but I believe you need a micrometer for that anyway, so if you can get one you could first ensure your parfocal ring is sitting right with it.

[Michael1971]

I just used a long cheshire, just arrived today, to check the rings around the primary. These are the reflection of the focuser tube if I get it right.

These need to be centralized and I made a few corrections there to.

Good idea to measure the distance of the ring, I eyeball it normally. Thats what I am going to do next.

Thanks!

[ollypenrice]

The images look like tilt to me, worse on one side. That's fairly classic.

Push fit into focusers has no place in fast optical systems. It's asking for trouble. You need screw fit at fast F ratios. A tilt adjuster would likely be a big help.

Olly

[tferrari]

The images look like tilt to me, worse on one side. That's fairly classic.

Push fit into focusers has no place in fast optical systems. It's asking for trouble. You need screw fit at fast F ratios. A tilt adjuster would likely be a big help.

Olly

Sorry to hijack the thread, but what options are available if the focuser has a compression fitting (in a fast optical system) but you want to use something threaded? Is it possible to insert an adapter into the compression fitting that adapts to a threaded setup? You could spend some time ensuring the adapter is centered and orthogonal and then leave it permanently in the scope? This would be helpful for me as I don't want to have to purchase a new R&P focuser so that I can use screwed connections. Thoughts?

[Michael1971]

Sorry to hijack the thread, but what options are available if the focuser has a compression fitting (in a fast optical system) but you want to use something threaded? Is it possible to insert an adapter into the compression fitting that adapts to a threaded setup? You could spend some time ensuring the adapter is centered and orthogonal and then leave it permanently in the scope? This would be helpful for me as I don't want to have to purchase a new R&P focuser so that I can use screwed connections. Thoughts?

I think you need something like this, http://www.teleskop-...ton-Auszug.html only I dont think its easy to reach focus this way, although with my system..

[tferrari]

I think you need something like this, http://www.teleskop-...ton-Auszug.html only I dont think its easy to reach focus this way, although with my system..

Thanks for reply. It looks like that still needs to be threaded into the focuser? There are no threads on my focuser, so I don't think this would work.

Any other ideas guys?

[Michael1971]

If you unscrew your compression fitting, if thats possible with your scope, don't you have threads there?

[tferrari]

The compression ring is recessed within the 'thickness' of the focuser draw tube. It basically just snaps into the spot that has been machined out. I suppose that I could take the tube to a machine shop and have them machine out a proper ID and create threads, but it might be easier to buy a whole new focuser.