Focus issues

[brisen (Brian)]

I am having issues with focus on a Celestron 11" HD and the reading I have done suggests that the OTA may not be cool enough. I have attached a couple of pictures of Antares which is the best focus I could get using live view focus on a laptop and prime focus. The second and third pictures are what I think is either side of the 'best focus' I could get. It was taken with a Canon 700 D. Is this thermal currents which are causing the star to look like a blob or is there some other possible cause? The pictures were taken about an hour after the scope was set up.

Any ideas would be greatly appreciated.

Brian

[Jon (Jonathan)]

Looks to me more like the collimation is way out of whack. What happens when you defocus the stars a fair bit? Do you get a round light circle with a round dark circle dead centre? If not, you have some sort of alignment issue, most likely the alignment of the secondary mirror, which is very easily adjusted.

[Joshua Bunn (Joshua)]

+1 to collimation. looks like its way out. try defocusing as Jon suggests to see what it looks like.

Josh

[raymo]

Firstly, need to be sure the problem is in the scope, and nothing to do with the camera, so does Antares [or any other star, for that matter] come to proper focus when the scope is used visually?
I don't think it is air currents, because they cause irregular aberrations,
not regular, sharply defined ones like you have.
The images are definitely out of focus because the diffraction rings are
clearly visible. Although horizontally stretched, they are concentric, which
suggests that collimation is probably not the problem.
I noticed that the right hand side of the aberration has a straight, vertical
line, which suggests the edge of something. Is it possible that the camera's
mirror is not moving completely out of the light path, either because it is faulty, or maybe the adaptor you are using is fouling it.
Just some food for thought. This one is intriguing. Looking forward to
your next post.
raymo

[Astroman (Andrew Wall)]

Is there any movement in the way the camera is attached to the telescope? It could be the weight of the camera pulling down on the adapter so the light path is not hitting the chip flat. But as Raymo has said, check visually to make sure it isn't the scopes optics.

[multiweb (Marc)]

Your collimation is way out. Fix that first. That's not camera tilt.

[Jon (Jonathan)]

Originally Posted by multiweb

Your collimation is way out. Fix that first. That's not camera tilt.

Yep, that's what I thought. Fortunately, it's very simple to correct. Google "collimate celestron SCT" and the are heaps of how-ton and videos. 5 minute job, unlike the more fiddly task of collimating a Newtonian.

[brisen (Brian)]

Thanks. I will have a go at collimating it after I have a look at the videos etc. I have only had it a couple of months and havent really had a chance to use the scope and it has been like this since new.

Brian