star testing collimation after cat's eye tools

[troypiggo]

I have the Cat's Eye kit and after some tweaking to my secondary holder, I can now quite easily get everything lined up with the hot dots and the reflections in the autocollimator.

1. With the autocollimator, looking through the centre hole and the hot dots are perfectly aligned, then look through the offset hole each pair may not be perfectly aligned. If I get them perfectly aligned in the offset hole, they're not in the centre hole. Wondering if this is normal, or if it could be because the hotdot may not be perfectly in the optical centre? If so, should I just average the difference?

2. I understand that ultimately a star test is the final step in the process. Given that the final step in the collimation iterations with the Cat's Eye kit seems to always be tweaking the secondary, is it the primary or secondary that you tweak with the star test? The primary would be easier and more forgiving, but the final adjustments to the secondary seem to minute that I was thinking that's the critical element?

[bmitchell82]

I observe the same in my setup Troy, although it is very minimal so I average between the two.

The template that they send out is for a 250mm mirror not 254mm mirror (strange) so i spent about 15 minutes moving it around with 2 sets of eyes judging.

As for a star test I think unless you have a solid set of spider vanes, its going to be hard to make fine adjustments to the secondary without sending it up the duff.

[Moon]

1.
Sounds reasonable. I also wonder if the secondary offset could somehow impact this? It would be great if someone could run this through an analysis program for us.
2. Are we talking imaging or visual? I always finish with the primary adjustment - with the CCD camera + coma corrector installed.
James

[Jason D]

With the XLK autocollimator, you should be able to achieve aligned images via both pupils. Secondary offset or having an off-center spot should not impact the ability to achieve aligned images via both pupils.

Try to re-iterate. Align the images via the offset pupil by only adjusting the secondary then align the images via the central pupil buy only adjusting the primary mirror.

http://www.youtube.com/watch?v=Vj12cx3tnsM

Jason

[bmitchell82]

Thanks for that little bit of information Jason.

I generally go back and forward between the cheshire and the offset pupil untill they virtually merge. and the center pupil i can only see the hot spot but no other reflections they essentially blacked out. Does this imply that my collimation is pretty much spot on?

[troypiggo]

That video clip was perfect. I had been doing the iterative method, but that first CDP method seems so much simpler. Will give that a go. Thanks, Jason.

[Jason D]

Correct.
If the cheshire and both offests show expected images then you virtually have achieved perfect axial alignment.

Jason

[bmitchell82]

Brilliant because thats what i see in the reflections! Its always good to have proper confirmation of what i have always believed to be correct.

Once again Thankyou Jason for your input

[cventer]

Evening All,

I am resurrecting this thread as I just took possession on my first reflecting telescope ( an f3.8 Orion Optics AG-10)

I have the catseye collimation tools. The Telecat XLS site tube/Cheshire cobo and the infinity XLK collimator. ( one with central and offset pupil)

I have watched videos and followed instructions. I think I am reasonably close. Quite a learning curve I must say having never dealt with reflecting optics before.

Where I am at is exactly the state described above by original poster. My radiation hotspot is centered in Cheshire and stacks are aligned in central view. Problem is in offset view they are not stacked . Once I adjust secondary to stack in offset view they are no longer stacked in central view and vice versa.

If I stack either central or offset with primary centered in Cheshire, my glatter laser puts laser dead center of the radiation spot.

I am just not sure why I cannot get offset and central view to stack as per images and instructions.

I notice in Jason's reply he says to align secondary via offset view and then stack central pupil view by adjusting primary. Is this right ? I thought you only adjusted primary when using Cheshire ? And autocollimator was only for secondary adjustment?

Any suggestions what I can try to stack Hot spots in central and offset views ?

[tlgerdes]

What I found caused the difference after consulting with Jim was the focal distance you are collimating at. Think about where your eyepiece or focal plane would focus at, it is possibly some distance above where you are collimating at. I found that once I racked the focuser out further (had to put an extension in), everything came together. This distance co-incided with where my cameras sensor would have been in focus.

[cventer]

Thanks. I posted on cloudy nights and Jim wrote same thing back to me. I have. 2 inc extension so will try this and see if it sorts out the issue.

One other question for you guys. When doing first course collimation to align secondary under focuser. How are you sure the rotation around center bolt on secondary is correct. Visually the secondary mirror in sight tube appears round but you cannot ate at least 5 or 10 degrees each side and it still appears round so this is not overly accurate. Does it matter ? Which tool confirms the secondary is projecting full image back up into focuser tube in the correct plane ?