Collimation problem

I have a C8 which I am having trouble collimating.When I get really close one of the screws seams to run out of adjustment and gets really tight while the other two screws are quite loose. I've tried winding them all out a couple of turns and starting again but allways seem to end up in the same place.Is there anything else I can do?
Looking forward to your wisdom,Cheers

Stephen,

I wonder if this issue might be related to a secondary mirror that isn't properly centred relative to the primary, hence the need to cock the secondary so far in one direction. Just a thought, though perhaps others with more experience might have opinions on this...

Do you see any vignetting of the defocused star images at certain spots in the FOV?

Best regards,
Tim

Stephen:

Funny you should post. Just last weekend I was dusting off my venerable Celestron Ultima 2000, and found that she was in need of collimation.

I use Bob's Knobs, and have collimated the scope a handful of times. I use it infrequently - usually when I have non-enthusiasts over for a "star party."

For some reason, I was having exactly issues you describe when attempting to collimate mine. If you can, collimation "best practices" suggest that you do so in straight through mode (no diagonal) with a moderately high power eyepiece on a defocused second or thrid magnitude star.

The problem I was having was as you described - I seemed to keep running out of room on the screws, just as things were looking close to collimated. Another issue I faced was the fact that I would get it to look collimated on one side of focus, but when I went to the other side of focus, it did not look collimated.

At any rate, I got frustrated, and switched scopes for the night. In the morning, I did a rough collimation by looking onto the tube and making sure the shadow of the secondary was concentric with the primary (it was not and a couple of careful tweaks later it was).

That night, I star collimated, and didn't have the issues I had the night before. Moral of the story: once you get things so far out of kilter that you are hittng the screw limits, it's probably better to rough center everything in daylight looking down the tube, then patiently recollimating the next night.