Please Help! I can't get my C-11 to focus!

Started by dmcgregor43, 06/22/2016 03:08PM
Posted 06/22/2016 03:08PM Opening Post
Hello Astromart.com, 

Please help me figure out what I'm not seeing here. I have a Celestron C-11 on a CGEM mount. I had a sharply perfect picture with the scope originally, until I installed a set of Bob's knobs to replace the collimation screws. After installing the knobs I collimated the scope to the best of my knowledge using the planets and a few stars, but nothing has brought that sharp picture back. I've cleaned the collector plate a couple times too. I am in Chicago and would really appreciate any help in person or via comments. 

Thank you Astromart.com

Posted 06/22/2016 04:15PM #1
If all you changed on your scope was Bob's Knobs, you're likely not collimated yet. You can't really use a planet to collimate, you must use a point source like a star. To check to see if you're collimated, point your scope at a fairly bright star, use a 15mm-20mm eye piece and defocus the image of the star until it looks like a bull's eye. If the center dot of the bull's eye isn't centered in the rings, you need more work on collimation. If the defocused star shows as a tear-drop shape, or a figure eight, then you need a lot of work on collimation.

You can get pretty good collimation on any clear night by getting the center dot of a defocused star image centered in the bull's eye pattern. However, to get collimation really dialed in, you need a night of very steady air so you can use really high magnification and only a small bit of defocusing to make sure the image is truly concentric.

After you've had a chance to evaluate your scope's collimation, let us know what you see. We might be able to help a bit more.