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Fix your decollimated vintage C90

Started by Doug Peterson, 12/31/2008 04:06PM
Posted 12/31/2008 04:06PM Opening Post
Most old C90s I have seen have decollimated primaries, and there is no user fix. You can send it to Celestron and it will cost $150 bucks, but after three attempts, I gave up. Very poor service. Didn't get it right and the scopes were covered with greasy paw prints. Finally had to figure out how to do it myself.

You remove the three filler screws from the rear cell.
The next may be hard to visualize without the scope in front of you. The spring clip is a open ring around the baffle tube right next to the mirror surface. It prevents the mirror from falling out. It also is a problem when you stick in the three 6-32 screws and contact the back of the mirror to tilt it: the spring clip prevents the mirror from moving, so you end up distorting the wavefront.

Remove the rubber ring and set screw and thread-off the entire front end. Protect the mirror with a rubber disk cut to lay over it, and pull the spring clip up the baffle tube away from the mirror a fraction of an inch. (An improvement may be to put an O-ring between the mirror and the clip). The mirror is now held only by the 3 silicone blobs on the back side, which you can't see. Now when you advance the screws you can tilt the mirror, you only need to do it a fraction of a turn, so the silicone isn't stressed.

Use a bright star out of focus at lower, then higher powers to center the dark hole of the baffle as seen in the eyepiece. At the highest powers, use the star image in focus and tilt the mirror till no coma. I have done this on three C90s and all of them now have superior optics, razor sharp.


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