Collimating the vintage C90

Started by Doug Peterson, 06/03/2006 10:19PM
Posted 06/03/2006 10:19PM | Edited 06/06/2006 12:20AM Opening Post
Having become exasperated by Celestron's inability to recollimate C90s at a hundred bucks a pop, I decided to try it myself.

First you remove the tripod block and then remove the small screw underneath, which is the stop preventing the rotating half of the barrel from coming off. If you have the Astro fork version, you can remove the setting circle and pull the scope and integral altitude bearing out in one piece; two screws removed then separate the bearing from the tube.

The 3 screws on the rear cell merely plug the holes; presumably Celestron uses longer screws during the collimation operation, which involves a silicone adhesive. Celestron told me the mirror is bonded to a plate, so inserting 3ea. 8-32x3/4" thumb screws I can reach the plate (or the backside of the mirror) and make minor tweaks which stresses presumably 3 pads of silicone. Since the amount of adjustment required is small, only a tweak is required.

Nevertheless astigmatism appeared. A spring clip around a depression in the primary baffle bears against the mirror front surface, protected by a thin ring gasket. Tilting the mirror against the clip must be causing the astig.

Next I made a rubber mask to protect the mirror and used a dental-pick type tip to pull the spring clip up the primary baffle tube a few mm, where it reseated itself in a 2nd depression. Now only the adhesive is holding the mirror in place for adjustment purposes, yet it is still positively restrained, the mirror will never escape the clip. This should be sufficient: Meade used only double sticky foam tape for years to hold secondary mirrors in place on SCTs and newtonians.

I tweaked the collimation dead-on with the thumb screws. These C90s were actually figured pretty well in terms of overall wavefront quality. Only surface roughness in the corrector aspheric front surface (these are Gregorys, like Questars) and chromatic abberation marred the image quality in the 3 or 4 units I have owned over the years. The Gregory configuration has strong edge of field coma inherent in the design, particularly with the ultrafast F1.75 primary, hence a small shift in collimation spells disaster. I have seen either primary decollimation or secondary baffle drift on virtually every vintage C90 examined.

So far my fix is working fine. The next step would be to put an O-ring in the groove meant originally for the spring clip, and re-do the adhesive: use the screws to drive out the primary, create 3 new pads of adhesive, replace, preferably letting the adhesive cure live during an artificial star test.

Anyone else have collimation experiences?

"--Granted, that's a worse case scenario. The destruction might in fact be ... limited to our own galaxy."