The re-building of a Celestron C11

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The re-building of a Celestron C11

Postby edward-fraini » Fri Sep 16, 2016 9:47 pm

As some of you are aware I have been working on a project to restore a C11. This includes cosmetically, mechanically, and optically. Every fastener was removed and the mirrors sent off to Spectrum for re coating.
I tore it down making careful marks as to how each and every piece was aligned. Lots and lots of marks. I also placed all the nuts, bolts and shims from each subset in a separate zip lock bags with notes on what I observed. All of this was very helpful in the re assemble…believe me! Next I set up to paint the parts. From a past Hobby I had all the gear to paint with an airbrush. Things went well till I got the mirrors back. I sent the primary mirror off with the focus arm and draw tube attached. It came back in pieces! Not what I expected. There was a lot of discouraging info out on the net about the probability I could get the primary remounted in a workable condition. I contacted Celestron and they punted on me. They offered to sell me a new scope :lol: .
So I launched out on my own. The key part of my plan was to use the Howie G. "Parallizer" approach. I measured the shims the factory put between the tube and the mirror hole and found them to be about 0.125 inches and all exactly the same. I used this information to create a situation that forced the two tubes, one being the hole in the mirror and one being the focus tube cylinder, to be parallel. At 120 degree spacing I used contact cement to place a vertical strip of .0625 inch of cork that was about .25 inches wide and one-inch long. Then I cut some strips of the same cork that were about 0.75 inches wide and glued them over the top of the first strip. So now I had created three vertical rails that, as I discovered doing a dry fit, fit the mirror hole snugly, forcing it to center and in theory forced the two cylinders to be parallel. Next I covered the strips and filled the space between them with red RTV, put a thin coat on top of the cork alignment rails, put a good bead at the joint point at the bottom, and put a thin coat on the base cork gasket. I set the mirror down on the shaft making sure the focus arm was lined up with the tail of the factory marked arrow that was on the bottom and gently press to set it. The collar was threaded on and snugged down, after an hour it was tightened a bit more.

After 24 hours I put the whole OTA together again making sure all my alignment marks made during the disassemble lined up properly. Here's an obscure fact I ran across - the secondary has a serial number on one edge and that should be positioned at 3:00 looking into the OTA. Or opposite the focus arm. The serial number on the primary ended up in this same alignment.

Next night in the field I started to test things out. This was my first attempt to collimate a SCT and I had about an hour of a hard lesson. The key was to start out with a low magnification and go really really lightly on the collimation screw movements so you don't lose your star. I progressed eyepiece to eyepiece till I ran out of sky, clouds moved in, and got a good collimation at about 200X. Moving in and out on the star resulted in a perfectly matched image on both sides of focus. We will be socked in with clouds for the next three or four days but I am planning to continue the collimation to a higher magnification.
I do believe I now have a beautiful scope I will use for a long time.
All and all my conclusion is that this turned out a success after looking pretty bleak for a while, thanks for the input that moved my project along. (**Clayton Jeter**)

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