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Remake a Aluminum C-11 into a Carbon Fiber beauty

Posted by Jerry Wise   03/16/2007 12:00AM

Remake a Aluminum C-11 into a Carbon Fiber beauty
A recent ad on Astromart for aftermarket Carbon Fiber tubes looked interesting and I decided to give the upgrade a try. The nearly $300 upgrade gives you an option of having a Carbon Fiber OTA on otas bought with the aluminum body. It would be better to go with the $50 or so upgrade for carbon fiber OTAs when buying the scope new if you can. Carbon Fiber is tough and can withstand dings and bumps well. It also has almost imperceptible thermal expansion which keeps the OTA in focus when the evening cool down occurs.

The carbon fiber tube arrived well packed and wrapped in paper. The carbon fiber shell really looked good with the inside already been pre-installed with high quality flocking fabric.


Before starting, place a towel or blanket on a table under a strong light. Have a medium Phillips screwdriver, set of small sockets, and rubber gloves ready. You will want the rubber gloves on in case you nudge the corrector glass or mirror during the change over.

At this time remove all external options such as Dovetail Plates, Finder Scopes and Diagonals. I also put a small mark on the topside of the front and rear castings for lining up later. You want everything to go back exactly as it came apart.

Corrector Plate Removal:

Place the OTA (Optical Tube Assembly) upright facing you in a chair. With rubber gloves on, remove the screws in the metal ring holding the corrector plate in. Make sure you drop nothing on the corrector plate. It’s strong but can be broken or chipped.

Mark the corrector plate position (as shown by the arrows above) to give you a reference for re-installation. The corrector plate should be aligned to the same position it was in relation to the primary mirror. This insures a proper alignment when you put it all back together. Gently lift the ring off the corrector plate. If it is snug, shape the tip of an old toothbrush into a flat screwdriver form by filing and use this to gently lift the edges of the corrector plate while exerting upward pressure on the secondary mirror holder.

Removal of the front casting:

Next remove the screws located on the portion of the front casting closest to the rear of the scope. They will be located evenly around the OTA. These screws have nuts on them so have the OTA in a horizontal position just in case you drop a nut. You don’t want the primary mirror catching it and if it falls it will hit the primary if the OTA is tilted up.

Two brackets on the top and bottom of the OTA located on this front casting. These are the brackets that reinforce the inside of the OTA when you mount dovetails on the top and bottom. You can easily see where there are three screws together and the center one is countersunk. The arrow shows their location inside the OTA (taken after CF install but best illustrates bracket location).

These must be removed but only on the front corrector casting. There are two more on the mirror base casting but they are secured without going through the tube material and are left in place. First remove the outside screws then remove the nuts on the center screws. While holding the bracket inside the OTA, back the center screw out and remove the bracket. Repeat for the bottom bracket. The front casting is now ready to come off. Gently rock it will pull it free. It should slip right off.

Removing the Primary Mirror housing/Base Casting:

Before removing the Base Casting, rotate the mirror focus knob clockwise until it stops. This will move the mirror all the way to the rear of the tube. This gives much better access to the nuts holding the rear casting on. With gloves on, reach down inside the OTA and hold each nut with a finger while backing out its corresponding screw. Carefully catch the nut when free and put it aside for reassembly. Do not let any fall onto the mirror. It could scratch the coatings and make you say ugly words. Only remove the screws closest to the front of the OTA. There are brackets corresponding to the brackets mentioned above on the back casting too but their screws do not go through the OTA tube material. Once the base casting screws and nuts are removed gently lift the old OTA tube out of the base casting.


Compare the drilled holes to the original aluminum OTA. My CF tube was an early one and a couple holes were a little off. The vendor has since corrected the problem. Once all the holes line up it is time for assembly. As mentioned above, the carbon fiber OTA comes with a high quality flocking material already installed inside the tube. Give the CF tube a good going over with a lint brush making sure all flakes or out of the flocking material before assembly. Use one of those rolling sticky paper fuzz removal tools and roll front to back a number of times until all foreign material is removed from the flocking. In the flocking seam there may be a few threads sticking up. Trim then back with small scissors.

Once the final cleaning is completed you are nearly done. The CF tube slips in nicely. Carefully install the screws around the mirror base casting and secure with nuts held with your finger on the inside of the CF tube. Use the original screws and nuts as they already have a flat black finish. Using other hardware will place shiny objects inside a professionally blackened OTA.

Gently replace the corrector casting and corrector plate. Make sure you align to the marks you made during disassembly. There were some small cork spacers around the corrector when you lifted it out. Put those back in the same position too.

Final Celestron C-11 XLT Carbon Fiber:

Once completed you have a nice looking C-11 with a shiny carbon fiber tube.


Was it worth the trouble? Depends on what you are looking for. I wanted the CF in order to stabilize focus in long imaging sessions. There is a certain “cool” factor to the look of carbon fiber too. Cooling has been mentioned as a negative on these tubes. Having the OTA in an observatory takes a away a lot of the cool down problems associated with carbon fiber. I’ve had two other carbon fiber scopes, an 8” Celestron Nexstar and 10” Meade RCX. Both were fine performers and were not that much harder to cool.

The installation was relatively straight forward and would have been a snap if the holes had been drilled to match the base casting which is now taken care of. If you plan to image a lot or like the looks of a tough “ding free” carbon fiber OTA, this upgrade is the way to go.

The scopes performs very well and holds focus all evening long now.