Disassembling a Celestron C11

Optical Tube Assembly


The process about to be described is one that I learned by doing. It worked for me. I make no claims about this process being correct or in any way approved by Celestron. I take no responsibility for any problems you may have or any damage you may incur in following this process. Be aware that Celestron probably will consider this act a voiding of any warranty you may have. You are on your own.

Why Disassemble?

  1. To clean the inside of the corrector plate and the mirror.
  2. To apply a heavy grease to the mirror slider to help reduce focus shift.
  3. To line the inside of the OTA with 3M flock paper to reduce internal reflections. The 3M flock paper can be obtained from Edmund's Scientific.
  4. To better secure the mirror by placing gobs of RTV between the focus arm and the mirror.
  5. To add a "shipping" bolt to secure the mirror in one place while at focus or even while shipping.
  6. To add a second handle at the top of the rear casting. Obtain the handle from Celestron.
  7. To add a cooling fan to the back casting.
  8. To see if the secondary mirror is loose.
  9. To replace you secondary mirror adjustment bolts with socket head bolts.
  10. Because you are curious.


  1. Obtain two blocks of wood about one inch square by about ten inches long. These blocks will be placed at the sides of the OTA to keep it from rolling around. The block sizes are not critical just so long as they do the job.
  2. Obtain a third block of wood about one inch square by about five inches long. This block will be used to raise the front of the OTA slightly for convenience and to help keep the corrector plate from falling forward out of the tube when you have removed its retaining ring.
  3. Obtain a towel to use as a pad on the workbench and over the wood blocks. This will prevent your OTA from getting scratched.
  4. Obtain something to keep the nuts, bolts, screws and other small parts constrained in one place.
  5. Remove all accessories, mounting plates, etc from the OTA.
  6. Sit the OTA on the towel-padded workbench. Place the wood blocks at the sides of the OTA to keep it from rolling. Place the third wood block under the front of the OTA to raise the front of the OTA a bit.

Remove the Corrector Plate

  1. Remove all the screws holding the corrector plate-retaining ring. As you remove the screws, note how they are not screwed in very tightly. When you replace these screws, you will need to tighten them just as lightly.
  2. Remove the retaining ring and set it aside. At this point it is best to try to keep one hand on the corrector secondary housing just to insure the corrector does not fall forward and out. Raising the front of the OTA should have helped keep in, but you can never be too careful
  3. Using a pencil, place a couple of marks on the metal housing and on the ground glass portion of the corrector plate so that you can return the corrector plate to the exact rotational position it is in at this time. This is very important. Make sure you marks are clear and that you can find them again. It may also help to mark the "Up" side of the OTA and note the upside of the secondary housing.
  4. At this time, note that there are four small cork spacers between the edge of the corrector and the metal housing. When you remove the corrector, these may fall out. Do not loose them! Mark their location and number them so that they may be returned to the same place.
  5. Gasping the secondary housing, ease the corrector plate and secondary housing forward out of the OTA and set it aside in a safe place.
  6. Remove the four small cork spacers and put them in a safe place.

Remove the Sheet Metal Tube From the Back Casting

  1. Using the focus knob, move the mirror as far towards the back of the OTA as possible.
  2. Look inside the OTA towards the back inside walls. Notice the little black hex nuts. These nuts are attached to Phillip's head screws that run through the back metal casting and through the sheet metal tube thus holding the tube to the casting. You will need to remove all of these nuts and screws.
  3. Place a small socket wrench on the nut. Be careful not to touch the mirror. I found it was easiest for me to perform this maneuver with my right hand. I rotated the clockwise a bit to get at each new nut.
  4. While holding the nut with the socket wrench, unscrew the screw. Remove the nut and the screw. Put the nut on the screw and put them in a safe place.
  5. Remove all the nuts and screws in a similar manner.
  6. Stand the OTA up on its front end away from you padded wooden blocks.
  7. Get the wooden blocks ready to hold the rear casting by rearrange them at right angles to their current location.
  8. Mark the casting and the tube so that you can easily relocate the casting onto the tube in the proper position.
  9. Have someone hold the sheet metal tube. Gently pull the casting off of the tube by pulling the casting straight up. Use caution. The back casting plus the mirror are a very heavy unit.
  10. Set the casting on its side between the wooden blocks. Adjust the blocks so that the casting does not roll.

Remove the Focus Control Housing

  1. Move the mirror all the way forward from the back of the casting with the focus knob.
  2. Loosen the set screw holding the decorative focus knob and slip the focus knob off.
  3. Remove the three screws holding the focus housing and remove the housing.
  4. Grasp the bearing and pull the bearing back. The mirror will move back as you do this.
  5. Look inside the back end of the bearing. There is a screw. Remove this screw while hold onto the bearing.
  6. Remove the bearing from the focus screw.

Remove the Primary Mirror

  1. Using the focus screw, push the mirror forward until it stops.
  2. Look at the front of the slider tube at the location where the mirror has stopped. A snap ring stopped the mirror. To get the mirror off, you will need to remove the snap ring.
  3. Move the mirror all the way back.
  4. Rotate the casting until you can see both ends of the snap ring. Using a screwdriver or other tool, ease one end of the snap ring out of the groove and forward. Do the same with the other end. Gently work the whole snap ring out of the groove and then remove it from slider tube.
  5. The mirror can now be slide off the front of the slider tube.
  6. Be careful not to touch the mirror surface. You may want to wear clean, white cotton gloves while handling the mirror.
  7. Set the mirror in a safe place. I found I could safely stand the mirror face down on the slider tube. If you do this, just make sure the table or workbench you are setting the mirror on is strong and will not be subjected to wiggles that could knock the mirror over.
  8. I did not disassemble the mirror from its mirror mount. I do not advise you to do this either. In order to reassemble this unit, you would need special equipment.


  1. Reverse the process.
  2. When replacing the focus housing, try to center things the best you can and tighten the three screws. Move the focus knob such that the mirror moves the full length. If you feel the focus binding at any point, stop, loosen the three screws a bit and then retighten them. This process should get the focus screw perfectly centered.
  3. When placing the casting back onto the tube, try to line up the boltholes before slipping the casing onto the tube. It can be a little difficult to rotate the casting on the tube. If there is a slight misalignment of the holes, a small Philips screwdriver can be used to wedge them into alignment.
  4. Place the nuts loosely on the bolts. Do not tighten any of the bolts until all the nuts and bolts have been replaced. The first two bolts you tighten should be 180 degrees apart. The next two 90 degrees from these first two and so on.
  5. Place the cork spacers onto the housing before replacing the corrector. You can wet them slightly on the cork sides with you tongue to so that will stay in place for a short while.
  6. Be sure to line up the corrector to the marks you previously made.
  7. Be sure not to tighten the corrector retainer ring to much. It does not take much force to hold the corrector from turning. Too much tightening can distort the corrector's optical figure.
  8. Have a cup of coffee.

  9. Back to the Observatory