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How to Remove a Celestron Telescope Corrective Lens

The corrective lens, or "corrector plate," is the circular optical element positioned inside the front of the tube of a catadioptric telescope. Removing a corrective lens may be required for cleaning both sides of the lens, removal of grit from around the ledge that holds the lens, or servicing of the secondary mirror assembly. The corrector lens is an element of precision manufacture and fit, and the manufacturer advises owners against removing it. Nevertheless, it can be done with sufficient care.

Things You'll Need

Tool kit supplied with the telescope
Black permanent marker pen
Latex or cotton gloves
Cotton swabs
Isopropyl alcohol

Instructions

- 1
  Position the telescope tube in the mount so that it points well above horizontal, then tighten the mount so that the tube is secure. This will prevent the retaining ring and corrector lens from falling out of the tube.
- 2
  Carefully remove the screws positioned around the corrector lens retaining ring, which is the metal ring that lies flat over the front edge of the lens. A hex key that fits these screws is included with the telescope tool kit. The retaining ring should have six to eight screws, depending on the telescope size and model.
- 3
  Remove the retaining ring, and set it aside with the screws and hex key.
- 4
  Put on the gloves and carefully remove the corrector lens assembly by gripping and lifting up on the secondary mirror housing in the center. If the assembly resists movement, then the lens is probably stuck to the cushioning strip that is glued along the ledge it rests on. Do not try to force the assembly; instead go to step 5.
- 5
  Dip a cotton swab in alcohol, then press the swab into the gap between the top of the corrector plate and the cell walls until you see alcohol spreading between the cushioning strip and corrector lens.
  
  Apply alcohol in this way at several positions around the corrector lens until the entire strip is damp. Wait for two or three minutes.
- 6
  Remove the corrector lens assembly. If it still refuses to move, repeat step 5. Rest the assembly flat on the secondary mirror housing, in a safe place, with the front facing up.
- 7
  Mark the position of each cork "lens spacer" (if any), and match it with the same mark on the spacer. Each spacer must be reinstalled at the factory-set position.

Astronomy