Which screws are safe to remove for mounting an accessory finderscope on my Celestron SCT?

May 5, 2009

There are three things to be careful about when turning or removing screws on your SCT tube:

  1. You must identify the screws holding the rear and front cells to the tube.
  2. The threading of any new screws must match the threading in the cell castings.
  3. Any screws replaced must not protrude into the tube, possibly causing problems with the mirror and its focusing motion.

The best way to identify which screws are actually the ones holding the scope together is to peer into the front end of your SCT. Do this in bright room light or use a flashlight. You’ll see screw threads sticking out into the tube around the primary mirror. You’ll also see three to a dozen nuts. The exact number varies with the model and size of your OTA. These nuts are on the screws that should never be removed to attach accessories. Loosening and removing these screws will drop the nuts in the tube, possibly damaging the optics. Be sure to note the corresponding locations on the outside of the rear cell and leave these screws alone.

The situation with the front cell is similar. Leave any nutted screws alone. Many older OTAs have screws that hold the front cell to the tube that aren’t nutted. Often these are the screws that are flush with the outer surface of the front cell, while the non-structural screws are button topped. Use caution when removing screws from the front cells on these old OTAs because you may inadvertently be removing the front cell!

Once you have found the right screws to remove, be careful replacing them with aftermarket screws. The threading must match. You can easily strip the holes by mismatching threading. Most OTA screws are English threading. Recent OTAs have metric collimation and retaining ring screws.

Length is also important. If the screws you replace--either to fill unused holes to keep dust out or to attach finderscopes, counterweight bars, etc.--are too long, they can protrude into the path of the mirror’s focusing motion or even come in contact with the mirror, damaging the primary or preventing the scope’s focusing.

Note: Opening the OTA will void your Celestron warranty, and using aftermarket screws may void the warranty as well.

Updated 12/27/13