I’m removing a Classic Celestron tube from its fork. What screw sizes will fill the holes on the tube and what sizes are used to attach accessories to the front and rear cells?

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	I’m removing a Classic Celestron tube from its fork. What screw sizes will fill the holes on the tube and what sizes are used to attach accessories to the front and rear cells?

Answer The older “Classic” Schmidt Cassegrain tubes (SCT OTAs) used English threading like 8-32, 10-24 and 10-32. These screws are still used in modern tubes as well. (Recent OTAs have metric collimation and retaining ring screws.)

Be careful plugging them with aftermarket screws. The threading must match. You can easily strip the holes by mismatching threading.

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. Lengths no longer than 1/2' should be ok, maybe even 1/4".

Article Details

Article ID:

2349

Created On:

Feb 18 2010 02:19 PM