We offer knobs for the Orion Optics OD-series and CT-series secondary in two different screw lengths. (Orion Optics in the United Kingdom and Orion in the US are two different companies.) Use our guidelines to determine the correct screw length.

A. Knobs for Orion Optics OD and CT Secondary

Part: ODsec35 or ODsec45
Fits: Orion (UK) OD-series secondary

Knobs: Black plastic, 13 mm (1/2”) diameter
Threads: Stainless steel (35 mm length); Zinc-plated steel (45 mm length)

  1. The knobs must clear the secondary housing, as shown in the photo. Order knobs with the correct screw length, either 35 mm or 45 mm. Knobs with 35 mm screws will fit most of these telescopes. (See Note 2)
  2. The newer CT-series usually requires the 45 mm screw length. These knobs look slightly different than those in the photograph on this page.


Screw length