Helical Focuser Adapter, Dust Covers, 114mm f/3.9 Newtonian Telescope

Last crawled date: 4 years, 2 months ago

A double-helical focuser adapter for a 114mm f/3.9 Newtonian telescope such as the Celestron Cometron 114mm, Orion StarBlast 4.5, or equivalent. These telescopes typically come with a plastic rack and pinion focuser that is very short, has no T threads for astrophotography, and is difficult to find an aftermarket replacement for.
Some 1.25" double-helical focusers are too tall and don't provide enough backfocus for this application even after shifting the main mirror forward. This adapter fits the short SVBONY 1.25" focuser that has an M36x1 thread and a locking thumbscrew on the focusing ring: (https://www.amazon.com/gp/product/B07HRMRPJJ). This focuser has a length of 43mm to 53mm, 10mm stroke.
To install the adapter, read and understand the following instructions and notes before making any cuts because it might be difficult to obtain a replacement focuser.
You will need to disassemble and remove the original focuser and cut the focuser base so that roughly 10mm of the tube remains. Cut the tube slightly longer and sand/file until the helical focuser ring rotates freely and the threaded base engages the adapter threads snugly. The adapter slides into the back of the focuser body in place of the focuser tube and the helical focuser threads into the adapter from the top end of the cut focuser body. If the opening in the telescope tube is too small for the adapter to pass, you might need to install the focuser body onto the telescope tube first and then install the adapter from inside the telescope tube.
Note that there is some wobble in the helical focuser body even after the lock screw is tightened. When the adapter is installed, the top rim of the plastic focuser body should be higher than the top of the adapter. The top rim should apply some upward pressure on the helical focuser ring to reduce or eliminate this wobble. Alternatively, you could fit the focuser such that turning the focuser body locks the focus ring against the plastic base.
During fitting, you might also need to take into consideration the resulting position of the focus index marks and the rotation of your camera.
Use flat black filament or spray with flat black paint before final installation.
The dust caps were designed for the Celestron telescope and might not fit other models. The large cap is used to cover the opening behind the mirror cell during storage, and the small cap replaces the inner cap on the factory front cover.