This is a helical Crayford Focuser for use with an amateur Newtonian telescope. It is made for 1¼″ eyepieces, but can be customized easily to accept 2" eyepieces. The mounting angle of the ball bearings is 5°, thus for each full rotation the draw tube (outer diameter D=35mm) moves inward or outward by tan(5°)πD= 9.6mm, which allows for very fine adjustments of focus. The angle can be customized (check out the customizer app) to allow for coarser or finer alignment.
The design consists of two parts: the actual focuser, and an adapter to attach the focuser to the cylindrical tube of the telescope. The shape of the latter depends heavily on the telescope, or may be not necessary at all for certain telescopes.
Besides the 3D printed parts, the following items are necessary to complete the focuser:
- 7 machine screws M3x25mm, one screw M3x16mm, preferably made of stainless steel.
- 3 screws M3, variable length, for mounting the adapter to the telescope
- M3 nuts & washers
- 6 (stainless) steel ball bearings type 623
- The draw tube. It should have an inner diameter of 1 1/4" or 32mm, and an outer diameter between 34mm and 35mm. The length depends on the position of the intermediate image in the telescope relative to the position of the focuser, and the eyepieces used with the focuser. In most cases it should be shorter than my version, which is fairly long (125mm, with 60mm usable travel). For suitable materials of the tube, I tried aluminum and stainless steel, but plastic may be possible, too. The roundness of commercially available steel/aluminum tubes is not sufficient for smooth operation of the focuser, so I improved the roundness by getting the outer diameter of the tube reduced slightly with a lathe in a nearby metal workshop.
I used the following OpenSCAD libraries for the design:
Update: The customizer shows more meaningful parameters now.