Starfield Optics 3" Dual-Speed Rack-and-Pinion Focuser for RC Telescopes (M117)

If you’re running a larger RC (or similar back end that uses the M117 standard) and you want a focuser that stays put under real imaging weight, this 3" rack-and-pinion upgrade is built for exactly that job. The key advantage of this design is that the drawtube support and the focusing drive are separated—so the tube rides on low-friction bearing surfaces while the gear train provides the movement. The result is a focuser that remains stable with heavy cameras, filter wheels, and reducers without the “give” you can sometimes see in lighter Crayford designs.

Why this focuser is a serious imaging upgrade

Geared drive with high load stability
Instead of relying on friction pressure to hold position, the rack-and-pinion drive provides positive, repeatable movement and strong holding power—useful when you’re hanging a substantial imaging train off the back of the scope.

Dual-speed control (10:1 microfocus)
Fast coarse focusing gets you close, and the 10:1 reduction makes it easy to land on critical focus without overshooting.

Rotation for easy framing
The included adapter assembly supports full rotation so you can square up a camera or reframe without fighting your mount orientation.

Big clear aperture for modern sensors
With a large light path and a 3" class focuser body, it’s designed to play nicer with larger camera formats and fast RC imaging accessories. (Exact clear-aperture figure below.)

Telescope connection

• Telescope side thread: M117 (via the included connection adapter)

• Optional plug connection: remove the adapter to reveal a 105 mm plug interface for custom fits/adaptations