For a basic understanding of the topic of telescope focusers, read our starting guide on telescope focusers, and then come back to this article where we individually talk about Rack-and-Pinion focusers, one of the cheapest types of telescope focusers.
The rack-and-pinion focuser type involves a gear (the pinion) engaging a toothed track (the rack). As you turn the focuser knobs, the pinion moves along the rack, moving the eyepiece in or out.
This design offers more than enough precision. But sometimes when I use it, it suffers from backlash, which is a slight play or movement in the focuser when changing the direction of focus.
Pros
- Fairly economical
- If well-designed, I’ve noticed that the deflection/backlash are minimal
- Can be made very stiff, offering virtually unlimited payload
- Easy to motorize
Cons
- Cheap models have lots of play when I use them.
- Gears can wear down/strip over time if made of cheap materials or to poor tolerances
- Can be hard to make fine adjustments, particularly if single-speed
- I need to lubricate it occasionally for good performance
Hybrid Rack-and-Pinion Focusers
Hybrid rack-and-pinions are similar to the design of the Crayford focuser, and thus these are termed “hybrid” focusers. The vast majority of good-quality rack-and-pinion units I’ve seen being sold today are hybrid focusers.
These support the side of the draw tube opposite the rack with a set of rollers to keep the draw tube parallel with the telescope’s optical axis.
Two Rack-and-Pinion Focusers I’ve Had Good Experiences With
- Orion 2″ Low-Profile Dual-Speed Hybrid Reflector Focuser: Also sold under the Antares brand and a few others, this hybrid focuser features a sliding extension tube that can be used to reach focus without the use of an additional separate piece of hardware. Unfortunately, with most eyepieces that I’ve tried it with, it ended up being a bad idea as there is plenty of play/wobble in this sliding auxiliary draw tube. But in any case, the focuser itself is excellent and the low profile helps me minimize the size of the secondary mirror when I build my own telescope.
- SVBONY 1.25” Rack-and-Pinion Focuser: This all-metal focuser is a great aftermarket upgrade I’ve used with many smaller Newtonians with plastic 1.25” focusers, such as the tabletop Dobsonians I often recommend to beginners. It even uses a brass compression ring to grip your eyepiece. But I do not recommend the 2” version due to its low-quality machining that I’ve noticed. It’s also sold under Skyoptikst with a refractor-compatible base plate.
