Helical Focusers: Sharing What I Know

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Written By: Zane Landers
Category: Learn

Helical focusers are essentially a big screw with a hole in the middle for our eyepiece. The “screw” threads of the draw tube turn in an identically threaded receptacle. Typically, a thumb screw or two is on the receptable part of the focuser body to adjust friction.

Helical focusers are easy to make to fairly fine tolerances. As such, some star diagonals, such as the T2 Prism sold by Baader Planetarium, as well as most binoviewers that I’ve used, have miniature helical focusers built in for fine adjustment as a supplement to our telescope’s stock focuser. So too does the Questar on its eyepiece holder.

The downside is that finer threads mean it takes longer to make a coarse adjustment. Coarse threads can wobble and deflect, affecting apparent collimation and making it hard to focus at higher powers.


  • Cheap
  • Hard to damage
  • Usually offers plenty of fine adjustment
  • Lightweight/low-profile


  • Can be time-consuming to make coarse adjustments
  • Deflection/wobble with heavier accessories
  • Entire eyepeice/diagonal/camera rotates (except in nonrotating designs)
  • Usually relies on using inserted eyepiece/accessory as a grip, which is not ideal in all circumstances
  • Hard or impossible to motorize

Nonrotating Helical Focusers

The rotation induced by twisting the entire eyepiece/drawtube assembly can be an irritant when trying to do photography. So, many small refractors and guide scopes designed for astrophotography have a non-rotating helical focuser, which essentially keeps the drawtube on a slotted track, preventing it from twisting as the rest of the focuser spins.

Helical Crayford Focusers

Helical Crayford Telescope Focuser Model HC-2
Helical Crayford Telescope Focuser Model HC-2. Image by Zane Landers for TelescopicWatch

Helical Crayford focusers swap the threaded draw tube and receptacle for a smooth draw tube and a set of angled rollers.

These focusers work very well, but it can be hard to design one that offers both fine adjustment and high weight capacity. Most helical Crayfords require me to compromise one for the other.

The KineOptics HC-2 and HC-1 are examples of quality helical Crayfords I can recommend. I’ve also made one myself fairly easily out of readily available hardware.

Zane Landers

An amateur astronomer and telescope maker from Connecticut who has been featured on TIME magazineNational GeographicLa Vanguardia, and Clarin, The Guardian, The Arizona Daily Star, and Astronomy Technology Today and had won the Stellafane 1st and 3rd place Junior Awards in the 2018 Convention. Zane has owned over 425 telescopes, of which around 400 he has actually gotten to take out under the stars. These range from the stuff we review on TelescopicWatch to homemade or antique telescopes; the oldest he has owned or worked on so far was an Emil Busch refractor made shortly before the outbreak of World War I. Many of these are telescopes that he repaired or built.

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