Focus in a telescope is achieved when light rays from a distant object, such as a star or planet, are converged (brought together) at a single point by the telescope’s lens or mirror. When this convergence point coincides perfectly with the focal plane of our eyepiece or camera sensor, the object appears sharp and clear.
Focusers help with this by moving the eyepiece back and forth, thus moving the focal plane of the eyepiece along with it.
Focusers themselves are a component of every telescope, but they’re often viewed as a bit of an afterthought. This is partly because most focusers work well enough that little is thought of them beyond fulfilling their basic job. Also, many mass-manufactured catadioptric telescopes don’t have an external focuser in the traditional sense.
Focusers are, however, just as essential and precise a part of your telescope as any other mechanical component and arguably nearly as much so as the optics.
A bad focuser results in poor high-magnification performance and considerable frustration for the user. With heavy loads like premium eyepieces or a camera setup, a low-quality focuser may sag or wiggle enough to be a genuine danger to your equipment.
What is a Focuser?
A focuser is an adjustable holder with which you attach your eyepiece, camera, or other relevant accessories or adapters to your telescope.
Almost all telescopes come with a built-in or attached focuser.
A typical external focuser on a telescope works by moving the eyepiece or camera in and out along the optical axis of the telescope. This movement alters the distance between the eyepiece and the telescope’s primary lens or mirror, which in turn changes the point where light rays converge to form an image. Accurate focusing is vital for producing clear and detailed observations or photographs of celestial objects.
Telescopes with an external focuser, like most refractors and reflectors, may be able to have their focuser swapped out for a different one, though new holes may need to be drilled if the screw patterns aren’t the same between the new and old focuser units.
Focusers can be motorized and controlled remotely to achieve precise focus without touching the telescope, thereby minimizing vibrations and ensuring precision adjustments for astrophotography (though generally unnecessary for visual work).
Focuser Size Formats
Telescope focusers come in various sizes, primarily determined by the diameter of the eyepieces (or other hardware) they can accommodate. The most common sizes are 0.965-inch, 1.25-inch, and 2-inch, with occasional larger sizes available for astrophotography or other specialized purposes.
- 0.965-inch Focusers: This is an older standard, often found on vintage or very basic telescopes. Eyepieces of this size are typically of lower quality and offer a narrower field of view. I see this size less common in modern astronomy equipment.
- 1.25-inch Focusers: This is now the most common standard in amateur astronomy. The 1.25-inch focusers and eyepieces offer a good balance between field of view, quality, and cost. Many telescopes only take 1.25” eyepieces due to physical limitations or a lack of need to go larger.
- 2-inch Focuser: Used by more advanced and larger telescopes, 2-inch focusers allow for eyepieces that provide a wider field of view, which I think is particularly beneficial for deep-sky observing. A 2” focuser is also necessary for deep-sky astrophotography with many common camera sensor sizes, as well as to utilize accessories like focal reducers, field flatteners, and coma correctors, which I need to employ for this task.
- Special Large Formats: Larger formats, such as 2.5-inch, 3-inch, or even larger, are typically used in professional or highly specialized amateur telescopes. These allow for even wider fields of view and can accommodate heavy-duty accessories, but they are generally overkill for most amateur applications.
Types of Focusers
With the exception of the internal focusers used on catadioptrics, telescopes today typically have one of three types of focusers. These are helical, rack-and-pinion, and Crayford focusers.
All do the same thing: move the eyepiece/camera and other relevant accessories back and forth relative to the light focused by the telescope’s objective lens or mirror.
In its most basic form, a focuser can consist of a tube in which the eyepiece slides back and forth or a draw tube to hold the eyepiece that slides back and forth inside a larger receptacle. I occasionally see some very old or homemade scopes using this system. In any telescope with a focal ratio faster than about f/10, it is extremely difficult for me to focus precisely at high magnifications with such a device; I cannot use heavy eyepieces, and leaning on the eyepiece too hard pushes it inward.
All in all, a proper focuser is certainly worth the trouble of implementing on any telescope.
Focuser Adapters & Clamping Mechanisms
There are two main ways most focusers employ for clamping an eyepiece.
- Compression Ring: This method uses a brass ring that evenly applies pressure around the eyepiece when an external screw is tightened, offering a secure grip without marring the surface of the eyepiece barrel. I generally prefer this to use with my higher-quality eyepieces, as it minimizes the risk of scratches and provides a more stable hold. The “rotating lock” and “ClickLock” adapters sold by Baader and other companies I’ve seen are more or less just slightly different designs of compression ring fittings with a twist-lock rather than thumb screw tightening mechanism.
- Thumbscrew: A more straightforward method, where one or more screws are tightened directly against the eyepiece barrel to hold it in place. While effective, this can sometimes damage the eyepiece’s finish. It may not provide as secure a grip as a compression ring, especially for heavier accessories.
Many older telescopes don’t have a proper thumb screw adapter and grip the eyepiece with a slotted metal flange made of soft brass or aluminum, or even just a 1.25” chrome pipe flange. Some eyepieces, such as the Tele-Vue Ethos oculars, still have an external thumb screw to clamp onto these 1.25” flanges securely.
This is fine with lightweight eyepieces but poses a problem with today’s heavy ultra-wide oculars. In any case, I often swap out the offending eyepiece holder or add a hose clamp around it to tighten the grip.
Installing a New Focuser on Telescopes
We can retrofit a new focuser onto most refracting or reflecting telescopes fairly easily, which is why I’ve taken the trouble to make recommendations for some aftermarket focusers in this article.
- On Refractors
Refractors usually require me to buy or make an adapter flange to fit the focuser body on the back of the tube. Installation is as easy as dropping in the replacement flange after removing the old focuser, which usually just takes a screwdriver.
- On Reflectors
If the new focuser and old focuser body do not have the same screw hole pattern, installing a new focuser on a Newtonian reflector requires me to drill a few additional holes in the tube.
This means I’ll want to remove the spider holding the secondary mirror and the primary mirror cell if I’m doing this on one of my solid-tubed telescopes that doesn’t come apart further. Any drill and hard metal bits make short work of the thin-walled metal, plastic, plywood, or cardboard tube walls.
However, enlarging the hole in a tube for a wider focuser drawtube is a different story. I often require a Dremel or some other small cutting tool to do it while making sure that the focuser is still centered on the secondary mirror.
- On Catadioptric Telescopes
The external focusers that screw onto the back of catadioptric telescopes need no explanation as to how they are installed.
However, Starlight Instruments also sells a Feather Touch planetary gear knob for many Schmidt-Cassegrain and Maksutov-Cassegrain models. Installing it required me to remove the old focuser knob and bearing assembly with a screwdriver and hex keys, but it was a fairly straightforward process.
My Telescope Focuser Recommendations
I Got The Best Quality from Starlight Instruments 2” Feather Touch
The Best Value Focuser I’ve Owned is GSO’s 2″ Dual-Speed Crayford
Dual-Speed Focuser Knobs
Many focusers offer dual-speed adjustment. This consists of a setup where a second small knob affixed to one of the focuser knobs operates on a different gear ratio, typically 1:10. Meaning that for every ten rotations of the smaller knob, the larger knob (and thus the focuser) rotates only once. This setup provides a finer degree of control.
This system allows for a compact design that can smoothly and accurately translate a high number of hand knob rotations into very fine movements of the focuser drawtube.
A dual-speed focuser is versatile, allowing me to quickly switch between rapid and fine focusing. This is particularly useful in dynamic observation sessions where both coarse and fine adjustments are regularly needed.
Schmidt- and Maksutov-Cassegrains can be adapted to have dual-speed focusers too, though the use of such a system by the manufacturer is rare (a few scopes, such as the Maksutovs offered by iOptron, do have a dual-speed knob by default). Starlight Instruments does sell Feather Touch dual-speed knobs for many commercial catadioptrics, however. A dual-speed knob from them is expensive, but certainly worth it if you do a lot of high-magnification observing or planetary imaging.
