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An Advanced Guide to Telescope Focusers

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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 these rays converge perfectly at the focal plane (where the eyepiece or camera sensor is located), the object appears sharp and clear.

Focusers themselves are a component of every telescope, but they’re often viewed as a bit of an afterthought. This is partly because many mass-manufactured catadioptric telescopes don’t have an external focuser in the traditional sense, and partly because most focusers work well enough that little is thought of them beyond fulfilling their basic job.

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?

StarSense Explorer 8 with GSO Dual Speed Crayford Focuser Attached
StarSense Explorer 8 with a GSO Dual Speed Crayford Focuser attached. Image by Zane Landers for TelescopicWatch

A focuser is an adjustable holder with which you attach your eyepiece, camera, or other relevant accessories or adapters to your telescope. 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.

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).

Almost all telescopes come with a built-in or attached focuser. 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 to accept mounting flanges if the screw patterns aren’t the same between the new and old focuser units.

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. This size is 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 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 you will likely 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.

How do I Know If My Telescope Is In Focus?

The easiest way to check the focus is by pointing your telescope at a bright star. Stars are ideal for this purpose because they are distant enough to be considered a point source of light.

A star in perfect focus should appear as a single, round point of light through the eyepiece. If it appears considerably larger than a point source at a normal magnification, you are probably out of focus. Slightly blurry/fuzzy stars can be caused by problems unrelated to being out of focus, such as being miscollimated or using too high a magnification. Our article, “Why is my Telescope Blurry?” goes into more detail. If you’re using a camera, the star should appear as the smallest possible point in your image. Larger, blurred spots indicate that the focus needs adjustment.

If you’re still confused, intentionally defocus the telescope slightly by turning the focuser. As you move out of focus, you’ll see the star’s light spread out. In a telescope with a central obstruction (like a Schmidt-Cassegrain or a Newtonian reflector), you’ll see the star expand into a ring with a dark center (the “donut” effect). In a refractor or an unobstructed reflector, the star will just bloom into a larger circle of light. Adjust the focuser until the diffraction pattern (ring or circle) is symmetrical around the center. Once symmetrical, move the focuser slowly to make the pattern smaller until it collapses back into a sharp point, indicating optimal focus.

Why Can’t I See Anything Through My Telescope?

It’s a common issue for both beginners and experienced astronomers to sometimes find that they can’t see anything through their telescope. This problem can often be attributed to issues related to focus travel, though you should make sure that you have taken the telescope’s caps off and started with a low-power eyepiece that doesn’t give too much magnification.

Point the telescope at a terrestrial object at least a football field away, or if a sufficiently distant or detailed one is unavailable, use the Moon. Slowly rotate the focuser, starting with one direction. It might take several turns before anything recognizable appears in the eyepiece, especially if the focuser is far from the initial focus point. You may need to go the other direction as well. Many catadioptric telescopes can have considerable focus travel and may need to be racked quite far to reach focus if they were used with an unusual setup by a previous owner or shipped with the focus racked all the way in from the factory.

If you’re unable to achieve focus, it could be because the eyepiece/camera/accessory or combination thereof you’re attempting to use is outside the focuser’s travel range. Some telescopes, especially those designed for astrophotography, might require an extension tube to increase focus travel with an eyepiece, and most refractors are designed to reach focus with a star diagonal in use.

If you find yourself unable to rack focus sufficiently inward (a common problem for astrophotographers), you may have hit a physical design limitation of the telescope requiring a different focuser or adjustments to the length of the tube, or you’re going to need a Barlow lens to bring the focal plane out, which may not work for your intended use case. This is inevitable with many binoviewers.

On the whole, most Newtonian-reflecting telescopes that are not designed primarily with long-exposure astrophotography in mind will not reach focus with a cooled astro camera, DSLR camera, or binoviewer at prime focus. This is often true of refractors. If you are doing deep-sky astrophotography, invest in the proper optical tube. It’s worth it.

Why do I See a Bullseye Through My Telescope?

Many telescopes have a central obstruction (the secondary mirror) that blocks some light. When such a telescope is out of focus, a distinctive “donut”-shaped image can be observed. This is because when the telescope is out of focus, the light rays from a star are not converging at the focal plane. Instead, they spread out, forming a circular pattern. The secondary mirror or obstruction in the optical path blocks the central portion of the incoming light rays. This results in a lack of light in the center of the circular pattern, creating the “donut” appearance. The size and shape of the out-of-focus disk can indicate how far the telescope is from being in perfect focus. As focus is adjusted, the “donut” will shrink and eventually collapse into a sharp point of light when proper focus is achieved.

The out-of-focus disk itself shows rings—diffraction rings—that can be used to star test a telescope.

Top Telescope Focuser Picks Overall

#1. Starlight Instruments 2” Feather Touch: Best Quality

With the departure of Moonlite from the manual focuser business, Starlight Instruments stands alone among American premium focuser manufactures. While the price is steep, the Feather Touch focuser is amazingly well-designed, lightweight, and compatible with a wide range of telescopes and accessories.

#2. Baader 2″ Diamond Steeltrack Low-Profile Dual Speed Focuser: Runner Up

Offering essentially the same design, features, and weight capacity as the Feather Touch, the Baader Diamond Steeltrack is an excellent focuser, though it’s a bit heavier and accommodates fewer aftermarket add-ons.

#3. GSO 2″ Dual-Speed Crayford Focuser: Best Value

GSO 2" Dual Speed Crayford
Image: Zane Landers, TelescopicWatch

This focuser is supplied with our top telescopes for beginners (Apertura’s AD series) and is great for almost any application and any type of telescope, offering plenty of weight capacity, motor focuser compatibility, and the high-quality design of a dual-speed Crayford focuser. A heavier-duty linear bearing version is also available.

#4. SVBONY 1.25” Rack-and-Pinion Focuser: Best for Small Telescopes

This sturdy all-metal focuser is easy to retrofit onto small Newtonian reflectors, which might have been supplied with a substandard plastic focuser; all that’s needed is to drill a few small holes.

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. You may 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 to focus precisely at high magnifications with such a device; you cannot use heavy eyepieces, and leaning on the eyepiece too hard may push 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 of clamping your eyepiece employed by most focusers and accessories.

  • 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. This is generally preferred for 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 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 and 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. This is fine with lightweight eyepieces but poses a problem with today’s heavy ultra-wide oculars. Some eyepieces, such as the Tele-Vue Ethos oculars, still have an external thumb screw to clamp onto these 1.25” flanges securely. In any case, however, you may wish to swap out the offending eyepiece holder or add a hose clamp around it to tighten the grip.

What is Back Focus?

Back focus (or back focal distance) is the distance from the mounting point of the eyepiece or camera (i.e., where the focuser holds the eyepiece or camera) to the focal plane of the telescope (where light converges to form a sharp image). It’s essentially the available “space” for placing camera sensors or eyepiece lenses to achieve focus.

In astrophotography, cameras, filter wheels, and other imaging accessories require a certain amount of back focus to function correctly. Insufficient back focus (i.e., insufficient focus travel inward) can prevent these devices from reaching the point where a sharp image is formed. This may also have to be balanced with spacing focal reducers, flatteners, and mechanical adapters or filters correctly, which can be a huge headache. 

Different telescope designs offer varying amounts of back focus, influencing what types of cameras and accessories can be used.  The moving-mirror focusers of most catadioptrics offer several inches of back focus; a typical refractor or Newtonian astrograph offers 2-4” of total focus travel by utilizing extension tubes, while the difference in focus between a fairly diverse set of eyepieces across both 1.25” and 2” formats rarely equals more than an inch, so most telescopes designed purely for visual observation only need about 1.5” of travel.

Impact of Temperature on Telescope Focusing—How to Compensate?

Thermal expansion is the phenomenon by which materials change their size and shape when their temperature changes. This, of course, affects telescopes given their minute precision, predominantly in the form of wrecking your image’s sharpness. In addition to the deformation of your telescope’s optics, the tube itself may expand or contract enough to put the telescope out of focus.

Telescope optics are made of glass, while the hardware associated with much of the optics and your focuser itself is predominantly aluminum, steel, or other common metals used in alloys such as zinc and copper. These materials all have a positive coefficient of thermal expansion, which means that they expand when heated and contract when cooled. The amount of expansion or contraction depends on the material and the change in temperature.

Telescope tubes can be made from a variety of materials, including aluminum, steel, and carbon fiber. Aluminum expands more than steel, which is why steel (otherwise inferior to aluminum in all aspects as a material) is used for many mass-produced telescope tubes. Carbon fiber expands even less than steel and is also lighter in weight than either material. However, its fragility and cost rule it out for many instruments.

The exact amount of focus shift depends on the temperature change, the coefficient of thermal expansion of the optics and tube materials, and the focal length of the telescope. For example, a 10-degree Celsius temperature drop might cause a focus shift of up to 0.1mm in a typical telescope with a 1000mm focal length. Whether this matters at all depends on the application and the focal ratio of the telescope. A telescope with a focal ratio of f/4 (so, a 10” imaging Newtonian, for instance) has a theoretical depth of field of about 9 microns, or 0.009 mm, in principle, but for most applications, the depth of focus is more like 3-5 times that, so no more than ~0.05mm or 50 microns. Regardless, even under the most generous figures, a 10-degree shift over the course of a night is going to completely defocus your telescope, wrecking any astrophotography sessions.

But what about a slower scope? A 100mm f/10 refractor isn’t ideal for imaging (Sky-Watcher sells an f/9 apo for the task, but it’s advised you use a focal reducer to speed it up) but has a depth of focus six times greater than that of the 10” f/4 Newtonian, assuming the same tube material. Of course, a 100mm f/9 or f/8 refractor is a bit more stringent, but you can use these scopes without refocusing in all likelihood.

What does this all boil down to?

Unless you live in a climate with little day-night temperature variation, if you’re doing astrophotography with a sizable telescope (4″+) and the telescope or final optical system has a fast focal ratio (<f/6.5), temperature changes over the course of a night will have an impact on your images. However, these climates are also typically the kind of humid places where you’ll need to wrap a lot of dew heaters around your imaging rig, the heat from which could cause focus to shift anyway. As such, you pretty much need a motor focuser with an f/6.5 or faster scope for longer imaging sessions, regardless of whether you are doing monochrome or color imaging. Even if your camera has large pixels, defocus will still be apparent.

For visual observers, all of what we just went over above is somewhat irrelevant. These minute focus changes take place over hours; you’ll probably adjust the focus more frequently for non-thermal reasons than for any pronounced changes in focus.

Of course, when a telescope is brought from a warm indoor environment and exposed to chilly air at night, the optics and the tube will both contract. However, the optics will typically contract more than the tube. This is because the optics are usually made of a material with a higher coefficient of thermal expansion. The contraction of the optics will mar the image far more than any actual change of the focal plane does; you won’t notice any shift in focus since any significant enough temperature change to cause one would also mean the image itself is fairly fuzzy when you first set the telescope outside. Our article on cooling down telescopes explains more in depth.

What is a Bahtinov Mask?

A Bahtinov mask is a focusing aid designed to fit over the front aperture of a telescope. It is typically made of a thin, opaque material like plastic or cardboard and features a specific pattern of slits. This pattern creates a diffraction effect on bright stars, which aids in achieving precise focus. You can buy a Bahtinov mask from sites like High Point Scientific for various telescope apertures, cut one out of plastic or cardboard, or 3D-print one yourself.

When you place a Bahtinov mask over the telescope’s aperture and point it at a bright star, the slits create a distinctive diffraction pattern of spikes, like the spikes created by the spider in a reflecting telescope. This unique pattern usually consists of three sets of lines: one central set and two symmetrical sets on either side. As you adjust the telescope’s focus, these lines will shift in relation to each other.

The key to using a Bahtinov mask is to adjust the focus until the central line of the diffraction pattern is exactly in the middle of the other two lines. When these lines intersect precisely at one point, you have achieved perfect focus. The process is typically easier to observe through a camera connected to the telescope, where the pattern can be seen on a screen, but it can also be done by eye through an eyepiece. The high precision of the Bahtinov mask makes it particularly useful for astrophotography, where focus needs to be exact. It is neither particularly useful nor necessary for most visual work.

Optimizing Focuser Smoothness

The user can, at least somewhat, improve the performance of most focusers. For starters, there’s lubrication. Many rack-and-pinion focusers are greased with substandard stuff or not at all; a good machine lubricant of any sort will improve motions considerably. Crayford focusers’ rollers (or ball bearings in a linear bearing unit) can be lubricated with spray lubricant as well, for optimal motion.

Most Crayford or rack-and-pinion focusers at least have two small screws adjusting the tightness of the pinion/roller relative to the focus shaft. These screws should be adjusted to be tight enough until the motion feels appropriate to you. If your focuser is a plastic rack-and-pinion unit where the opposing side of the draw tube is unsupported by any roller or screw, you can put strips of tape on the drawtube or inside the focuser body to shim things until there is minimal play. If there are any screws opposite the roller/pinion assembly, spend some time playing with the tightness of these (simultaneously with any screws around the pinion) until things feel right. It’s not an exact science. Just be sure that any opposing screws are tightened equally relative to each other, and be sure not to over-tighten. Test the movement with the eyepiece or camera setup you would actually use—a focuser that seems buttery smooth unloaded will probably collapse in on itself with a 1-kilogram eyepiece inserted.

If your focuser accumulates dirt, be sure to clean it. You may want to even take apart, clean, and re-lubricate your focuser from time to time for optimal performance if it frequently gets dirty or wet.

If you have a catadioptric telescope with an internal focusing rod, you can partially dismantle the back end and grease the rod as well as the focus knob assembly; just be careful not to get anything near the primary mirror! Spending a few minutes racking the knob all the way to either end of travel will typically improve the focuser’s movement by helping to re-distribute lubricant and sync the threads.

Another trick is to add a lever arm or a larger-diameter focusing wheel to one of the focusing knobs, giving you more focusing precision by utilizing the principles of angular momentum. This can vary from a machined or printed “donut” that grips the original knob to literally grabbing a spring or C-clamp and using the end of that as a makeshift handle. I have employed both of these options on a variety of telescopes with great success.

Can I Install a New Focuser On My Telescope?

You can retrofit a new focuser onto most refracting or reflecting telescopes fairly easily, which is why we’ve taken the trouble to make recommendations for some aftermarket focusers in this article

Refractors usually just require you 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.

Installing a new focuser on a Newtonian reflector may require you to drill a few additional holes in the tube if the new focuser and old focuser body do not have the same screw hole pattern. This means you’ll want to remove the spider holding the secondary mirror and the primary mirror cell if you have a solid-tubed telescope that doesn’t come apart further. Any drill and hard metal bits will make short work of the thin-walled metal, plastic, plywood, or cardboard tube walls. Enlarging the hole in your tube for a wider focuser drawtube, however, is a different story; a Dremel or some other small cutting tool is required, and you’ll want to be sure the focuser is still centered on your secondary mirror.

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 involves removing the old focuser knob and bearing assembly with a screwdriver and hex keys, a fairly straightforward process.

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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