In a perfectly focused telescope, light rays from a distant object are bent or converged by the lens or mirror in such a way that they meet at a single point on the focal plane. When a telescope is out of focus, the light rays do not converge correctly at the focal plane. Instead, they either converge before reaching the focal plane (under-focus) or fail to converge by the time they reach it (over-focus), resulting in a blurred image. 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 talk about the types of telescope focusers individually.
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Helical focusers are essentially a big screw with a hole in the middle for your 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, have miniature helical focusers built in for fine adjustment as a supplement to your 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.
Helical Focuser Pros
- Hard to damage
- Usually offers plenty of fine adjustment
Helical Focuser Cons
- 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, 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 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 the user to compromise one for the other. The KineOptics HC-2 and HC-1 are examples of quality helical Crayfords; you can also make one yourself fairly easily out of readily available hardware.
This 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 can offer more than enough precision but sometimes suffers from backlash, which is a slight play or movement in the focuser when changing the direction of focus.
Rack-and-Pinion Focuser Pros
- Can be made very stiff, offering virtually unlimited payload
- If well-designed, deflection/backlash are minimal
- Fairly economical
- Easy to motorize
Rack-and-Pinion Focuser Cons
- Cheap models have lots of play
- 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
- Needs to be lubricated for good performance
Hybrid Rack-and-Pinion Focusers
Many high-end rack-and-pinion focusers 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. This is 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 sold today are hybrid focusers.
Rack-and-Pinion Focusers – Our Top Picks
Explore Scientific/Bresser 2.5” HEX Focuser
This unique focuser uses a hexagonal draw tube to help keep things concentric. Otherwise, it’s a fairly standard – if high quality – 2” rack-and-pinion, great for both imaging and visual purposes and with two supplied thread-on extension tubes.
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 which can be used to reach focus without the use of an additional separate piece of hardware. Unfortunately, using it is probably a bad idea with most eyepieces as there is plenty of play/wobble in this sliding auxiliary draw tube. In any case, the focuser itself is excellent and the low profile helps with minimizing the size of the secondary mirror if you are building your own telescope.
SVBONY 1.25” Rack-and-Pinion Focuser
This all-metal focuser is a great aftermarket upgrade for smaller Newtonians with plastic 1.25” focusers, such as the tabletop Dobsonians we often recommend to beginners. It even uses a brass compression ring to grip your eyepiece. Also sold under Skyoptikst with a refractor-compatible base plate. We do not recommend the 2” version due to its low quality machining.
Named after the Crayford Manor House Astronomical Society in England where it was invented, this focuser outwardly resembles a rack-and-pinion but does not use gears to drive the draw tube along. Instead, it relies on a smooth, non-toothed friction system.
In a Crayford focuser, the drawtube is moved by the pressure of a roller bearing (replacing the pinion gear) gliding against a flat area on the drawtube body (analogous to a rack), which is either a separate metal plate or simply machined into the body of the drawtube. When the focus knob is turned, this roller (connected to the knob via a rod or shaft) exerts pressure on the drawtube, causing it to slide in or out smoothly along the focuser body. The tension and pressure can be adjusted to accommodate different weights of eyepieces or cameras.
The main advantage of a Crayford focuser is the high precision and smoothness of the focusing movement. This is due to the absence of gears and teeth, which eliminates backlash (the slight play or jerkiness often found in rack-and-pinion or helical systems).
Crayford focusers tend to be more durable as they have fewer moving parts and no teeth to wear out over time. Since there are no gears, there’s usually no need for frequent lubrication, making the focuser easier to maintain. Like the Dobsonian telescopes it is often attached to, the Crayford focuser is an oddly simple but precise piece of engineering, relying only on the principles of friction to achieve smooth movement.
Crayford Focuser Pros
- Can be made very stiff, offering virtually unlimited payload
- If well-designed, deflection/backlash are minimal
- Fairly economical
- Easy to motorize
Crayford Focuser Cons
- Can develop wear to draw tube over time
- Can be pushed in accidentally if focuser lacks a locking mechanism
- Some models can be finicky to balance tension adjustments for smooth motion
- Extremely heavy loads may cause focuser to sag or rack in/out on its own
Crayford Focusers – Our Top Picks
GSO 2″ Crayford Focuser for Reflectors – Dual Speed
This is the focuser used on the Apertura AD Dobsonians, the Zhumell Z8/Z10/Z12, and most other GSO reflectors/Cassegrains not otherwise equipped with the linear bearing version of this focuser. While not perfect, this is a surprisingly good focuser, especially if you take the time to add some lubricant and tweak the tightness of the various grub/tension screws. Available for refractors as well.
JMI EV1 Focuser
More expensive than the GSO focusers, this high-quality dual-speed focuser is considerably lighter than GSO’s offerings and allows you to rotate the base of the focuser without any tools to put the knobs where you want them or rotate your camera.
JMI EV3 Focuser
Essentially a single-speed EV1, the EV3 (and the identical EV2) is made to a bit higher of a quality standard than import focusers like GSO’s; however, the price is quite steep.
GSO 2″ Crayford Focuser for Reflectors – Single Speed
A stripped-down version of the dual-speed GSO Crayford. There isn’t much of a reason to buy this new; the dual-speed unit is well worth the slight increase in cost. However, if your telescope has a fairly slow f/ratio (say, >f/8) you can get away with it. Available for refractors too, with various flange options.
Skyoptikst 2” Single-Speed Crayford Focuser
A basic single-speed Crayford focuser; not much different from the GSO single speed unit but with lower quality control and a bit more slop from worse machining tolerances on some units.
GSO 1.25″ Crayford Focuser for Reflectors – Single Speed
Essentially a scaled-down version of the 2” unit offered by GSO, this focuser can be swapped in for the plastic 1.25” rack-and-pinion focusers found on many Newtonian reflectors.
Linear Bearing Crayford Focusers
In a standard Crayford focuser, the drawtube’s movement is facilitated by a smooth, flat surface milled into the drawtube and a single cylindrical roller (coinciding with the focus knobs) to drive it along. The opposite side is supported by a set of smaller rollers.
In a linear bearing Crayford focuser, this system is upgraded by replacing the milled flat face of the drawtube with a machined metal rail and replacing the roller cylinders with a set of ball bearings, which are precisely aligned to ensure that the drawtube moves in a perfectly straight line. Like other Crayford designs, linear bearing focusers often feature adjustable tension and a locking mechanism to securely hold heavy equipment, as well as dual speed options.
Linear bearings can handle heavier loads compared to standard Crayford focusers. This makes them ideal for use with heavy eyepieces, large cameras, and other accessories.
The main issue with linear bearing focusers, however, is that the ball bearings or rail can accumulate dirt or grime easily and then produce bumpy or stiff movement. It is quite time-consuming to dismantle and clean a linear bearing focuser to fix this issue, which is almost inevitable on some designs.
Linear Bearing Crayford Focusers – Our Top Picks
Starlight 2″ Feather Touch 10:1 Crayford Focuser
This premium focuser by Starlight Instruments is a top-tier choice for serious astronomers, as with most Starlight products. Its ultra-precise fine focusing capabilities make it ideal for both observational astronomy and astrophotography. If you own a top-quality Dobsonian or refractor, this or the linear bearing variant is the focuser for you.
For imagers, a Feather Touch might be overkill. Available with various travel options, integrated coma corrector, lightweight variant, different anodization colors and more.
Baader 2″ Diamond Steeltrack Low-Profile Dual Speed Focuser for Newtonian Reflectors
Similar to the Feather Touch but weighing a bit more, the Baader Steeltrack is another excellent option for those demanding the highest precision and weight capacity out of a manual focuser.
GSO Dual Speed Linear Bearing Crayford Focuser
Essentially just an upgraded version of GSO’s regular dual-speed Crayford and stock on their R-C imaging telescopes, this focuser works well if properly lubricated and adjusted but tends to quickly develop slop and issues with dirt grinding inside the bearing mechanism. We would not generally recommend this focuser over the regular GSO dual-speed Crayford and certainly not for non-imaging applications.
Internal Focusers of Catadioptrics
Catadioptric Cassegrain-type telescopes, which combine lenses and mirrors in their optical design, often use an internal focusing mechanism that’s quite distinct from the external focusers found on most refractors and reflectors. This internal mechanism is a key feature of popular telescope designs like the Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain (Mak).
In a typical SCT, or Maksutov-Cassegrain catadioptric telescope, internal focusing is achieved by moving the primary mirror back and forth. The mirror is mounted on a threaded rod connected to the focus knob. When you turn the focus knob, the primary mirror moves along the rod, either toward or away from the secondary mirror. This movement changes the convergence point of the light entering the telescope, allowing for precise focus adjustments without altering the telescope’s external dimensions. Standardized threads on the back of the telescope allow you to attach a camera adapter, eyepiece, focal reducer, visual back, or star diagonal that screws directly to the rear of the tube without an intermediary 1.25” or 2” visual back adapter.
There are a number of pros to this configuration, but quite a few drawbacks.
- Compact Design: Since the focusing mechanism is internal, there is minimal hardware sticking out the back of the tube. This keeps the tube as physically short as possible and allows maximum clearance away from any fork mount.
- Sealed Optical Tube: The closed optical tube reduces the accumulation of dust and debris on the internal optics, making maintenance easier.
- Low Maintenance: Lubricants don’t seep out of the focuser or dry up, and dirt can’t clog the focusing mechanism nearly as quickly as an external focusing device.
- Image Shift: A common issue with this system is image shift. As the mirror moves, it can slightly change its alignment, wobbling on its rod and thus causing the image to appear to rock back and forth. This can be particularly problematic for astrophotography or for close-up planetary observation.
- Mirror Flop: In larger aperture catadioptrics, gravity can cause the mirror to slightly change its angle as the telescope is moved over a long period of time (e.g., during an astrophotography session), affecting focus and image quality. This is known as mirror flop.
- Lack of Precision: It can be hard to make precise focus adjustments when observing at high magnifications with a telescope that has an internal focuser and no additional fine focus knob.
Image shift is almost entirely a unit-to-unit variation, but it can be reduced by carefully lubricating and perhaps re-assembling the focus assembly; even just racking the focuser back and forth repeatedly helps a lot. For instance, I have an Explore Scientific 5” Maksutov that arrived with so much image shift that Jupiter appeared to fly out of the field of view even at 80x. After racking the focuser back and forth with the scope in my lap on the couch for a few Seinfeld episodes, it has been practically nonexistent.
To address mirror flop during long exposures, many high-end catadioptric telescopes come equipped with mirror locks. These locks can secure the primary mirror in place once focus is achieved, preventing any movement and thus maintaining a stable image and precise focus. However, it’s important to use mirror locks cautiously, as over-tightening can cause mechanical strains or misalignments. The main purpose of these locks is more for imaging than for frequent tightening and loosening during a night of visual observation.
External Focusers for Catadioptrics with Internal Focusers
Besides upgrading the focus knob to a dual-speed version, you can also screw an aftermarket Crayford or rack-and-pinion focuser to the back of many catadioptric telescopes. Adding an external focuser to an SCT or Mak can greatly enhance the focusing capabilities of these telescopes, especially for astrophotography or high-power observing. However, it’s essential to weigh these benefits against the additional cost, potential mechanical complexities, and the specific requirements of your observing or imaging setup.
External focusers for SCTs and Maksutovs are typically designed as add-ons that attach to the rear cell of the telescope, where the visual back or camera would normally connect. By using an external focuser, the primary mirror of the SCT, or Mak, remains stationary, minimizing the image shift associated with mirror movement.
However, using an external focuser is a last resort, and we generally don’t recommend it. An external focuser on the back of your scope increases the distance from the telescope’s optical system to the eyepiece or camera, which can affect the use of some accessories as well as the overall performance of the telescope. Adjusting the internal spacing of the optics to accommodate the external focuser can change the spherical aberration correction and the actual focal length of the system.
An external focuser attached to the back of an SCT or Mak designed with an internal focusing system will also, almost inevitably, result in vignetting and prevent the use of a focal reducer, so its only benefits are for planetary observing/imaging or other purposes where the telescope’s full field of view is not required. However, the loss of sharpness typically induced by the massive spacing changes inside the telescope may negate this use case.
This all being said, there are plenty of Cassegrain-type telescopes with no internal focus system and with a conventional focuser attached to the rear instead. Many Maksutov-Cassegrains made in Russia, for instance, use a fixed Crayford or helical focuser on the back of the telescope. The 152mm Maksutov-Cassegrain offered by Bresser/Explore Scientific and the various R-Cs/Cassegrains sold by GSO have Crayford focusers on the back end as well.
Dual Speed Focuser Knobs
Many focusers offer dual-speed adjustment in the form of a planetary gear. 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 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. The term “planetary” in this context refers to the gear system used. It’s called a planetary system because the gears move in a way that resembles planets orbiting around a sun. This gearing 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 the observer 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.
Top Dual Speed Focuser Upgrades
- GSO Dual Speed Focuser Upgrade: Also fits branded Apertura, Zhumell, Agena, TPO, and some Orion focusers.
- Two-Speed 10:1 Focus Knob Mechanism for Explore Scientific and BRESSER 2.5-inch HEX Focusers: Fits Explore Scientific FirstLight and Bresser Messier Dobsonians, 152mm Mak, some refractors.
Electronic motor focusers, also known as motorized focusers, have become increasingly popular among astrophotographers. Electronic motor focusers can be either built into the telescope or added as an aftermarket accessory. They typically consist of a small motor attached to the focuser’s shaft, controlled either via a hand controller or through a computer interface. In more advanced setups, the focuser can be integrated into a computerized control system, allowing for automatic focusing based on inputs from imaging software or sensors.
Since there’s no need to touch the telescope to adjust focus, motorized focusers can help reduce vibrations for planetary imaging or observing at high magnifications. You can also automatically adjust the focus to compensate for temperature changes throughout the night (which can expand or contract the telescope tube and shift the focus), as well as between different color filters if you are imaging with a monochrome camera and filter wheel.
For visual observers, the cost of motor focusers, the lack of rapid adjustment, and often the deprivation of any ability to adjust focus manually at all tend to make them less desirable compared to a quality dual-speed Crayford or rack-and-pinion. However, for astrophotography, these devices are a godsend.
Motor Focusers: Our Top Picks
ZWO EAF (Electronic Automatic Focuser)
This versatile electronic focuser is compatible with a wide range of telescopes and is particularly popular among astrophotographers for its precision. It can be controlled manually with a hand controller, through a ZWO ASIAIR, or through a PC, and is known for its smooth, accurate focusing.
PrimeLuceLab Sesto Senso 2
A highly precise robotic focusing motor, the Sesto Senso 2 is compatible with a variety of telescopes and is known for its build quality and reliability. Its robotic micro-movements make it ideal for high-resolution imaging.
Pegasus Astro FocusCube
This motor focuser is compatible with a wide range of telescopes and focuser designs.
Celestron Focus Motor for SCT, EdgeHD & RASA
This focuser is designed specifically for Celestron’s Schmidt-Cassegrain, EdgeHD, and Rowe-Ackermann Schmidt Astrograph (RASA) telescopes. It offers seamless integration with Celestron’s telescope systems; in addition to third-party software, it can run off of Celestron’s CPWI software or even your mount’s NexStar+ hand controller.