Today I will talk about an important functional unit of the telescope – the focuser. The purpose of the focuser is to sharpen the image in the eyepiece or on the photodetector matrix. Typically, beginners when buying a telescope rarely pay attention to the type and characteristics of the focuser, but a high-quality focuser makes astronomical observations and obtaining astrophotographs more comfortable and productive.
As a rule, a focuser is a movable tube into which either an eyepiece, or a star diagonal with an eyepiece, or a photo / video camera are inserted.
The diameter of the focuser is usually indicated in inches. Among amateur telescopes, the most common are focusers with landing diameters of 1.25 “, 2”, rarely 0.965 “, and even less often – 3”. Accordingly, with 1.25″ focuser, you can use 1.25″ eyepieces, with 2″ focuser – both 2″ eyepieces and 1.25″ (through a special adapter). There are not so many eyepiece focusers under 3″ eyewear – perhaps the famous Explore Scientific 30mm 100 degrees. The focuser 0.965″ is usually equipped with the simplest telescopes with an aperture of up to 50mm.
The main advantage of the 2” focuser in front of 1.25” is the ability to use various 2” accessories – wide-angle eyepieces, coma corrections, field flatteners\reducers, cameras with a large diagonal, etc.
2. Types of the focusers
The most common types of focusers are rack and pinion and Craifford. More rare ones are threaded, helicoid, Clement’s focuser and sliding focuser.
The rack focuser consists of two tubes, one of which is inserted into the other. On a tube with a smaller diameter there is a gear rack, on a tube with a larger diameter there is a wheel with a gear wheel. When the wheel (focuser knob) is rotated, the small sleeve moves and the eyepiece or camera matrix moves. This type of focuser is installed on most budget models of telescopes (refractors up to 102 mm, reflectors up to 150 mm). Advantages – simplicity and cheapness, the lack of rotation of the photodetector. The main disadvantage is backlash, but a quality-made rail is very, very convenient to use.
Owners of 3D printers can print the basis of such a focuser:
The Crayford focuser is an advanced version of the rack focuser, where the rail is simply absent – the metal axis on friction moves the focuser tube. The main advantage is smooth focusing, lack of backlash, lack of rotation of the photodetector. However, in the Crayford focuser, slippage is possible – for example, when using heavy eyepieces or cameras.
If desired, you can make a simple Crayford focuser from improvised materials.
A helical (threaded) focuser is two tubes, one of which is screwed into a sudden thread. Advantages – simplicity and low weight. Cons – backlash, the need for a generous thread lubrication with thick grease, as well as the rotation of the camera when focusing. This type of focuser is often used in finders and telescopes.
The helical focuser is something like a screw in a nut: rotate the nut – the screw moves back and forth. There is no rotation of the camera, but there are noticeable backlashes. It is used in many photo lenses (for example, in the famous Helios-44-2, from which you can make a good helical focuser for a telescope if you wish). The main advantage is that minimal dimensions are possible, which is critical when the focus point of the telescope is slightly out of the optical tube.
Clement focuser is a pretty exotic type of focuser. It is a kind of door hinge that rests on two steel plates. The main advantages are rigidity, lack of backlash, high load capacity. Cons – the difficulty in manufacturing, dimensions and high cost. However, if desired, it can also be made independently.
Sliding focuser is an even rarer way to focus. I met its use only on some amateur telescopes and on several factory telescopes. When focusing, the eyepiece moves with the secondary mirror unit.
I would also like to mention focusing with the main mirror of the telescope. This focusing method is used in most Schmidt-Cassegrains and Maksutov-Cassegrains. A small pusher moves the main mirror back and forth, and, accordingly, the removal of focus outside the pipe changes.
With this focusing method, image shift and a slight misalignment are possible, so some amateurs will equip their telescope with an additional focuser:
3. Nubmer of speeds
Focusers can be single-speed and dual-speed. In the dual-speed focuser, along with the main focusing wheel, there is one more with a gearbox (usually with a reduction ratio of 1:10). A dual-speed focuser is much more convenient when focusing at high magnifications.
However, more accurate focusing can be achieved by replacing the focusing wheel with a larger one:
When using a conventional focuser of any type, the focus knob must be rotated by hand. This can cause additional vibration of the telescope and, accordingly, shaking of the image. It is also not always convenient to be near a telescope – for example, when focusing on a laptop screen (astrophotography of planets, the moon, the sun). In all these cases, a wonderful device comes to the rescue – an electro focuser. It is a small motor that joins the focuser shaft in one way or another. Using a special remote control, you can control the focus and adjust the speed of rotation. Focusing is smooth and without any hesitation.
For example, simlpe electric focuser Synta (Sky-Watcher) and using with different telescopes:
There are also more functional models of electrofocusers that can be controlled from a computer (for example, Robofocus). Such devices can provide automatic focusing and focus adjustment of the telescope throughout the shooting session.
When choosing a telescope, be sure to pay attention to the type of focuser and its characteristics. If, with similar optical parameters of the telescopes, there is a choice between a rack focuser and a Craifford, it is, of course, better to take a telescope with a Craifford focuser. It will allow you to use a wide variety of optical accessories, has a smoother focusing and greater payload.