Skip to content
Reflector — Refractor — Advanced — Planetary — Astronomy — Eyepieces — Telescopes — Beginner — Astrophotography

A Comprehensive Tutorial on telescope focus adjustment mechanisms

Convergence of Light Rays from Distant Objects in a Telescope: This process occurs when the rays emanating from a stars or planets are collectively focused at a single point within the telescope.

,   and  Administrator
3 min read
Convergence of Light Rays in a Telescope: When starlight or light from planets is brought together...
Convergence of Light Rays in a Telescope: When starlight or light from planets is brought together at a focal point by the optical system of the telescope.

A Comprehensive Tutorial on telescope focus adjustment mechanisms

Telescope Focusers: An Invaluable yet Often Undervalued Components

In the world of astronomy, a telescope's focusers are indispensable, yet often overlooked, components. They play a vital role in achieving sharp and clear images of distant celestial objects, serving as the interface between the telescope and the eyepiece or camera.

A telescope's focuser, in essence, is an adjustable holder designed to attach an eyepiece, camera, or other accessories to the telescope. Most telescopes come equipped with either a built-in focuser or an attached external focuser, which can sometimes be swapped out based on the telescope type.

Focusers perform the simple yet crucial function of moving the eyepiece or camera along the telescope's optical axis—a movement that alters the distance between the eyepiece and the telescope's primary lens or mirror, thereby impacting the convergence point of the light rays. This convergence point, when it coincides with the focal plane of the eyepiece or camera sensor, produces a sharp and clear image of the distant object.

In many cases, a subpar focuser can lead to poor high-magnification performance and frustration, especially when handling heavy loads such as premium eyepieces or camera setups. A low-quality focuser may sag or wiggle excessively, posing a genuine threat to the equipment's longevity.

Properly chosen and installed focusers are just as crucial to a telescope's performance as any other mechanical component and arguably even more so than the optics themselves.

Telescope focusers come in various sizes, primarily determined by the diameter of the eyepiece or other hardware they can accommodate. The three most common sizes in amateur astronomy are 0.965-inch, 1.25-inch, and 2-inch. Rarer sizes can be found for specialized purposes, such as deep-sky astrophotography.

1.25-inch focusers are now the standard in amateur astronomy and strike a good balance between field of view, quality, and cost. Most telescopes take only 1.25" eyepieces due to physical limitations or a lack of need to go larger.

2-inch focusers, used by more advanced and larger telescopes, allow for eyepieces that provide a wider field of view, beneficial for deep-sky observing and necessary for deep-sky astrophotography with many common camera sensor sizes. Additionally, 2-inch focusers are necessary to utilize accessories like focal reducers, field flatteners, and coma correctors.

Enlarging the actual hole in a tube for a wider focuser drawtube can be challenging and may require careful planning and precision drilling. Notably, telescopes with an external focuser can have their focusers swapped out if the screw hole patterns match between the new and old focusers. New holes may need to be drilled if the screw pattern does not match between the new and old focuser units.

Focusers can be categorized into three primary types: helical, rack-and-pinion, and Crayford. All types perform the same function, which is to move the eyepiece/camera and other relevant accessories back and forth relative to where the telescope's objective lens or mirror focuses the light.

Upgrading a telescope's focuser offers several advantages, notably, improved mechanical stability, reduced play, lower profile designs for better illumination, dual-speed or fine focusing capability, load capacity, and smooth operation. These upgrades can significantly enhance focus precision, load handling, and operational smoothness, particularly relevant for serious visual observing and astrophotography.

Some high-quality focusers include the Omegon Newton Hybrid Crayford Dual Speed 2" Focuser and the QHY Q-Focuser Electronic Automatic Focuser (High Precision Version). Both focusers boast robust construction, load tolerance, dual-speed focusing, and compatibility with a wide range of telescopes and imaging equipment.

  1. For better focus precision in telescopes, the role of focusers, often overlooked components, is indispensable in achieving sharp images of distant celestial objects.
  2. Focusers serve as the interface between the telescope and eyepieces or cameras, altering the distance between the eyepiece and telescope's primary lens or mirror to converge light rays onto the focal plane.
  3. Properly chosen and installed focusers are essential for a telescope's performance, often even more so than the optics themselves.
  4. In amateur astronomy, the standard focuser size is 1.25-inch, offering a good balance between field of view, quality, and cost.
  5. For deep-sky astrophotography, 2-inch focusers are necessary for ancillary accessories like focal reducers and field flatteners and provide a wider field of view suitable for larger telescopes.
  6. New technology has introduced data-and-cloud-computing-focused telescopes, requiring upgrades to advanced focusers for better mechanical stability, reduced play, and smoother operation, ideal for both visual observing and astrophotography.
  7. High-quality focusers like the Omegon Newton Hybrid Crayford Dual Speed 2" Focuser and the QHY Q-Focuser Electronic Automatic Focuser (High Precision Version) are available, boasting robust construction, load tolerance, dual-speed focusing, and compatibility with various telescopes and imaging equipment.

Read also:

    Latest