How Telescopes Work: Understanding The Main Components of Telescopes


Telescopes are precision instruments that are constructed using several distinct working parts. These optical devices work on the principle of collecting light, magnifying it and then redirecting it. As a result of this, a close-up image of the object on view is produced.

However, for the telescope to work correctly, every component should be skillfully designed and assembled according to strict specifications. So, it is not a surprise that professional telescopes cost quite much. However, there are also available a lot of high-quality telescopes for amateur astronomers with lower price tags. Let’s find out how these tools work and deliver amazing views of planets and stars located at far far away.

At the first optical light arriving from the space is gathered and focused by a prism, lens, or mirror, or sets of those. These optical components refract or reflect light into a common focal point, allowing the camera, computer or human eye to view an exact image of the object. The different components forming a telescope are explained below.

The Optical Tube

The optical tube is the cylindrical part, just above the mount. It is the biggest part of the actual telescope excluding mount or dome. It holds all the lenses or mirrors that gather light when viewing objects. The exact positions of the lenses inside the optical tube depend on the telescope.

For example, a refracting telescope has the objective lens placed at the front, while a reflecting telescope has the objective at the back. The optical tube size depends on the size of individual lenses or mirrors.

Larger lenses and mirrors make a larger optical tube. However, some telescopes are powerful and portable. The optical tube is also a protective casing that protects internal parts from breaking or damage.

The mount

The mount supports and keeps the telescope stable, making also it ultimately a very important part. Actually, the mount is usually the weakest point of amateur telescopes as most manufacturers try to save production costs choosing low-quality mounts. This could make device almost useless, that could be otherwise quite good. So, in such case, you could maybe improve a mount by yourself or buy a better mount separately.

The Objective Lens or Mirror

The lens or mirror is the main and most important part of a telescope. The lens is normally a curved sheet of glass inside the optical tube that has a perfect axial symmetry used to refract or reflect light rays.

A single telescope may have multiple lenses to reduce optical inaccuracies or aberrations in the image produced. The type of mirrors or lenses and their arrangement determines most features of the telescope such as the magnification ability, image clarity, and the resulting color distortion.

A refracting telescope has two convex lenses, positioned oppositely at the telescope ends. The largest lens called objective lens is placed at the front of the telescope. It has an opening called aperture that determines how much light is allowed into the scope; though it is dependent on the size of the lens, larger lenses allow more light. More light in the telescope allows a view of more objects in the nighttime sky. The refracting telescope works on the principle of bending light.

Reflective telescopes also called catadioptric telescopes have a series of curved mirrors instead of lenses. The mirrors work in conjunction with the objective lens producing a crisper image without optical aberrations. Reflecting telescopes are mainly found in so called “light-bucket” telescopes, that have larger objective lenses than the refracting telescopes. They work on the principle of bouncing back light.


The focuser is positioned outside the optical tube. Its function is focusing the image produced further. An ideal focuser allows for tiny adjustments to produce a clear and sharp image.

There two types of focusers: The Crayford focusers and the Rack and pinion focusers. Rack and pinion focusers are commonly used on beginner-level telescopes. For Crayford focusers, they provide sharp images while making fewer adjustments than the rack and pinions. This convenience has made Crayford focusers to take over rack and pinion focusers in designing amateur telescopes.


The eyepiece also called the telescope’s ocular, is a cylindrical shaped attachment at the back (for a refractor telescope) or the top (for a reflector telescope).

It focuses the light gathered by the reflective mirrors or objective lens the final image as seen by the eye. It fits into the focuser providing a greater field of view and higher magnification. A telescope may have one or two eyepieces, depending on the type.

Every eyepiece has a certain focal length. The magnification produced is calculated by dividing the telescope’s focal length by the eyepiece focal length. The maximum achievable magnification for a telescope is identified by multiplying the telescope focal ratio by 7.

The eyepieces vary in terms of diameter, but the typical length is either 1.25 or 2 cm. The eyepiece comes with a particular field of view, that determines the visible sky view when using it.

There are different types of eyepieces such as Plossls and Kellners. When choosing an eyepiece, ensure it fits on the focuser and checks its magnification. It is advisable that you purchase a Barlow lens along with your eyepiece, it is fitted behind the eyepiece to increase magnification.

Overview of components

Main parts of Newtonian telescope. Image credit: Szőcs Tamás CC Wikipedia

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Be sure to read also NASA’s “How Telescopes sees” article!

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