What is a telescope? The basics explained

We sometimes get the question ‘what is a telescope? Telescopes have been indispensable in the toolbox of stargazers since the early seventeenth century. They help you see craters on the moon, admire Saturn's rings or observe distant galaxies. But how does such an instrument actually work? In this blog post, we explain in an accessible way how a telescope collects and magnifies light and what types exist.

Why do you need a telescope?

First of all, why would you want to buy a telescope? Well, without a telescope, the universe remains shallow. You see a few bright dots, but miss the details and depth that make the universe so fascinating. With a telescope, you bring space to you literally and figuratively.

A telescope is not a luxury (perhaps just an affordable luxury?), but a gateway to wonder. It addresses two fundamental limitations of the human eye:

• Light collection: the larger the lens or mirror, the more light is captured. This allows you to see objects that are normally invisible, such as nebulae or the moons of Jupiter.
• Enlarge image: distant celestial bodies appear closer, allowing you to distinguish craters, rings or cloud bands.

So you are not just buying a pair of scopes but investing in an experience. A telescope makes it possible to spot planets yourself, study the moon in detail and discover deep star clusters or nebulae. An unforgettable way for young and old to get to know the universe.

What is a telescope and what does a telescope do?

A telescope doesn't just magnify objects; it mainly collects light. The more light that comes in, the more details you see. After all, stars and planets are extremely far away and emit little light. A telescope makes that light visible and magnifies the image, so you can see things that remain invisible to the naked eye.

There are roughly two types of telescopes:

• Lens telescopes (refractors). - use glass lenses
• Mirror telescopes (reflectors). - use curved mirrors

Both systems have their advantages, about which more later.

The basics: focal length and aperture

Two numbers are crucial in a telescope:

• Opening (in mm) often the first number (e.g. Celestron Starsense explorer 114/1000 - then 114 is the aperture in mm) how big is the lens or mirror? More aperture means more light, so more detail.
• Focal length (in mm) - determines how long the light path is in the telescope. This then determines the magnification.

You calculate the magnification like this: Magnification = focal length telescope ÷ focal length eyepiece

So a telescope with 650 mm focal length and a 10 mm eyepiece yields 65x magnification.

But note that more magnification does not automatically mean a better image. Often, less is more. A sharp image at 120x is more valuable than a blurry spot at 300x. Maximum useful magnification of one is lens aperture x 2. So for example, if a telescope has a lens aperture of 130, the max magnification is 260. Anything above that constitutes a blurry image.

Refractor versus reflector

So what is the difference between these two telescope types?

Refractor (lens telescope)

• Light passes through a glass lens
• Seals light directly: low maintenance
• Good for moon and planets
• Often a bit more expensive per millimeter opening
• Compact and easy to use

Reflector (mirror telescope)

• Light is reflected off a curved mirror
• More opening for less money
• Good for deep-sky objects (nebulae, galaxies)
• You usually look from the side, through a mirror
• Needs occasional collimation (alignment of mirrors)

There are also combination systems such as Maksutov-Cassegrains or Schmidt-Cassegrains, but they are less common for absolute beginners.

How does a telescope work optically?

When light from a distant star or planet enters your telescope, something special happens.

• Refractors use a convex lens to refract and concentrate parallel light rays into a focal point. After the focal point, the light diverges again, and the eyepiece (the lens you look through) bends this light so you see a magnified, upright image.
• Reflectors use a concave mirror to concentrate the light. A smaller second mirror sends the light sideways to the eyepiece. So you are looking from the side.

For both types, the larger the aperture (lens or mirror), the more light and therefore the more detail. However, the quality of the image is also affected by the atmosphere.

The influence of the atmosphere

A telescope on Earth looks through a thick layer of air. That layer moves continuously - with temperature differences, moisture and turbulence. This causes:

• Seeing: the stability of the sky. Poor seeing gives a dancing, blurred image. You can often check this through websites like clearoutside.com.
• Transparency: the brightness of the sky. Haze, moisture or light pollution reduce contrast.

Stars twinkling violently = poor seeing. Do you see a lot of Little Bear stars? Then the transparency is good.

This is why professional telescopes are often located on high mountains or even in space. The Hubble Space Telescope, for example, is suspended outside the atmosphere and can therefore take extremely sharp images, to the level of a firefly in Tokyo as seen from New York.

What do you see through a telescope?

It depends on the combination of aperture, magnification, and conditions. With a good beginner's scope, you can see the following:

✓ Craters and details on the moon
✓ The rings of Saturn
✓ Cloud bands on Jupiter and its four largest moons
✓ The phases of Venus
✓ And under good conditions: star clusters and nebulae

Note that you will not see colorful Hubble images, but subtle shades of gray or pale yellow/orange detail. Visual viewing is different from astrophotography.

Common mistakes

• Buying a telescope with “800x magnification” on the box. Nonsense. No beginner's telescope will give a usable image at that kind of magnification.
• Cheap toy telescopes: these often cause frustration due to poor optics and shaky tripods.
• No patience: learning to focus and find objects takes practice.

Which telescope do we recommend for beginners?

Some excellent choices from our range:

• Celestron StarSense Explorer 114AZ - User-friendly, works with your smartphone
• Sky-Watcher Explorer 130P AZ-GTe - Electronically controlled, excellent optics
• Sky-Watcher Heritage 100P - Compact, lightweight and powerful for its size
• Bresser Messier 130/650 Dobson - Affordable, visually powerful and stable

👉 View our selection guide for beginners for a list of the best entry-level models.

Need help with your choice?

Still unsure which telescope is best for you? Ask your question to AstroBob, our smart chatbot at the bottom right of the site. Or send us a message/call us and we'll be happy to think with you.