Telescopes have been astronomy’s essential tools for centuries. Without them, we would be limited to what our eyes can see, and we wouldn’t know what marvels lie beyond our atmosphere.

But for the beginner astronomers like me, telescopes can also be a jungle of names, numbers, abbreviations… Let’s explore this jungle together!

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What is a telescope?

A telescope is simply an optical device that has two functions:

  1. Gathering light from a target
  2. Magnifying the target

That’s it! Of course, there are many ways to achieve these two goals, using various designs, materials and sizes. But in the end, all telescopes fulfil these two basic functions.

It’s all about gathering the light

The main and perhaps most important function of a telescope is gathering as much light as possible. Most people actually think that a telescope simply magnifies the target, allowing us to see things that are too small for the naked eye. But that’s half true: the telescopes can also gather more light than what our eyes can see, and thus make dim objects much brighter.

How is this possible? The light coming from an object, like a galaxy, doesn’t suddenly get brighter because you’re using a telescope. So why can we observe a galaxy with a telescope, while our eyes can’t?

Well, as far as light is concerned, size actually matters: our pupils are just a few millimetres wide, while a telescope’s front element can be as wide as a pool!

Let’s think of light as rain, and telescopes as buckets. If you want to collect water from the rain, a big bucket is better than a small one. If it rains for one hour, the big bucket will collect more water due to its wider surface.

Well, it’s the same with light: as a wave going in all possible directions, the best way to capture light is to have the widest telescope possible.

The mathematics behind this is fairly simple and really shows why a wider telescope gather more light. In the end, it’s just about calculating a surface. Let’s look at some examples:

  • My Pentax 200mm f/4 lens has a front element about 5 cm wide (200/4). Since it’s circular, the area that collects light is: π x (5 cm/2)² ≈ 20 cm²
  • My Canon 300mm f/4 lens has a front element about 7.5 cm wide (300/4). Since it’s circular, the area that collects light is: π x (7.5 cm/2)² ≈ 44 cm²

Which means that, by just increasing by 50% the width of the front element of the lens, we more than doubled the surface gathering light (exactly (1+50%)² = 2.25).

Now, imagine a telescope that is 50 cm wide (i.e. 10x wider than the Pentax lens). For such telescope, the surface of the front element would be almost 2,000 cm² which is 100x bigger than the Pentax lens!

This is why space agencies and scientists use very large telescopes, so they can gather as much light as possible to observe very dim targets. And sometimes, dim also means far away.

The James C. Maxwell Telescope in Hawaii. Photo by Nikita Andreev on Unsplash

Naming a telescope

The light gathering capability of a telescope is measured by what is called the aperture. For photographers, this term sounds familiar; it’s a ratio (e.g. f/5.6) that indicates how much light enters the camera, and is controlled by a diaphragm inside the lens. The aperture of a lens is often associated to its focal length (e.g. a Canon 200mm f/2.8 lens).

However, in astronomy, the aperture refers more to the size of the front element, and telescopes are often named using the aperture and the focal length (e.g. Meade 150/750, where 150mm is the aperture and 1200mm is the FL). Alternatively, some scopes have their aperture measured in inches; therefore, a 6″ inches telescope is equivalent to a 150mm telescope.

The aperture, the focal length and the f-ratio are all linked together by the following formula:

[f-ratio] = [focal length] / [aperture]

This is true for both camera lenses and telescopes (although camera lenses usually have a variable aperture, so the f-ratio indicated is the maximum one). For instance:

  • A Canon 200mm f/2.8 lens will have a front element of about 71mm in diameter: 200/2.8 = 71.4 mm
  • A Meade 150/750 telescope, will have a an f-ratio of 750/150 = 5, or f/5.
  • A Celestron 6″ f/10 telescope, will have a focal length of about 10 x 6 inches = 1500mm
Understanding telescopes

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