Sunday space: What are telescopes and how do they work?

When most people think of astronomy, one of the first things that comes to mind other than stars or galaxies is the astronomer's greatest tool: the telescope.

But what is a telescope and how does it work?

A telescope is "a cylinder-shaped device for making objects that are far away look closer and larger, using a combination of lenses, or lenses and curved mirrors".

This is a good definition, but it doesn't tell us how these pieces of equipment work.

There are two main types of telescopes: refractors and reflectors.

These differ by whether they are primarily using lenses or mirrors.


Refractor telescopes consist of at least two lenses in a tube: the first, larger lens is called the objective and the smaller second lens is known as the eyepiece.

The light from a distant star goes through the objective lens, which has a convex shape - shaped outward.

This brings it to a focus just before the eyepiece.

This essentially means that all of the light entering the telescope comes to a single point.

Sunday space: What are telescopes and how do they work?

The eyepiece then refocusses that point of light onto the back of your eye at a given magnification.

Refracting telescopes have a bunch of problems with them, however. Building really large, precise objective lenses gets really hard and requires a very long tube for the light to focus properly.

They also suffer from chromatic aberration - light at different colours will focus at different points.

To get around this, most modern telescopes are reflectors - they replace the objective lens with a concave or inward shape mirror known as the primary.

As shown in the accompanying image (a popular configuration known as a Cassegrain reflector), light bounces off the primary to a smaller convex mirror known as the secondary, which then focusses the light into a camera or an eyepiece.

These types of telescopes can be made very large without the problems associated with refractors.

When I take people stargazing, many people ask me about the magnification of the telescope we use.

While this is a valid question, the magnification is determined by the eyepiece rather than the telescope itself.

You can obtain a really high magnification on a really small telescope, but not get a great image.

You can think of this like a digital photo: you can zoom into an image on the computer, but only get blurry pixels if the image is not high resolution.

The image quality is really set by the primary mirror or objective lens.

The bigger in diameter the primary, the better the resolution you can achieve, and so zooming in with a better eyepiece will give you more detail.

A bigger telescope also acts as a bigger "light bucket" and so can see fainter stars and galaxies.

So, in terms of telescopes, bigger is definitely better.