The Extremely Large Telescope

UK involvement in the ELT

What is a telescope?

Our eyes can see large distances. For example, the Andromeda Galaxy is the furthest object we can see by just using our eyes (at a distance of 2.5 million light-years). But, because it is so far away, something as big as a galaxy looks to us like a tiny fuzzy spot in the sky. Why does this happen?

As an object gets further away less of it’s light will reach your eye. This happens because the light coming from an object is dispersed with distance. For example, if our eyes/detector saw all of the light in a given direction, we would only see 25% of it if we were twice as far away, and only about 10% if we were three times that distance.

The further the object is from us, the less space it takes up on our eye’s retina (the light-sensitive tissue at the back of the eye) making the image smaller. To think about it in the language of a digital camera, an object that is too far away does not cover enough pixels on your retinal sensor for you to see the detail.

Therefore to see a distant object in more detail, we need to have a "bigger eye" to collect more light from it and create a brighter image. We can magnify part of that image so that it stretches out over more pixels on your retina. This is how telescopes help us study far way objects.

A telescope is an instrument that helps us study objects far away by making them appear much closer. They work by gathering light from the object and magnifying it. There are two main types of telescopes, depending how the light is collected:

Refracting Telescopes

These telescopes use lenses, which are curved pieces of glass, to collect and focus the light to form an image. As the light passes through the lens, it gets bent or refracted (in the same way we see a rainbow when light passes through a prism) and converges to a point called focus where the image is formed. A second lens called eyepiece then magnifies the image.

Reflecting Telescopes

These telescopes use curved or concave (curved inwards) mirrors instead of lenses to focus the light. The light from a distant object falls on the primary mirror, where the light is collected and reflected to the secondary mirror. The secondary mirror then reflects the light to a focus where it is then magnified by the eyepiece.

Why are all big telescopes reflectors?

Chromatic aberration: This is the main problem when using refractive telescopes. As the light passes though the lens it gets diffracted. But each wavelength or colour is diffracted by different angles and therefore has a different focus point. This causes the image to have fringes of colour along the boundaries between dark and bright areas of the image.

Dispersion: As the light passes through the lens, some of it gets lost through reflection and by absorption of the lens and therefore loses efficiency of its light collecting power.

Lens production: The production of a glass lens usually results in defects, such as small air bubbles trapped within the glass, which will affect the quality of the image. This makes lenses difficult and expensive to make and thus not practical for big telescopes.

Sag: Because lenses are needed to be held in place by their edges, the centre of large lenses are deformed by gravity. This affects their optical path and therefore distorts the images they produce. This is especially true for lenses bigger than 1 metre in diameter.

Do Bigger Mirrors Give us Bigger Images?

In the same way as a larger bucket allows us to collect more rainwater, the larger the mirror in a telescope, the more light we can collect from distant objects. This then allows us to look at them in greater detail.