Telescope optics

When looking up in the night sky, your eyes are doing exactly what any telescope does - Gathering light. However the aperture or opening of our eyes isn't very large so the amount of light that enters our eyes is limited. By using a telescope, you can artificially increase the amount of light entering your eyes thus brightening a part of the night sky. This page explains the basic principles of optics in relation to a telescope.

A pinhole camera
Imagine a sheet of light sensitive cells on your skin. Because they are all exposed to light coming from every direction, the only thing you can 'see' with this would be the brightness of your surroundings. Now, imagine a light shielding box is covered over the sheet of light sensitive cells and a tiny hole is made facing out. With this each light sensitive cell only has a line of sight towards one particular area of your surroundings. This is a basic pinhole camera.



Using a lens
However the aperture or opening is very small so not a lot of light can enter, making the image quite dark. Increasing the size of the hole would allow more light to enter but this would decrease the resolution. This can be solved by using a lens. A lens focuses parallel rays of light onto a spot or focal point, effectively creating a mosaic of pinhole camera holes that work together to form a single image. The bigger the lens the more light it can pass trough. The distance of the focal point from the lens (also known as the focal length) can be changed by changing the shape of the lens.

Faint objects in the night sky
The lenses in our eyes create a much bigger opening than that of a pinhole camera, but for the very dim objects in our night sky this isn't enough. A telescope artificially increases the surface area that collects light thus making the object you are looking at appear brighter. This is why astronomers are always in need of bigger telescopes so more light is gathered at the same time.