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Ray Diagrams

Telescopes collect light from distant objects in space. They bring the light together at a focus. The focused image is viewed using the eye. Or the light can be measured using an instrument.

Not all telescopes focus light in the same way. Refracting telescopes use lenses. Reflecting telescope use mirrors.

We can use ray diagrams to compare types of telescope. These show the path that light takes.

On the left, 3 parallel blue, vertical lines travel upwards before hitting a yellow rectangle (labelled "Mirror) that is angled diagonally. The blue lines then travel horizontally away from the mirror and reach an elongated oval (labelled "lens") in the middle of the image. Leaving the lens, the lines all meet at a point some distance away that is labelled "Focus".
This work by The Schools' Observatory is licensed under All rights reserved

The ray diagram in the picture shows 3 'rays' of light entering the telescope. Each ray hits a mirror and is reflected off at 90 degrees. The 3 rays then pass through a glass lens. The light is refracted by the lens. Each ray exits the lens at a different angle. 

Together the mirror and lens 'steer' the rays so that they all meet at a chosen focus point. The focus is where the rays cross each other. A mirror can be used in place of the lens, but it must have a curved surface.

 

Explore and compare ray diagrams for different telescopes:

Refracting Telescope
On the left, horizontal, blue, parallel lines hit an elongated oval (labelled "Lens") at one end of an outlined yellow rectangle. The lines then all meet at a point some distance away and spread out again slightly, now hitting a much smaller oval contained within a small tube protrusion at the other end of the rectangle (labelled "Eyepiece"). The lines are horizontal again and reach a graphic of an eye.
This work by The Schools' Observatory is licensed under All rights reserved

The light enters the telescope and is focused by the main lens. Another smaller lens in the eyepiece magnifies the image. This makes it easier for us to see the image.

Primary Focus Telescope
An outline of a yellow rectangle with its left side missing is in the middle of the image. Blue, horizontal, parallel lines travel past an eye graphic in the middle of the shape and reach the other end where there is a curved white line labelled "Curved Mirror". The lines then meet at a point just before the eye, which is labelled "Focus".
This work by The Schools' Observatory is licensed under All rights reserved

The light enters the telescope and is reflected off the large, curved mirror at the back. The light is focused inside the telescope. This make sit difficult to see the result.

The telescope would need another mirror to move the focal point to somewhere more useful. Have a look at the ray diagrams for Newtonian and Cassegrain telescope to see this in action.

Newtonian Focus Telescope
An outline of a yellow rectangle with its left side missing is in the middle of the image. Blue, horizontal, parallel lines travel past a small diagonal line (labelled “Flat Mirror”) in the middle of the shape and reach the other end where there is a curved white line (labelled “Primary Mirror”). The lines then meet at the flat mirror and travel vertically down through an opening on the rectangles side, reaching a graphic eye.
This work by The Schools' Observatory is licensed under All rights reserved

The light enters the telescope and is reflected off the concave primary mirror at the back. A smaller, flat mirror reflects the light outside the telescope. 

The light is focused at a point where it can be viewed or measured.

Cassegrain Focus Telescope
An outline of a yellow rectangle with its left side missing is in the middle of the image. Blue, horizontal, parallel lines travel past a smaller filled in rectangle (labelled “Secondary Mirror”) in the middle of the shape and reach the other end where there is a curved white line (labelled “Primary Mirror”). The lines then meet at the secondary mirror, which has a curved front, before traveling horizontally through an opening on the far end of the rectangle, meeting at a point in front of an eye graphic.
This work by The Schools' Observatory is licensed under All rights reserved

The light enters the telescope and is reflected off the concave primary mirror at the back. A convex secondary mirror reflects the light out through a hole in the primary mirror. The light is focused outside the telescope, where it can be viewed or measured.

Most modern professional telescopes are this type, including the Liverpool Telescope.

Part of
Light
Telescope
Telescope Design
Telescopes
Reflection, Refraction and Diffraction