The Primary Mirror
The most important part of the telescope is the large light gathering mirror referred to as the primary mirror. Mirrors for astronomical telescopes are very difficult to make and the larger they are the more difficult it is to get everything correct.
Most mirrors are made from glass although, in the past, some have been made from aluminium of even liquid mercury! The mirror for the Liverpool Telescope is made from glass which is still considered the best material to use for most telescopes.
Not window glass!
The glass itself is special because it is not affected by changes in temperature, at least those changes experienced at the top of mountains where observatories are located. On La Palma, where the Liverpool Telescope will be sited, the temperature can vary between -10 and +25 degrees Celsius.
 The Mirror |
 The mirrors concave surface |
 The highly reflective surface |
Materials generally expand and contract with changes temperature, think of a thermometer where you can see the movement of mercury as the temperature alters. If the mirror in the Liverpool Telescope expanded and contracted the telescope would produce poor images of the stars. Light would be reflected in slightly different ways as the temperature changed resulting in distortion in the image.
The mirrors concave surface
The mirror's surface has a special shape
The surface of the mirror used by the Liverpool Telescope is slightly curved. You can see the effect of this curvature by looking at the reflections of people's heads in the picture. The diagram shows how the glass is shaped in cross-section. This type of mirror is called a concave mirror.
It is a very long process often taking between 6 and 9 months to grind the glass to create the right curve. The polishing process is very specialised because the glass surface must be made as smooth as possible.
How smooth is smooth?
The polishing process makes the surface of the mirror so smooth that any bumps are less than 1/10 of the wavelength of light. Putting it another way, if the mirror was the size of England there would be no hills more than 5 cm high!
Light Path Diagram
An extremely thin coating
To make the surface of the mirror highly reflective, so that as much light as possible from the stars is reflected, the final polished surface is coated with a thin layer of aluminium. This layer is only about 10 atoms thick. The coating lasts for about 2 years after which time the mirror is completely removed from the telescope. The old coating is removed and a fresh layer of aluminium applied using a special mirror coating plant built next to the telescopes on the observatory site.
You have probably noticed that the mirror seems to have a large hole in the centre. This is meant to be there!
Once light from the stars has been reflected from the primary mirror it travels upwards towards the top of the telescope. Here there is a secondary mirror which reflects the light a second time. The light then travels back down the telescope and straight through the hole in the centre of the primary mirror. Just below the primary mirror is an electronic (digital) camera placed just at the point where the starlight comes to a point (focus).
Modern astronomical telescope do not use photographic film, instead, and electronic image is created that can be displayed on a computer. You will be able to see these images (image data) yourself when the Liverpool Telescope is operating. You can even request that the telescope points at an object in the sky that you are interested in and then down load the digital image onto your computer at school or home. Check out the GO OBSERVING pages!
