How the mirrors are made.

'Light Bucket' is a common way to describe a mirror amongst astronomers. Mirrors are very important in reflecting telescopes, but present the biggest problems in design, manufacture and installation.

Whenever a large mirror is made it must be ground perfectly smooth. This process can take many years to complete. The Mt. Palomar telescope in California, was first used in 1947 and has a mirror that is five metres across. This makes it one of the biggest reflecting telescopes in the world. The mirror it uses took eleven years to grind to the right shape.

To avoid the need for large mirrors, scientists devised techniques to be more efficient with the light collected in their mirrors. Originally the film used could only react with 1% of the light from the telescope. Now they can use close to the theorhecttical limit of 70%. This is done by using digital light detectors.

Today the latest technique is to melt the glass into the parabola shape in the kiln. This is relatively easy because a spinning liquid creates a parabola due to the nature of our universe. This means that less glass has to be ground away to create a smooth surface in exactly the right shape.

Using a spinning liquid to create a parabola is also used by a university in Canada. They have a rotating bowl of mercury. This technique was used the mid 1800's but because they could not cushion it from vibration it was not practical. Today air cushions are used that eliminate vibration. The surface of the liquid is also protected to by applying a thin layer of chemical that acts as a shield to air movement. A mercury mirror is also much cheaper than a glass mirror, but can not be tilted away from the vertical axis at all.

Another technique to make the mirrors weigh less is to make the back of the mirror in a honeycomb shape. This reduces the weight significantly and the strength of honeycomb is surprising. When there is less weight, the weight force of the mirror is less and eliminates the need for strong mountings. An interesting study into the strength of honeycomb structures can be found at:

http://www.hexcelcomposites.com/products/honeycomb/hexweb_attrib/hw_p06.htm