That little telescope was an astronomical game changer

Years ago, one of the UK observatories had a replica of a telescope made by Isaac Newton in the late 17th Century. It was marked “For use in case of emergency,” jokingly suggesting that if the big telescope failed, astronomers could use this one.

Actually, that little telescope was an astronomical game changer, and is now the standard design for most optical telescopes intended for astronomy, whether in observatories or in backyards.

At some point in our young lives most of us have been given toy binoculars. They were usually made of brightly-coloured plastic, with plastic lenses. The binoculars worked, but wherever there was a sharp junction between bright and dark areas, like the edge of a roof with the sky in the background, we saw colours: red on some edges and blue on others.

This is effect is known as chromatic aberration. It is due to different colours of light passing through lenses at different speeds. This means they come into focus at different places. Today, using composite lenses made from different types of glass, this problem can be corrected. However, back in Newton’s day chromatic aberration was a major problem.

Newton’s idea was that if the light collected by a telescope did not have to go through glass on its way to becoming an image, chromatic aberration would not happen, so he used a concave mirror. With a lens the light is brought to a focus behind the lens, forming an image there. With a concave mirror the light is focussed to a point in front of the mirror, which puts the image at a very inconvenient place, so Newton added another small mirror, near the focal point.

This sent the converging light, before it actually forms the image, out of the side of the telescope, where the image can be easily viewed, at the expense having that small mirror blocking some of the light.

By putting the reflecting coating on the front face of the mirror, the light goes through no glass and all the colours are sent to the same focus. There is no chromatic aberration. The concave mirror in Newton’s telescope was not made of glass at all. He used a metallic alloy called “speculum.”

After all, the mirror material need not be transparent. Today mirrors are made of special glasses or ceramics.

Another great advantage of reflecting telescopes is that mirrors can be made really big. Since light has to pass through lenses as freely as possible, the only place a lens can be supported is round the edge. This means there is a limit to how big it can be made before it distorts under its own weight, or even breaks. It is easy to support a mirror. We can put as much support structure or other hardware as we like behind it, because the light does not go there. This leads us to another advantage. Lenses need to be high up, at the top end of a telescope; mirrors go at the bottom end, where it is easier to deal with huge weights, and the telescope is never top heavy, making it safer and easier to control.

The biggest mirror we can make is around 5 m in diameter. As the diameter increases, the thickness has to increase too, to keep it in shape as the telescope is pointed in different directions. Today we have found out how to make far larger mirrors.

We can make them out of relatively thin single pieces of material, or out of many hexagonal panels. Behind the mirror are lots of computer-controlled actuators, which continually adjust the mirror to keep it in shape.

Mirrors of around 10 metres diameter are in use now, and a telescope with a 30m mirror is under construction.

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On the 21st the Sun reaches the northernmost point in its yearly travels, and at noon is higher in the sky than at any other time of the year. That gives us the day with the longest period of daylight. Mercury lurks low in the dawn glow, with Venus higher and brighter. To Venus' right, lie Mars and Jupiter, close together, then Saturn. The Moon will be New on the 28th.