The greatest telescopes in history

Hale was not satisfied. His constant call was for ‘more light!’ and if a 100-inch telescope, why not a 200-inch?

What are, or were, the world’s greatest telescopes? I do not refer particularly to size; I mean the telescopes that in their time reigned supreme and led to ‘quantum leaps’ in science. I can list six: Galileo’s original refractor, Isaac Newton’s first reflector, the Rosse ‘Leviathan’ at Birr Castle, the Mount Wilson 100-inch, the Palomar 200-inch and the Hubble Space Telescope. All are still in working order, although the first of them has not been turned skyward for a long time!

We begin in the winter of 1609–1610 in Holland: a telescope had been built by a Middleburg spectacle-maker, Hans Lippershey. It was probably not the first, but reports of earlier telescopes are vague, and Lippershey’s is the first about which we have definite information. News of it reached the Italian scientist Galileo Galilei in Pisa and “sparing neither trouble nor expense,” he made a telescope for himself.

Up to then, astronomical observations had been made solely with the naked eye, the limitations of which are obvious. But in a few weeks, early in 1610, Galileo made a series of spectacular discoveries that changed our outlook forever. He saw the craters of the Moon, the phases of Venus, the “myriad stars” of the Milky Way, and – perhaps most importantly of all – the four main satellites of Jupiter still known as the Galileans.

All these discoveries led to the realisation that the Sun, not the Earth, is the centre of our planetary system. Galileo was accused of heresy and condemned by the Inquisition, and it was almost four centuries before the Vatican admitted he had been right.

Reflected glory

Next, the tiny reflector made by Isaac Newton and demonstrated to the Royal Society in 1672. Galileo’s telescope was, of course, a refractor, and apart from various other defects it produced false colour – a lens does not refract all wavelengths equally. Newton could see no way around this problem, and so he decided to collect the light with a curved mirror. It worked and his prototype telescope is still in the possession of the Royal Society, or so we think. Suggestions that it may be a later copy have been officially discounted.

Now we come to 1845 and the 72-inch reflector made, more or less single-handed, by the Earl of Rosse – an Irish nobleman whose home was Birr Castle, not far from Athlone in Ireland. He was not a professional scientist, but he was deeply interested in astronomy, and began to make reflecting telescopes.

He made a conventional 36-inch, and then decided to attempt a telescope twice this aperture. The mirror would be made of speculum metal, an alloy of copper and tin; glass mirrors of any real size were too difficult to make by mid 19th century manufacturers. Lord Rosse built a forge, cast the mirror and made a mounting for it.

The resulting telescope was extraordinary: the huge wooden tube was mounted at the bottom between two massive stone walls. It could swing for a limited distance to either side of the meridian, covering a narrow strip of sky; you had to wait for the Earth’s rotation to bring your target into view.

Almost as soon as Lord Rosse had completed the telescope, he found that the ‘starry nebulae’ were spirals, shaped like Catherine wheels. We now know them to be galaxies, far beyond the confines of our Milky Way.

After 1909 the telescope fell into disuse, but it has now been restored and is fully operational once more. I am particularly glad about this; indeed, I believe that the idea of restoration came from a suggestion I made in about 1960, when I was spending a purely social weekend at Birr Castle. Until 1917, the ‘Leviathan’ remained the largest telescope in existence, although it had been overtaken in performance by great refractors such as the Meudon 33-inch in France, and the Yerkes 40-inch in the US.

Now we move on to George Ellery Hale, a young American astronomer who wanted to build powerful reflectors, and had the happy knack of persuading friendly millionaires to finance them. He set up the 100-inch reflector on Mount Wilson, California, that was in a class of its own for over 30 years. It could show faint objects and fine detail beyond the range of any other instrument.

Spiralling to an answer

In 1923, Edwin Hubble used it to identify Cepheid variables in several of Lord Rosse’s spirals, including M31 in Andromeda. Up to that time, astronomers had argued about the nature of the spirals: were they parts of the Milky Way, or galaxies in their own right? As soon as Hubble found his Cepheids, the question was answered – they were far away in intergalactic space. This work could not have been carried out with any other telescope.

Still Hale was not satisfied. His constant call was for “more light!” and if a 100-inch, why not a 200-inch? Again he planned and succeeded, and in 1948 the 200-inch was set up on Mount Palomar. Sadly Hale did not live to see it completed, but it is only right that the telescope should be named after him.

It ruled supreme for several decades, and was used by Walter Baade to show that the Universe is much larger than had been thought. There had been an excusable error in the Cepheid scale used by Hubble: the stars were twice as luminous, so twice as far away, than believed. Again, only the 200-inch was capable of making these measurements.

And finally to 1990, and my last selection – the space telescope named after Edwin Hubble. It is ‘only’ a 94-inch, but operating from above the atmosphere means it is untroubled by pollution and can receive wavelengths that are blocked by air. All of us know the magnificent images it sends, but ‘pretty pictures’ make up only a part of its scientific programme. It will not last indefinitely, but let us hope that it continues to function for a long time yet.

These, then, make up my list: Galileo, Newton, Rosse, the Hale 100-inch and 200-inch, and the Hubble. If someone makes a list in, say, the year 2200, I wonder what theirs will be?