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The Great Telescope of Birr Castle, Ireland

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In the 19th Century, a rich, Irish land-owner was interested in astronomy and decided to build a telescope in his garden. The result was the biggest telescope in the world, a record it held for over 70 years. The rather unusual telescope, recently restored, is still on display in the gardens of Birr Castle, although it is no longer used for serious astronomy.

Birr and William Parsons, the Third Earl

Birr is a town in County Offaly, a rich agricultural area in the centre of Ireland. In the 19th Century, most of the land around was owned by the Parsons family, and indeed at that time the town was called Parsonstown. The head of the household held the title of Earl of Rosse. The Parsons family lived in a mansion known as Birr Castle. This had been a castle in the past but by the 1800s was a big house with mock castellations on some parts to make it look like a castle. The mansion had, and still has, large grounds around it which were the gardens of the family.

William Parsons (1800-67) was the Third Earl of Rosse. He was educated at Trinity College, Dublin and Magdalen College, Oxford, receiving a degree in mathematics with first class honours in 1822. He acted as the representative for King's County (the name at that time for County Offaly) in the House of Lords, but gave up a political life in 1834. In 1836 he married Mary Wilmer-Field, a rich heiress from Bradford, Yorkshire, and his parents moved out of Birr to Brighton, leaving him as owner of the Castle and grounds.

By this time, Parsons was already interested in astronomy and was putting all his time and efforts into building a big telescope. He had started experimenting with methods of construction in 1827, but it was not until 1839 that he made his first successful instrument. It was impressive: a 36-inch Newtonian reflector, the biggest in the world at the time although not as big as the one Herschel had used earlier in the century1. His second telescope can still be seen in the Castle gardens to this day. Nicknamed 'the Leviathan of Parsonstown' and built in 1842-45, it was again a Newtonian reflector, but this time with a mirror six feet across (1.83m). This made it the biggest telescope ever made by far, a record it retained until it was dismantled in 1914.

The Telescope

The first requirement for a reflecting telescope is its mirror. Parsons' mirror was 72 inches (1.83m) in diameter and a world record holder at the time. The giant mirror was known at the time by its title in Latin, the speculum. The technology of working a giant block of glass into a parabolic mirror had not yet been developed, so a metal alloy, known simply as 'speculum metal', was used. Parsons did many experiments and eventually settled on the best composition for speculum metal: a mixture of four parts copper to one part tin. He designed a steam-powered machine which allowed a piece of speculum metal as big as six feet across to be ground into the necessary parabolic shape and then polished. This was a slow process taking months. The first mirror was cast on 13 April, 1842, but after about a month of grinding it cracked. The pieces were recast and, second time round, a working mirror was produced, weighing about four tons (4,100kg).

Speculum metal, unfortunately, oxidises easily causing a tarnish on the surface which ruins the image - this meant that the mirror had to be re-polished regularly. This was a major operation. The mirror had to be removed from the telescope and brought to the polishing machine, a task which involved 25 men. The polishing took six hours, and then it had to be manhandled back to the telescope again. Parsons cast a second mirror, slightly thinner and weighing 3½ tons (3,600kg) as a spare. One mirror could be used while the other was being polished.

The tube to hold the mirror was 58 feet long (17.7m) and varied between seven and eight feet in diameter (2.1-2.4m), being widest at the midpoint and tapering at each end. It was made in a similar manner to a barrel, with wooden staves bound together with metal rings. The tube and mirror together weighed about 12 tons (12,200kg). Parsons decided that this was much too heavy to allow the normal sort of telescope mounting. His solution supported the tube, but restricted its motion: two giant parallel stone walls were built, aligned in a north-south direction. There was a complicated arrangement of ties, trusses and beams between them which supported the telescope tube and allowed it to move up and down, but unfortunately its side-to-side motion was very limited.

Because the Earth turns on its axis, the stars appear to move in the sky - telescopes have to move slowly to track the stars so that the image appears stationary. The tracking abilities of the Great Telescope were limited by its construction. The telescope was aimed at a point in the sky, and then the planet, star or nebula would move across the field of vision. When it came into view, the telescope could then be moved using a series of cranks and handles to match the motion of the object being viewed. The object would stay in view for about an hour, then it would disappear beyond the range of the telescope until the following night.

The telescope was built before the invention of photography, but even when cameras became commonplace, they were never used with the Great Telescope - its construction was not really suitable for long-exposure tracking needed for good star photos.

We tend to think of a telescope as a tube in which we point one end at a star and look in the other end. In a reflecting telescope, however, the light is reflected back up the tube by the giant mirror. It then strikes a small, flat mirror mounted at the top of the tube along its central axis. This turns the path of the light through a right angle, and the observer looks in through a small eyepiece at the side of the tube to see the image. This means that in a giant telescope like the one at Birr, the observer must position themselves at the top of the tube, up to 60 feet (18.3m) off the ground. The telescope mounting had a curved wooden walkway at the top of one of the walls which allowed the observer to stand at the necessary position to look into the telescope. The various cranks and winches needed to move the massive tube required a team of workmen to operate, so a night's observation was a group project with a lot of shouted instructions.

By the time Parsons had finished building the telescope, he was generally considered the world expert on the design and construction of reflecting telescopes. Anxious to advance the cause of science, he published complete accounts of everything he had done, to help others build bigger, better telescopes.

First Tests

Despite now being the world's premier telescope builder, he did not as yet know much about the skies. He had to learn to be an observer. Parsons was friends with Ireland's foremost professional astronomer, Dr Thomas Romney Robinson, the director of Armagh Observatory, who he had invited a few years previously to inspect the 36-inch telescope. Now Romney Robinson returned to Birr to try out the new 'Leviathan'. First viewing was on 15 February, 1845. Unfortunately the night was cloudy, but there was a gap in the clouds just long enough to see the double star of Castor, and the M1 Crab Nebula. Romney Robinson was very impressed with what he saw. The telescope was truly a world-class instrument. Unfortunately, the skies continued to be cloudy that month and the next, so very few observations were made.

In April 1845, disaster struck Ireland. Over the preceding century, the population had doubled due to the introduction of a new wonder-food, the 'lumper' potato. In 1845, a blight struck the potato crop, wiping it out. There was widespread famine. Half a million people died of starvation, and another half million fled the country, emigrating to America in the hope of a better life. This started a trend of depopulation, with emigration robbing the country of another three million people over the next century. As a wealthy landowner, Parsons was kept busy for the next three years, first distributing food to the starving, and then reorganising and rebuilding the country. He did not get a chance to make any more observations until 1848.

Spiral Nebulas

In 1848, Parsons resumed his observations using the Great Telescope. He found that the mirror needed to be re-polished, but once that was done the instrument was as good as ever. He decided to make Birr into a centre for science. Astronomers from all over the UK were invited and got a chance to use the telescope. Parsons himself gradually became an expert in using the equipment and made many discoveries.

The telescope was used to make a few observations of the planet Jupiter, and of the Moon, where it could see far more detail than any other device in the world. The telescope's real strength, however, was its use at low or medium magnification to view very dim objects, due to its massive light-gathering ability. This made it ideal for the examination of the mysterious blurry objects known as nebulas2.

Look up in the sky at night and you will see a few thousand stars. On a clear night you'll also see a pale glowing band that goes the whole way around the sky - this is known as the Milky Way. Examine this through a telescope and you will find that it is made up of millions of very distant stars. In the middle of the 19th Century, it was thought that the whole of the Universe consisted of the stars of the Milky Way arranged in a shape like a mill-stone, with the Sun at the centre.

The biggest topic in astronomy at the time was explaining the objects known as nebulas. The word 'nebula' is Latin and means 'cloud'. A nebula looks like a cloudy grey spot. Not much was known about them, although lists had been made of the locations of a few hundred of them. Nobody knew what they were made of - were they collections of stars, like the Milky Way itself, or were they made of glowing dust and gas?

Parsons looked at the object known in the Messier Catalogue as M51 and discovered that it has a spiral structure, like a giant whirlpool in space. This was probably his greatest discovery with the telescope. The term 'spiral nebula' was born, and M51 was named the 'Whirlpool Nebula'. He undertook a programme of examining all the known nebulas, which would occupy the next 30 years. This was too much for one person to attempt, so he hired the services of an assistant. Many great names in Irish science and engineering served their time as assistants at Birr: Johnstone Stoney, the inventor of the concept of the electron; his brother Bindon Stoney, designer of Dublin Port, and the inventor of the diving bell; and Robert Ball, a mathematician who developed the theory of screw displacement.

Parsons discovered that many of the nebulas had a spiral structure, but many others were spherical, and some were an irregular shape. He was able to divide them into ones which were made of collections of stars and ones in which the telescope could not distinguish individual stars. For example, he could see individual stars in the Whirlpool Nebula using the Great Telescope, although these were not visible using smaller telescopes. The Great Nebula in Orion, on the other hand, resisted all attempts to break it up into individual stars. Parsons suspected (wrongly) that all nebulas were made of stars, and that this would become evident when even bigger telescopes were made. We now know that some nebulas, such as the Great Nebula in Orion, are in fact made of glowing dust and gas. The word 'nebula' is now used to mean only that type of object. Objects made up of stars are now known as 'clusters'.

Parsons also suspected, correctly in this case, that the spiral nebulas were much further away than the furthest stars in the Milky Way and were in fact collections of stars as big as the Milky Way itself. Without any way of measuring the distance of the objects he was observing, he was unable to prove this. It was only in 1923, long after Parsons' time, that this was shown to be true by Edwin Hubble. The so-called spiral nebulas are in fact collections of stars very far away - a new word was invented: 'galaxy' from Galaxias, the Greek for Milky Way. The Milky Way itself was now considered a galaxy, but the Whirlpool Nebula and the other spiral nebulas were also galaxies, collections of billions of stars entirely separate from our own galaxy. The term 'nebula' stopped being used for the spirals, and M51 is now the Whirlpool Galaxy.

Laurence Parsons, the Fourth Earl

William Parsons died in 1867, but his son Laurence Parsons (1840-1907) carried on the work on the nebulas. He continued to use assistants, the most notable being John Dreyer. In 1877, Laurence considered the work on nebulas complete and published full details of all the discoveries made at Birr. Dreyer moved on, taking a job first at Dunsink Observatory near Dublin, and then becoming director of Armagh Observatory, where he published the New Galactic Catalogue, the definitive study of all the major nebulas, using data from Birr and other observatories.

Laurence Parsons continued to do valuable astronomical work, but his main interest was measuring the surface temperature of the Moon. This was done using equipment which was not really compatible with the 72-inch Great Telescope. Astronomy had moved with the other sciences; photography and spectroscopy were now the tools of the trade and neither of these could be used with the 'Leviathan'. It was no longer suitable for anything other than casual stargazing. Laurence kept the Great Telescope in working order but did not use it again.

Decay and Restoration

After Laurence's death in 1907, his executors decided to dismantle the telescope. In 1914, the mirror was removed and sent to London where it became an exhibit in the Science Museum - you can still see it there to this day. By 1925 the telescope's mechanism had become dangerous so it was taken apart, leaving only the two walls and the giant tube lying on the ground between them. It remained in this state until the 1990s.

In 1971 Patrick Moore wrote a book called The Astronomy of Birr Castle and this gradually began to revive interest in the Great Telescope. In the 1990s, work finally began on restoring the telescope. First the mechanism and walkways were replaced, then a new, much lighter aluminium mirror was installed to replace the original speculum metal one. Once again the telescope can scan the skies.

Visiting the Telescope

Birr is about 140km from Dublin. The estate of Birr Castle takes up about half the town so you can't miss it. There's an admission charge to the gardens of the castle. The castle itself is still a family home so it is not open to the public.

The gardens are well worth a visit. As well as the telescope, there are broad meadows, an arboretum, a riverside walk, a beautiful lake, and formal gardens laid out with hornbeam alleys. The best times to visit the gardens are probably in spring, when all the flowers are in bloom, and during autumn, when the changing colours of the leaves provide a different sort of beauty.

There are occasional demonstrations of the mechanism of the telescope - it's best to check to see when these are arranged for. Even without a demonstration, the telescope is an impressive work of Victorian engineering. The walls are made from the same grey stone as the Castle and its stables, and are complete with mock-castellations and Gothic arches to match the architecture of the Castle. The mechanism of the telescope is simple enough that it can be appreciated by any budding astronomer.

As a tribute to Parsons' discovery of the Whirlpool Galaxy, there is a copse of trees laid out in the shape of his sketch of the galaxy. You can walk the length of one spiral arm on a curving path lined with lime trees. At the centre is a boulder engraved with a galaxy symbol. Then you can walk out from the centre along the other spiral arm.

The gardens also feature a science exhibition at the entrance; this is dedicated to the science of Birr Castle - the telescope, the pioneering photography of William Parsons' wife, Mary, and the steam turbine invented by his son Charles.

There's a café at the entrance, where you can get snacks or lunches.


There are now many much larger telescopes around the world, and most of them are in locations where the skies are clearer than central Ireland. The Great Telescope of Birr is no longer useful in making astronomical discoveries. It is, however, a piece of history. It can continue to advance the cause of science by inspiring people to look at the sky, to question what they see, and to try to explain the mysteries of the Universe.

1Herschel's 49-inch reflector had been decommissioned the previous year.2The plural of nebula is often given in its Latin form as nebulae.

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