The Earl of Rosse and the Leviathan of Parsontown

by Lenny Abbey

(Visit the Birr Castle web site.)

The 72" as it appears today.
(By permission, Birr Castle.)

With the advent of stellar astronomy (an event which took place in the front yard of the greatest amateur astronomer of all, William Herschel), it became clear that a telescope's most important quality was its ability to gather light. Herschel built telescopes up to 48" in aperture, but the largest of these behemoths was difficult to operate, requiring the assistance of several well-muscled workers. Understandably, these helpers were not necessarily interested in staying up all night to turn the crank of a windlass. The 48" made several important discoveries, but Herschel soon returned to his favorite, a 24" instrument.

The real race for aperture was launched by William Parsons, the third Earl of Rosse. The Earl was a man of vast wealth, who owned extensive estates in Ireland. He was a passionate amateur astronomer, and like most amateurs, he needed a larger telescope. Money was not a hindrance for the Earl. He wanted to build the world's largest telescope, and he wanted it to be used by professional astronomers who would be able to properly access the discoveries which would be made. He decided to attempt a mirror six feet in diameter.

There were two great engineering roadblocks which had to be overcome. First, the mirror would be very heavy. Modern methods of coating glass surfaces with silver or aluminum were yet to be invented, and speculum metal was the only known material with all of the properties required for a precision reflecting optical element. In order to produce such a large and heavy piece of metal, he had to build a foundry with three peat-fired furnaces on his rural estate at Birr Castle, near Parsontown. His workers were successful in casting and polishing a 3½ ton mirror on the third try. Two mirrors were made, so that one could be used in the telescope while the other was being resurfaced. It was necessary to repolish one of the two giant mirrors every six months due to the tarnishing effect of the damp Irish climate.

The second problem was that the tube had to be 58 feet long in order to accommodate the f/10 optical system. Lord Rosse's first efforts in amateur telescope making had produced instruments which were mounted along the lines of Herschel's design – monstrous scaffolding supported by a rotating platform. This gantry arrangement would not do for such a long and heavy tube. Bearings capable of supporting such tremendous weights were not available. The problem was solved with a type of mounting which had not been used before (and has not been used since). Two parallel 70-foot high masonry walls were erected in a North-South direction, and the tube was slung between them. On the bottom of the mirror cell was a large steel ball which fit into a socket set into the ground midway between the walls. The top of the tube was maneuvered by cables and wooden supports strung from one wall to the other. The cables, struts, spars, and supports were constructed so that equatorial tracking motion was possible. The space between the two walls was adequate for tracking objects for one hour before and one hour after meridian passage.

The project began in 1843, but had to be suspended in 1845 due to the Great Potato Famine. By 1847 conditions had improved, and the telescope was finally placed into service.

The team of professional astronomers which the Earl had engaged was headed by Sir Robert Ball. When Sir Robert left, the position was taken by J. L. E. Dreyer, who began to compile the NGC catalog while at Parsontown.

The telescope proved to be of amazing optical quality. But its light-gathering capacity was beyond all expectation. For the first time it was possible to detect stars as faint as 18th magnitude.

The first subject of detailed examination was the Moon. Minute craters and rills which had never before been glimpsed were charted. Delicate new details were seen on Jupiter, Saturn, and Mars.

After about one year of intensive study of the solar system, the telescope was turned to the stars. The big question which astronomers were asking in the mid-Nineteenth Century was "Are the nebulae composed of minute stars which we cannot resolve because of their extreme distance?" The great telescope provided the first opportunity to try for a meaningful answer. It was not known at this time that there were different kinds of nebulae – some composed of stars, and some composed of gas and dust. Examination under high powers at Parsontown revealed that some hitherto unresolved nebulae were indeed composed of faint stars, but some continued to resist every effort at resolution.

During this research, the telescope's great discovery was made. Some nebulae which were seen as dim blobs in lesser instruments were revealed to be giant spirals. The first of these spiral nebulae to be revealed was M51 – known today as the Whirlpool Galaxy. The Earl was the first to suggest that these spirals could actually be rotating masses of stars. Within a few years, over a hundred more spirals were observed. Of course, none of them were resolved into stars. That would have to wait on yet larger instruments.

Other important observations with the telescope were the confirmation of the satellites of Mars (which had been discovered by Asaph Hall in Washington), and the beginning of Dreyer's observations of the NGC objects.

After the Earl's death in 1867, his son, the Fourth Earl of Rosse continued his father's work. He fitted the telescope with a clock drive, and expanded the research with other (smaller) special-purpose instruments. Research on the heat content of the Moon was carried out over a 22-year period.

The last observations were made with the 72" in 1878, and it was dismantled in 1908. It is interesting to note that this instrument was not surpassed in size until the 100" Hooker telescope at Mt.  Wilson was placed in service in 1917.

There are two interesting footnotes to the story of the Leviathan. The third Earl's youngest son, Charles, inherited his father's interest in massive engineering projects. In the early 1890's he invented the steam turbine, which had a profound influence on the history of the Twentieth Century – it made the modern battleship possible.

In 1935 Charles (by that time Sir Charles) bought out Sir Howard Grubb's optical shop. Since that time, Grubb - Parsons has manufactured many very large telescopes, including the 98" Isaac Newton Telescope. Thus, the two largest telescopes to be constructed in the British Empire, the 72" at Parsontown, and the 98" (now relocated to the Canary Islands) were the fruit of one family's genius.

Unlike the other telescopes described in this series, the Leviathan has a future! The present Earl (the sixth) has established an astronomical museum at Parsontown, and a fund which has as its goal the reactivation of the 72" telescope. This goal has now been achieved, and the telescope will be made available to amateur astronomers who would appreciate this opportunity to participate in the unique and important history of Birr Castle.

Visit the Birr Castle web site for a wonderful and extensive set of images of this project.

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