The Birr Telescope, the world's largest telescope of the 19th century, was the design and construction marvel of Ireland's Lord Rosse.
Located in County Offaly, in the heart of Ireland, the Birr Telescope reigned for more than seventy years as the largest telescope in the world. Also known as "The Leviathan of Parsonstown," this giant reflecting telescope was the 19th century vision and creation of William Parsons, the 3rd Lord of Rosse. On February 15, 1845, three years after construction began, the Birr Telescope provided the most magnified view of the sky possible at the time. Until the completion in 1917 of the 100 inch Hooker telescope at Mount Wilson in California, the Birr Telescope beckoned to astronomers of the day as the tool to aid their study and exploration of the heavens.
The 72 inch mirror, mounted in a 56 foot long tube, weighed 3 tons. The lower end of the telescope was set on a cast iron universal joint, which sat on a masonry base. Chains controlled by a hand winch, raised and lowered the far end of the telescope. Support for the chains, as well as protection from the wind, was provided by two massive stone walls, one on each side, 50 feet high and 70 feet long. Using the telescope was a not for the faint of heart, since the observer had be as much as 60 feet in the air on a scaffold like device, which accommodated the movement of the telescope as it tracked an object in the sky.
Technology of the day made it impractical for Lord Rosse to have the parts for his telescope manufactured outside of Ireland, and facilities didn't exist nearby. He not only designed the telescope, but he manufactured the telescope. He made the machinery necessary to manufacture the parts. This was truly a telescope of Ireland.
The mirror is the most outstanding feature of the Birr Telescope, and the greatest testament to Lord Rosse's skill as a designer. The technology didn't exist to cast a metal coated glass mirror of that size. Speculum metal, an alloy of copper and tin, was used. The casting was accomplished with three crucibles, each 24 inches in diameter and weighing 1/2 ton. Surrounding bogland supplied the turf which produced the heat needed to melt the metal. The process was a difficult one. The higher the tin content, the more reflective the mirror. Yet a higher tin content could cause the mirror to become brittle and break. Lord Rosse built a foundry in the bottom of the Birr Castle moat to cast the mirror. Two mirrors were made so that one would always be available while the other was being polished. The Birr telescope mirror remains the largest metal mirror in the world today.
The innovative design which allowed for the construction of a large telescope caused the Birr telescope to have a restricted range. The wall that protected it, also prevented it from swinging around in a circular motion. Its range was limited to a north/south direction along the meridian. An hour was the longest time in which the motion of a star or nebula could be tracked. Many prominent telescope makers of the day ridiculed the Birr Telescope and Lord Rosse. Astronomers, on the other hand, flocked to Birr Castle from all over the world for a chance to see the skies in more detail than anyone else of that time. Lord Rosse would go on to identify many celestial objects. It was Lord Rosse, using the Birr Telescope who first described the spiral nature of nebulae and recognized them as galaxies outside our own, containing stars and thus disproving the prevailing theory that they were merely objects made of dust within our own galaxy.
After Lord Rosse died, his son continued to maintain and use the telescope. However, after his death, the Birr Telescope fell into disuse and disrepair. Parts of it were dismantled. The mirror was taken to the Science Museum in London, where it resides to this day. Attempts to restore the telescope during the 1960's fell short. In 1994 with the help of private, business, and governmental funding and determination, restoration of the Birr Telescope was started and finally completed in 1999. It is hoped that it will occasionally be used for it's original purpose, bringing the heavens into closer view.
