Fig 1: Sketches of the Orion Nebulae, on display in the Armagh Planetarium.
It takes a good telescope to see deep into space, but it takes a Great Telescope to change how we see the entire universe. In 1845 such a telescope was built by the 3rd Earl of Rosse, in the grounds of his castle in the town of Birr, County Offaly. He didn’t build it alone however; he was inspired by earlier instruments, such as the 4-foot reflector made by Sir William Herschel. That telescope had been so powerful that it helped Sir Herschel discover a new planet for the first time in thousands of years. He was also aided by more recent developments, such as the new technology employed by Thomas Grubb in his 15-inch telescope he built for Armagh Observatory in 1834.
The Great Telescope used a 6-foot mirror to gather light. It was the largest mirror ever produced at that time, and it took many attempts to get it right. The mirror was made of Speculum, made of copper and tin, that could be polished to a very reflective surface. The problem facing the 3rd Earl and other instrument makers was that Speculum is also very brittle, and was so heavy it could crack under its own weight. The cooling process had to be done extremely slowly, by putting the molten metal mirror into an oven, and turning down the temperature over a long period. How long? Well, the first attempt slowly cooled over six long weeks, during which the oven was watched constantly. When it was removed, it cracked because the centre was still molten, and only the outside had cooled sufficiently. The second attempt was given sixteen weeks and was successful.
When it was assembled the telescope was immediately turned towards one of the great astronomical questions of the 19th century. For decades some objects in the sky had been found to be ‘nebulous’, they were full of fuzzy, gaseous clouds, that could not be resolved into clear images by other telescopes. Resolving these nebulae and discovering their structure became extremely important, and in the Spring of 1845, Lord Rosse began to investigate them.
He turned his attention to one particular nebula, designated as Messier 51, or M51. When he turned the telescope to investigate it in April 1845 he was shocked to see a spiral in the eyepiece in front of him on those cold nights. He sketched what he saw, precisely measuring the size of the arms with a Micrometer, and on 19 June 1845 he presented it to the scientific community at the annual meeting of the British Association for the Advancement of Science. The drawings immediately produced considerable debate, with astronomers across Europe drawing battle lines for and against the validity of his claims.
The drawings would later be vindicated by the invention of astrophotography, which along with ever larger telescopes, confirmed that the artistic hand of the 3rd Earl had sketched them with extreme accuracy.
Fig 2: One of the earliest drawings of M51, showing the long spiral arms of the galaxy.
However, in the 1840’s photography had not yet been applied to the science of astronomy. The accuracy of the drawings that the 3rd Earl, and the astronomers such as T.R. Robinson who worked with him, was crucial to their reputation. In his writings the 3rd Earl went to great lengths to describe his method of drawing, and of the construction of the telescope. He compiled many of his drawings from smaller sketches taken over many nights, to make a composite of the images. There was significant debate in his team about whether it was better to draw white chalk on black paper, or to use pencil on white paper. The advantage of black paper and white chalk was that it was easier for the observer, seated on the observing platform twenty feet above the ground, to see the dark.
The original drawings still survive to this day and have had a considerable influence on how we understand the universe. They have remained in the care of the Rosse family for generations, and recently the Heritage Council of Ireland have performed restoration projects on them to prepare the drawings for exhibition in Europe and the United Kingdom. Through the Astronomical Observatories of Ireland, a project that has brought together Birr Castle, Dunsink Observatory and Armagh Observatory to celebrate the Irish contribution to the history of astronomy, these drawings have now come to Armagh for a temporary exhibition. The exhibition will run until 27 April 2025 and is hosted in the Armagh Planetarium Copernicus Hall.
source: armaghplanet.com