On June 17, 1800, William Parsons the 3rd Earl of Rosse was born. He was one of the first astronomers to suffer from aperture fever – the desire to have a bigger and larger telescope – and he had the means to act on this. He built what was the largest telescope in the world for 70 years and is one of the largest still working historic scientific instruments.
Parsons started modestly – learning to polish metal mirrors and he built a 36 inch telescope first. He used this to find the Crab Nebula – a supernova remnant that he sketched and named in about 1844.
He then set to work on a much larger telescope – a 72 inch reflecting telescope. Work began in 1843. He had three foundries built in order to cast the metal for the mirror and had two mirrors made (although it took many attempts to get them cast). The mirrors were five inches thick and weighed almost 3 tons. At this weight a mirror is likely to deform, so needs to be supported in a cell. The whole telescope weighed over 12 tons, so was supported by a pair of walls built on either side of it. It was necessary to re-polish the mirrors every six months due to the tarnishing effect of our damp Irish climate.
credit seds.org
The purpose of the telescope was to resolve the nebulae in the catalogues of Charles Messier and John Herschel. Nebula as a term refers to any object that appeared nebulous or fuzzy in the early telescopes. With greater aperture – a larger diameter mirror- more detail could be resolved, and so determine if the nebulae were unresolved star clusters, or actually nebulous regions of space. Given that the Messier catalog includes what we now know to be galaxies, globular clusters, open clusters, planetary nebulae and actual gas cloud nebulae, trying to work out what each one truly was would have been a daunting task.
Parsons discovered that several nebulae had a spiral structure, most notably M51 – the Whirlpool Galaxy which he resolved into stars. But other spiral nebulae could not be resolved, so the question of their exact nature had to await even larger telescopes like the 100″ telescope at Mt. Wilson in 1917.
After William Parsons died in 1867, the 4th Earl (Laurence Parsons) continued to operate the six foot telescope. From 1874 to 1878, J. L. E. Dreyer worked with the telescope and began to compile his New General Catalogue of Nebulae and Clusters of Stars – the source of the NGC initials used in the names of so many celestial objects.
The six foot telescope remained in use until about 1890. After Laurence Parson’s death in 1908, the telescope was partly dismantled, and in 1914, one of the mirrors with its mirror box was transferred to the Science Museum in London.
The telescope was reconstructed in the 1990s and can be seen at Birr Castle.