Telescopes and astronomy part 2 BY Dan
1781 and Herschel was watching the late night skies, and to his surprise he noticed a small body moving slowly across. At first he thought a Comet, and then further observations revealed it was a planet.
“Georgium Sidus” – after King George III – is what William lobbied for his new discovery; however it eventually was named “Uranus” after the Greek God of the sky. Soon after this discovery, he was knighted by the king and was promoted to Court Astronomer. (The symbol for the planet Uranus features the capital letter ‘H’ in his honour).
While Herschel was cataloguing different stellar bodies and making new and exciting discoveries, he also performed simple experiments to determine the temperature of colours of sunlight that passed through a prism, which in-turn lead to the discovery of infrared radiation. He also proposed the name Asteroids for large bodies discovered in 1801, where his last published paper catalogued 145 double-stars.
1845 gave birth to the Leviathan of Parsonstown (reflecting telescope), built by William Parsons III, Earl of Rosse. The Earl decided he wanted the world’s biggest telescope however, as mirror technology was still in its infancy, he would have to make them himself (Herschel didn’t leave any records of the method he used to make mirrors). Over 17 years, Rosse managed to make 15, 24 and 36 inch mirrors. A massive monster contained a 72 inch metal mirror weighing in at about 4 ton, as well as a backup which was kept polished when the other mirror was used. This telescope was the first to discover a spiral nebula; as much as this was a great achievement, it could only be moved up or down and very slightly left to right, which in turn limited observations.
Also, where the telescope had been constructed there were numerous problems including dampness and wind as the major factors; but it was discovered that atmospheric distortions are present that in turn make the stars “twinkle”. This lead to astronomers planning where to build future telescopes to minimise any distortions from Earth.
1895 saw the construction of the biggest achromatic refractor ever built: the Yerkes Telescope. Astronomer George Ellery Hale persuaded a Chicago tycoon (Charles Tyson Yerkes) to finance the project. Yerkes, a convicted embezzler with a reputation for dishonest deals, grew to the idea for his name to be attached to the largest telescope in the world. He would spend what needed to be spent, on the condition it was the largest in the world.
Construction began at Lake Geneva, Wisconsin, which is quite low for an observatory but in the winter the temperature could go as low as minus 20 Fahrenheit (-28.889 C), cool enough for skies to be clear. Hale employed Alvan Clark and his son, the team of elite telescope builders (who designed the Naval Observatory) to construct the telescope.
A 40 inch (101 cm) lens doublet was placed in a 60 foot (18m) telescope weighing in at 20 tons. It was perfectly balanced so that the slightest touch of hand could position the telescope to follow observations throughout the sky. Yerkes is the last of the great refractors; lenses couldn’t really get any bigger – weight would make the lens sag or the lens would absorb too much light.
Refracting telescopes remained in use, but for future advances, astronomers would use the refractors nearly-constant competitor: the reflecting telescope.