Two concave metal mirrors were made for the telescope, each with a focal ratio of f/10. The first was cast in a London foundry on 31 October 1785, and was made of speculum (an alloy of mostly copper and tin) with arsenic to improve the finish. It weighed 1023 lb after being cast, but it was found to be 0.9 inches thinner at the centre than at the edge (where it was around 2 inches thick). Over a year was spent grinding and polishing the mirror; however, Herschel found it to be "much too thin to keep its figure when put into the telescope" (despite weighing half a ton). A second mirror with twice the thickness of the original was cast a few years later, and this was used rather than the original. The first casting on 26 January 1788 cracked on cooling, but the second casting three weeks later was successful. The mirror was in use by 24 October, but continued to be polished more until 27 August 1789, when Herschel was happy with it. However, this second mirror required more frequent polishing due to the fast tarnishing nature of the metal, and the original mirror was used when the second was being polished. The mirrors remained the largest in the world until 1845.
