This 17.5-foot hemispherical bearing is the structural heart of the 140-foot (43-meter) telescope in Green Bank, West Virginia. It supports the full rotating weight of 2700 tons on the .005-inch thick oil film between it and the main hydrostatic pad supports. Cast by General Steel Industries it was then finished and polished by Westinghouse. Nearly 17 tons of metal were ground off during this process of getting the surface smooth with no bump more than .003 inches. The bearing still weighs in excess of 150 tons– one of the largest steel castings ever poured, certainly the largest ever cast of 3.5% nickel.