Polishing the mirror

It took from 1936 to 1947 to grind the 200-inch mirror. According to the Palomar Observatory Web site, "In the optics lab at Caltech, the front surface of the mirror is ground to the approximate concave form required. Using successively finer polishing grit, the opticians then carefully smooth the surface, constantly using optical tests to compare it to a perfect paraboloid shape. It is slow and painstaking work. To make the final mirror, almost 10,000 pounds of glass are polished away, including the top two inches which contain 'scar tissue' left over from the casting and annealing process."