Lenses are used in refracting telescopes. Mirrors are used in reflecting telescopes. The mirrors are ground and polished so that a precise concave surface remains to be coated with a shiny, reflective finish. Mirrors concentrate light and focus all colors of the spectrum in an image. No lens can bring to focus all of the colors in white light. Further, lenses absorb some of the light that passes through, thus weakening the image. Mirrors are easier to make in large sizes than lenses and can concentrate the light of the dim images from deep space. Reflecting telescopes are currently used for space exploration.
On March 25th, 1934, in Corning, New York, the largest disk, made of a special glass containing borax (borosilicate glass), was poured. It was 16.6 feet across, and referred to as the 200 inch disk. It was 26 inches thick, and weighed 20 tons. Its ultimate destination, when ground, polished and coated with a layer of bright aluminum, was the Hale Telescope on Mt. Palomar in north San Diego County, California.
During the manufacture, the furnace was so hot that several cores in the brick mold broke away from the metal anchor rods and floated on top of the molten glass. Though the accident meant that the disk would never be used as the great mirror, it wasn't considered a complete failure. It was used to test the annealing process for the pouring of the second disk, and for experimenting with packing and crating methods to be chosen for shipping it to California. The first disk, displayed in Corning, New York, is pictured to the left. You can see the reinforced back of the disk that looks like a giant waffle.
The second disk was cast six months later, cooled, packed and shipped by railroad to California. It took 7½ years to grind and polish nearly 5 tons of glass from the flat side of the original 20 ton blank. The reason the job took so long to finish was because a world-wide war (World War II) interrupted the process for about 5 years. Skilled people and materials were diverted to the war effort.
Today, engineers build accurate telescope mirrors by manufacturing relatively small pieces (boules) and then fusing them together to make the disk. Using this technique, the world's largest telescope mirror blank (8.31 meter, or 27+ feet, diameter, 9 inches thick, 33 tons) was made from ultra-low expansion glass for the Subaru telescope located on top of Hawaii's 14,000 foot high Mauna Kea mountain. It took nearly three years to make before it was shipped off to be ground, polished, and coated with a reflective surface. The telescope has been in operation since 1999. Careful, precise grinding and polishing is important to the success of a mirror meant to be the heart of a reflecting telescope.