The Modern Era of the Telescope

Fused quartz has been experimented with also; in fact, it was first thought to make the 200-inch of this substance. A number of mirrors and optical flats, of various sizes, have already been made of it. It has one sixth the coefficient of expansion of pyrex, and is absolutely impervious to the most extreme climatic temperature changes. The cost of manufacture, however, will keep it out of the hands of most amateurs for the present.

At the close of the 19th century, experiments began on the deposition of metal films on glass by an evaporation process in high vacuum. These have resulted in the replacement of the silver mirror coating by a more durable one of aluminum. Although a number of metallic elements have been evaporated, some of them having a higher reflective index than aluminum for certain wave lengths, the efficiency of aluminum over the entire spectral range is higher than that of any other metal. It is but slightly less efficient in the visible spectrum than a fresh silver coat, with a reflective index of about 88 per cent, but it retains this reflective quality almost indefinitely.

Its absorption in the ultraviolet is considerably less than that of silver, and so it is decidedly superior for photographic purposes. The first mirror to be coated by the
evaporation process was the 15-inch of the Lowell Observatory. This was done in 1931 by Cornell University researchers, using chromium.

The largest refractor in the world today is the 40-inch telescope at Yerkes Observatory, Williams Bay, Wis. The lens, which has a focal length of 63.5 feet, was completed in 1895 by Alvan Clark. This is probably the ultimate in size for a refractor, because of flexures imposed by the weight of larger lenses, and also because of a serious light loss by absorption in the thick lenses.

The largest plate-glass mirror in use is the 100-inch, ratio f/5.1, at Mount Wilson Observatory. The glass blank was cast at the St. Gobain Glass Works in France and shipped to Pasadena, where it was made into a mirror by George Ritchey.
The 200-inch reflector, f/3.3, of the observatory on Mount Palomar, is by all standards the greatest of telescopes, a far cry from Newton's first reflector, 1 1/3" in diameter.

Reflector or Refractor