The Modern Era of the Telescope

Almost the only obstacle now in the path of the reflector was the problem of casting and annealing larger glass blanks. These first glass mirrors, and even those of quite recent years, were of ordinary1 plate. The chief source of trouble was the high coefficient of expansion, with the resulting injurious effect on the figure caused by temperature changes.

(Similar difficulties had been encountered with speculum metal, which had more than double the coefficient of expansion of plate glass, but the effects were no more pronounced due to the much thinner disks that were used. The thinner disks, however, were quite sensitive to flexure.)

Extensive experiment was undertaken in the United States, following the first World War, toward the development of a low-expansion glass. This resulted in a product called pyrex, first used for baking dishes. As is well known, pyrex was used in the casting of the 200-inch mirror blank, at Corning, N. Y., in 1934. Its
coefficient of expansion is only one third that of plate glass, materially reducing deformation due to temperature changes. It is harder than plate glass and more difficult to grind, but it is also more resistant to scratches.

Following the successful pouring of the 200-inch disk, a clamor for this new glass went up from amateurs everywhere. The Corning Glass Company responded by providing suitably annealed disks in various sizes. It is fortunate that the homogeneity of the glass does not have to be taken into account. The melting point of pyrex is high, about 2,900° F., and even then it flows like cold
molasses. As a consequence, folds, striate, bubbles and stones, in no way impairing its qualities as a foundation for a mirror, are to be found in almost any blank.

Modern era of the telescope continued