Man's physical universe

xanabras

TELESCOPES 67

eye, as compared with 160,000 times for the Mount Wilson 100-inch

telescope. The aluminum coating should also greatly increase the

total reflected light to which the much-improved photographic emulsions

are sensitive, so it is hoped that the new telescope will penetrate

three times as far into space as formerly, opening for investigation an

unexplored sphere about thirty times the volume now within the range

of the Mount Wilson 100-inch reflector.

The 200-inch mirror for this telescope was made of Pyrex glass,

borosilicate glass, and weighs 20 tons. It was cast from a batch of

65 tons of glass after heating it in a furnace for forty days. It was

placed in an oven to cool on December 2, 1934, and removed on December

8, 1935. Then followed many tests to determine whether it would

be satisfactory or whether another mirror would have to be cast. The

mirror was then shipped to California on April 10, 1936, to be polished

to an accuracy of "two millionths of an inch." In the final stages,

polishers could not work on the mirror more than an hour each day,

because it was necessary to test so frequently. Places on the surface

that were too high had to be lowered, for there was no means of changing

those that were too low, and the lowering could not go too far.

Often two or three minutes' work was all that could be done without

testing to see whether or not enough had been done. In order to make

these tests, the surface had to be cleaned off and dried and then set up

for the test. In this process temperature changes were inevitable; and

hence, with a large mirror, several hours elapsed before a reliable test

reading could be made.

The construction of the mounting for the 200-inch mirror has been

a tremendous engineering problem.

Five hundred tons of steel had to

be fitted together to tolerances in some places as close as two millionths

of an inch. The whole mounting is floated on oil and can be moved

by exerting only 1/650,000 of a horsepower.

Many Other Giant Telescopes Are in Use.

Important as this large telescope is, it must be remembered that

there is a tremendous number of observations to be made and that all

observations must be checked as many times as possible by different

observers using different instruments located at different places.

Other great instruments now in u.se are: a new 76-inch reflector at the

University of Toronto, Canada; a 72-inch reflector at Victoria, British

Columbia; a 69-inch reflector at the Perkin Observatory at Delaware,

Ohio; an 85-inch reflector at Base Lake, Michigan; and an 80-inch

reflector in the $840,000 McDonald Observatory on Mount Locke,

Texas. There are at least nineteen other giant reflectors, ranging from

a