Telescopes from the Ground Up

An astronomer’s final telescope fulfills a long-held dream

American astronomer George Ellery Hale was supposed to be retired. But why stop when there was so much more to see, when fantastic views of distant galaxies could be within reach? Already responsible for so many of America’s high-powered telescopes, Hale began raising money once again in 1928, this time for a reflecting telescope with a 200-inch Cassegrain mirror.

He assembled a team of the day’s leading astronomers, technicians, and engineers. The glass mirror they intended to build would be bigger and heavier than any piece of glass ever made before. The team knew from previous telescopes that even small temperature changes made the glass expand and contract, causing changes in the mirror that would distort the view while the telescope was being used.

So they turned away from ordinary glass and considered other materials — stainless steel, composite metal, fused quartz, and a new type of glass being made in Corning, N.Y., called Pyrex.

Rock solid

First the team tried fused quartz, which is harder than glass, more difficult to scratch, and polishes brightly. Most importantly, it would barely expand or contract when the temperature changed.

But the quartz didn’t work out. It was too likely to form bubbles, which would pit the surface of the glass during the grinding process, and the best anyone could do was a 66-inch disk. In 1934, the team turned to Pyrex, typically used to make ovenware. Molten Pyrex didn’t bubble up as badly. Also, once solidified, Pyrex expands and contracts less than ordinary glass when the temperature changes.

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Huge Reflectors
Map of Mt. Palomar, California, USA, where the 200 inch Hale reflector is located.
Image of the 200 inch Hale telescope within its dome at Mt. Palomar observatory.Enlarge picture
The Hale
200-inch Reflector
Year completed: 1948
Telescope type: Reflector
Light collector: Aluminum-coated glass mirror
Mirror diameter: 200 inches
(5.0 m)
Light observed: Visible
Discovery Highlights:
  • Discovered visible evidence of quasars — very bright objects at very great distances that were later found to be supermassive black holes at the centers of distant galaxies.