Telescopes from the Ground Up

A rejected piece of glass holds the key to a powerful telescope

The 60-inch telescope at California’s Mount Wilson Observatory was big, but it wasn’t big enough to suit astronomer George Ellery Hale. Hale wanted a telescope that could collect more light. So even as the 60-inch reflecting telescope was being built, Hale was looking for the funding for a reflector with a 100-inch mirror.

He found it in Los Angeles businessman John D. Hooker, who wanted his name attached to the largest telescope ever built.

Bubble, bubble,
toil and trouble

Only one glassmaker was willing to attempt the daunting feat of making a 100-inch disk — the same French company that had provided the glass for the 60-inch reflector. The reluctance was not without cause: when the 100-inch disk arrived, in 1908, astronomers thought it was worthless. It was full of air bubbles, and some of the glass had crystallized — which meant it probably wouldn’t stand up to the grinding and polishing that would be needed to make it into a mirror.

The glassworks built a new furnace and oven and kept trying, but none of the disks seemed to be of telescope quality. World War I broke out, and the experimentation stopped. Now, no one had time for giant glass disks.

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Huge Reflectors
Map showing the location of Mt. Wilson in southern California, USA.
See major discoveries of this telescope.
Image of the 100 inch Hooker Telescope within its dome.Enlarge picture
The Hooker
100-inch Reflector
Year completed: 1917
Telescope type: Reflector
Light collector: Silver-coated glass mirror
Mirror diameter: 100 inches
(2.5 m)
Light observed: Visible
Discovery Highlights:
  • Edwin Hubble used this telescope to establish that "spiral nebulae" are in fact galaxies outside our own Milky Way and that they are moving away from us, indicating that the universe is expanding.
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