J.S. Plaskett, C.A.R. Lundin, and George A. Decker view the completed mirror of McDonald Observatory's first telescope in 1935. The 82-inch (2.1-meter) mirror was cast on December 31, 1933. Flaws in the original casting and problems with polishing the mirror to its proper shape delayed completion of the telescope until 1939. The telescope is still in use today. [McDonald Observatory]
You are here
In late 1933, McDonald Observatory was starting to come together. Crews broke ground for the Observatory’s first telescope in November, and the contractor, Warner and Swasey, was building the telescope dome and tube at its headquarters in Cleveland. And 80 years ago today, work began on the final major piece: the telescope’s main mirror.
A telescope’s mirror gathers and focuses the light from distant objects. A larger mirror gathers more light, allowing it to see objects that are fainter and farther away. The mirror for the new McDonald telescope would be about 80 inches in diameter — the second-largest in the world.
McDonald’s director, Otto Struve, chose to have the mirror made of a new type of glass, known as Pyrex — the same material used in oven-safe cookware. It doesn’t expand and contract much as it heats and cools. That made it perfect for a telescope, which must adjust to cool night air.
With Struve watching on, the mirror’s melted glass was poured into a mold and placed in a giant oven, which would allow it to cool slowly.
Even so, when the mirror was removed from the mold four months later, it was covered with cracks. Struve insisted that the makers start over, so the mirror was recast. After that, it began a long process of grinding and polishing to achieve the proper shape. That process took a lot longer than anyone expected. Even so, it eventually led to the completion of the big new telescope — and the birth of McDonald Observatory.
Script by Damond Benningfield, Copyright 2013