I left my fingerprints on the 200 inch mirror!
by Richard Overy

During a recent visit to our eldest son David, who lives in the US, near Rochester in upstate New York, he took us on a trip down to Corning, to the famous Corning Glass Works.

There we visited the Corning Glass Centre which includes the Museum of Glass and The Hall of Science and Industry. This is dominated by the massive first Pyrex glass casting of the 200 inch mirror for the Hale Telescope at Mt. Palomar, made in 1934. It really is an awesome piece of work when you get right up to it: 5m in diameter, 70 cm thick with a 11m central aperture and a mass of some 14 tonnes.

This first casting was unfortunately a failure as several of the cores forming the honeycomb-like rib structure of the back simply fused to the casting. Several internal flaws can also be seen, probably caused by too rapid or uneven cooling which may not have been sufficiently well controlled. It is actually quite rough to the touch.

This was in fact actually the second casting of the mirror. The first was an attempt to use fused quartz because of its very low coefficient of expansion. However, due to the high temperature required to melt the quartz, some of the bronze bolts holding the core-blocks in the mould melted, allowing the cores to float around in the mould. It was then decided to use Pyrex, the next-best material available at that time.

The second Pyrex casting was more successful and is the one now in use. But it also very nearly ended in disaster when the Chemung river which flows right past the Glassworks came down in flood and threatened to engulf the foundry where the casting was in the process of cooling down. Only by hastily sandbagging the whole building up to window height was calamity averted.