by Prasad Ganti
The book “The Perfect Machine” is written by Ronald Florence. A fascinating book about the design and construction of the 200 inch telescope at Mount Palomar during the years leading up to the second world war and its immediate aftermath. George Hale, known for his research in solar astronomy, championed the telescope right from the proposal stage all the way into the advanced construction of mirror and the mounts. He died just before its completion. New York Times urged that the telescope be named in his honor.
George Hale was known for his earlier efforts to build the big telescopes for the Yerkes and Mount Wilson observatories. The 200 inch at Mount Palomar was a logical successor to the 100 inch at Mount Wilson which Hale had built. Hale also had other impressive credentials, as a co-founder of Caltech, an officer of the National Academy of Sciences and the National Research Council, and the founder of Journals of Astrophysics. Hale conceived of the design, sold it to the scientific community and other stakeholders at large and got the funds needed to support the project. Lot of detail was written about the casting of the huge mirror. It really stretched the technology envelope of those times. GE first started off the mirror work with fused quartz. Fused quartz had such a low coefficient of expansion, that it would be far more efficient to grind and polish than plate glass. GE ran into trouble using this new material. Corning was assigned the task of casting the mirror. They used Pyrex which is basically borosilicate glass with extremely low coefficient of expansion. It required heating to very high temperatures. And then cooling in a very slow and controlled manner, called annealing, taking months to do so. The tolerances for the cast were very tight. Any imperfection leads to discarding the mirror disk and starting all over again. There were several attempts before the mirror could be finally cast.
Once the mirror disk was ready, it was shipped in a special freight train from Corning factory in New York across the country to Caltech in Southern California. Caltech had an optics and machine shop for grinding and polishing the mirror to its final shape. The train became a spectacle. People lined up all along its route to catch a glimpse. Several trains had to be held up or rerouted to provide clear tracks for the train with this special cargo.
At Caltech, the laborious work of grinding and polishing the mirror and fabricating the mount took several years interspersed by World War II. An equatorial mount was designed with a yoke holding the mirror and the yoke itself moving on a horseshoe bearing. There were three big motors to guide the telescope precisely. One for declining axis, tipping the telescope to be in the great yoke. One right ascension gear was for slewing the telescope, moving at relatively high speed when the observer wanted to point the instrument at new area of the sky. The third gear was for the slow-moving drive that would keep the telescope moving synchronously with the motion of the earth.
The mirror and the mount had to be transported on a huge trailer along a carefully selected route. New roads were laid to the top of Mount Palomar in San Diego county. The assembly of the mirror and the mount within the dome was an extraordinary engineering feat by itself. When it opened in late 1940s, it was the biggest telescope in the world and was to be for decades to come.
Overall, the project was a gigantic engineering marvel, no lesser than the Manhattan project or the construction of the Hoover dam. A grand intersection of physics and engineering. To this day, the Hale telescope holds its own against the newer ones with larger segmented mirrors and the simpler alt-azimuth mounts controlled by sophisticated computers.