Astronomical Engines, Big Complicated Machines

Making the 200 Inch Mirror for the Hale Telescope

Inspecting the 200 inch mirror blank

Greetings, everyone.

Today I’m going to talk about the making of the 200 inch mirror for the Hale telescope. I was originally going to talk about the telescope as a whole, but that would require an article that’s far too long for the bulk of our readers to deal with. There is a lot of history associated with the Hale telescope, in its creation and construction, and in the myriad discoveries that have been made using it. On top of that, I’ve been fascinated by the instrument for decades, and I know enough about the thing to bore a zombie into a stupor.  What I’ve decided  to do, is to write two or three articles about the telescope, with lots of references so that those of you who want to know more about the Hale telescope can do so, while the casual reader won’t be driven off by the extent of my voluminous verbosity. That’s the plan, anyway.

The Inception of the Hale Telescope

George Ellery Hale was an astronomer with big ideas and the knack for getting funding to build them. To get into his background is beyond the scope of this article, so I’ll leave it at that and suggest the reference section for more information.

In 1928, Hale secured funding when the International Education Board, an organization associated with the Rockefeller Foundation, gave $6 million to the California Institute of Technology to build a 200-inch telescope. The project was a massive undertaking, and the design came out of several committees and advice and input from industrial firms. Of the many challenges, the biggest was how to make a mirror blank of sufficient size and with the necessary properties.

Making the Disc

General Electric got the first shot at it, and they tried to make a mirror blank out of fused quartz. After $600,000 worth of failures, Hale terminated that experiment. He then approached the Corning Glassworks of New York about casting a disc from Pyrex glass. Corning started development in 1929, but it took until 1934 for them to get to the point to where they were ready to try pouring a 200 inch Pyrex disc. Here is a silent movie of the process:


The first attempt ended in failure when sections of the mold broke loose during pouring. The ruined disc is still on display in the Corning Museum of Glass.

The first mirror blank casting, which was a failure.

The second pour was successful, and the glass blank spent the next year in an annealing oven, so that it could cool slowly and prevent damage to the disk.

The successful second mirror blank casting.

The mirror blank was then packed into a special shipping container, loaded onto a special padded railcar in an upright position, and spent the next two weeks traveling only during the daylight hours, and never faster than 25 miles per hour. It’s destination was Caltech’s optical shop, where the grinding and polishing of the mirror blank would take place.

The railcar and shipping container for the 200 inch mirror blank.

Grinding and Polishing

Once in Caltech’s optical lab, the front surface of the mirror blank is ground to the approximate concave shape required. Then the mirror was polished using finer and finer abrasives to produce the desired parabolic shape. All during this process, optical tests were performed constantly, comparing the shape of the mirror to a perfect parabola. Almost 10,000 pounds of glass were removed from the mirror blank during grinding and polishing, including two inches of ‘scar material’ created by the casting process. The grinding and polishing started in 1936 and continued until 1941 when it was interrupted by World War II, because most of the engineers and scientists, as well as their laboratories, were reassigned to war-related projects. Drat the luck. After the war, work restarted in September 1945 with a new crew of technicians, as most of the prewar workers didn’t return to the project.

In 1947 the polishing process is complete, and the 40 ton mirror is shuttled up Mount Palomar over the course of two days, and installed in the telescope. The first results were encouraging, but it took another two years to finish polishing, aligning, and adjusting the mirror. Full time scientific observations began in 1949.

Creating the Mirror Surface

Some of you have expressed interest in how the 200 inch mirror is re-silvered periodically. It just so happens that I came across a time-lapse video of the process as it occurred in 2006.



That’s all for now.  Feedback is always appreciated, regardless of my grousing.

For more information, check out the links in the reference section.



The American Institute of Physics website

The California Institute of Technology Astronomy website

The University of San Diego’s San Diego History website

The Corning Museum of Glass website



  • skitter

    It doesn't seem like it should be that complex an undertaking, but scale clearly had other ideas. A year of annealing? Even for ridiculous standards of homogenous composition, and keeping the temperature gradient near zero, that's incredibly tedious.

    • The Professor

      I don't know enough about the chemistry of how large volumes of glass change as it cools at different rates. I suspect that it has to do with keeping the thermal expansion properties uniform throughout the disc. One of the main concerns of the engineers was to keep the thermal movement of the mirror to a minimum as the temperature changed in the observatory from day to night. Having a mirror that changed shape unevenly would be a disaster for observations.

      • OA5599

        Objects in mirror are closer than they appear.
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        • The Professor

          I just noticed that you made it to The Century Club. Congrats! They say that there will be scotch and cookies, but it's Costco scotch, and I have no idea where those cookies came from.

          • OA5599

            Thanks. Don't tell anyone, but I've been crashing the Century Club parties for a couple of months now, but it's good to now be a member in good standing.

            Unfortunately, my browser settings don't permit third-party cookies, so I'll have to settle for cheap scotch only.

  • Number_Six

    I had to re-read that part about the decade-long polishing twice.