Here is a video that they produced that explains it:
Here is a direct link to the video on YouTube.
Last March Robert Cassetti, who is seen in the video, happened to give me a wonderful tour of the fabulous glass collection at the museum. If you are at all interested in unique and amazing glass artistry I encourage you to explore all 60 pieces that are highlighted for the museum's 6oth anniversary.
*By the way, the 200-inch mirror that is actually in service at Palomar is my favorite piece of glass. However, it is not on display, nor does it weigh 20 tons.