Ovens…
Big Astronomy (and with it, the science of astrophysics) requires big mirrors to see farther and fainter across the universe. If you visit such places as Palomar Observatory or Gemini, you see big mirrors that act as giant light buckets to capture as many photons as they can from dim, distant objects. Mirror-building is getting to be quite an exact science, as we all found out from the Hubble Space Telescope, which has a mirror that was ground perfectly, exactly slightly wrong. (Precision works both ways, you know.) Since then, mirror builders have turned out ever more accurate and correct mirrors that are helping us see amazingly far out through the cosmos. Not only are they using single large mirrors, but some facilities are using arrays of smaller segments, joined together to create one large observing “surface.” For this discussion, I’m going to focus on the single big mirrors because another big one has just come out of the hopper (or oven, actually) in Arizona.
How do they create these big ones?
When astronomers need a mirror, the first thing they do is have one made out of glass, which has to be cast in a furnace. If you’ve ever seen glassblowers at a street fair, it’s roughly the same principle in that they use molten glass to create their fantastic art. In the case of a mirror, a huge amount of molten glass is created in a furnace and then cast into what is called a “mirror blank.” That blank is then polished to high precision and to the correct curvature and put in the telescope. This, by the way, is pretty much the same way glass mirror blanks for small telescopes are made, too. If you ever go to a star party event such as Stellafane (held each summer in Vermont), you’ll see lots of people polishing their mirror blanks.
The latest “big” astronomy mirror to be cast is going into the Large Synoptic Survey Telescope at Cerro Pachón, Chile. It just came out of the ovens at the University of Arizona’s Steward Observatory Mirror Lab. It’s a 51,900-pound blank that contains a 27.5-foot (8.4-meter) primary mirror and a 16.5-foot (5-meter) tertiary mirror. Both were cast in the same mold, which is quite an accomplishment. This picture shows the blanks and quite possibly the last time that human hands will be allowed to touch the mirror. Now it will go in for polishing and shaping, before making the long journey south to its final installation.
The LSST will be the widest, fastest, deepest eye on the sky, aided by an incredible array of digital imaging instruments. If all goes well, science operations will begin in 2015 and astronomers will be able to do time-lapse digital imaging across the entire available night sky every three days. With this sort of large-scale mapping, they will be able to chart the structure of the universe and track events as they occur (such as supernova explosions). One of the most important science objectives for LSST is to explore the nature of dark matter and dark energy.