The James Webb Space Telescope, Hubble’s successor, is an infrared telescope. Warm materials glow in the infrared, and for this reason Webb optics have to be kept cold — all the way down to 40 Kelvin (or -233C or -388 F). Unfortunately, our technology doesn’t allow us to polish mirrors while working at 40K. Thus, the conundrum for Webb was that the mirrors had to be polished at ordinary temperatures but still had to be the right shape at 40K. When temperatures change so dramatically, mirrors warp and deform.
About 10 years ago, a study took place to select the best material for the Webb mirrors. Instead of glass, we decided to use a substance called Beryllium. We found that by using computer models, we could accurately predict the ways Beryllium would deform. The plan was to polish the 18 mirror segments that make up our primary mirror at a warm temperature, but give them exactly the wrong shape that would deform into the right shape once they were brought down to 40K.
It sounds like a bold plan and, while confident in our testing, we were all a bit concerned about whether this would work well and on the first try. One can always try again — but this entails extra cost and delays, so we were hoping to avoid it.
Finally, in January 2010, the first demonstration mirror segment for Webb went through the full polishing process and was frozen to 40K at the X-ray and Cryogenic Facility at the Marshal Space Flight Center in Huntsville, Ala. We measured the segment at the end, and found it had deformed to the right shape. This is a major success for the Webb project and lets us move on to developing the rest of the primary mirror segments.