James Webb’s mirrors are almost but not quite cooled

NASA’s James Webb Space Telescope is nearing completion of the seventh and final step in its alignment process. With its MIRI instrument now cooled to its operating temperature, the telescope is approaching its final, chilly overall temperature as it mirrors cool as well.

“Now that the instruments are at their operating temperatures, the telescope mirrors will also continue cooling down to their final temperatures, but they are not quite there yet,” writes Jonathan Gardner, Webb deputy senior project scientist at NASA’s Goddard Space Flight Center. “The primary mirror segments and the secondary mirror are made of beryllium (coated with gold). At cryogenic temperatures, beryllium has a long thermal time constant, which means that it takes a long time to cool or to heat up. The primary mirror segments are still cooling, very slowly.”

One of the problems that designers of space missions need to address is that most materials change shape as they cool. If the mirror segments were made of glass, for example, they would warp as their temperatures changed, meaning the careful work of aligning the mirror would be lost. That’s why the mirror is made of beryllium, which has a property called low thermal expansion, meaning it changes shape very little when heated. That means that even as the primary mirror segments cool, they don’t affect the process of aligning the telescope.

As well as the 18 segments of the primary mirror, which currently vary in temperature between 34.4 kelvins to 54.5 kelvins, there is also the secondary mirror to consider. This small, round mirror sits on the end of a long boom arm and is currently at a cooler 29.4 kelvins due to being located further away from the heat sources.

The mirror segments are now cool enough, at below 55 kelvins, that they won’t prevent MIRI from taking science readings. However, the team hopes that they will cool further, by 0.5 to 2 kelvins, which would allow MIRI to take even more accurate readings. The exact temperature which they reach is related to the way that the telescope and its huge sunshield are pointing at the sun. The angle at which the telescope is relative to the sun depends on the target that it is looking at, and this angle changes the telescope’s temperature over time.

When Webb begins science operations this summer, it is expected that its average temperature will drop a bit more as the direction in which it points is changed.

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