The seventh and final step in the alignment process of NASA's James Webb Space Telescope is nearing completion. The telescope is approaching its final, chilly temperature as it mirrors cool, as it has cooled to its operating temperature.
The telescope mirrors are cooling down now that the instruments are at their operating temperatures, but they are not quite there yet. It takes a long time to cool or heat up when it's at cryogenic temperatures. The primary mirror segments are cooling slowly.
Designers of space missions need to address the issue of materials changing shape as they cool. The careful work of aligning the mirror would be lost if the mirror segments were made of glass. The mirror is made of beryllium, which has a property called low thermal expansion, which means it doesn't change shape when heated. Even as the primary mirror segments cool, they don't affect the process of aligning the telescope.
The 18 segments of the primary mirror vary in temperature from 34.4 kelvins to 54.5 kelvins, and there is also the secondary mirror to consider. The small mirror sits on the end of a boom arm and is currently at a cooler 29.4 kelvins due to being further away from the heat sources.
The mirror segments are cool enough that they won't prevent MIRI from taking science readings. The team hopes that they will cool down by 2 kelvins, which will allow MIRI to take even more accurate readings. The way that the telescope is pointing at the sun is related to the temperature that they reach. 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.
As the direction in which it points is changed, it is expected that the average temperature will go down.
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