Is the Webb telescope in the end temperature stage

(ORDO NEWS) — The Mid-Infrared Instrument (MIRI) of NASA‘s James Webb Space Telescope is currently cooled to less than 7 Kelvin.

With the cooler in its final state, the Webb team is working with the MIRI instrument this week as part of the seventh and final phase of telescope alignment.

During operation, the detectors and electronics generate heat, which is balanced by a cryocooler to maintain a stable and very cold MIRI operating temperature.

Near-infrared devices get hot during operation and must dissipate heat, although for these devices this is done by passive cooling; heat from detectors and electronics is radiated into space.

Now that the instruments have reached their operating temperatures, the telescope mirrors will also continue to cool to their final temperatures, but they have not yet reached that.

The primary mirror segments and the secondary mirror are made of beryllium (coated with gold). At cryogenic temperatures, beryllium has a large thermal time delay, which means it takes a long time to cool or heat up. The primary mirror segments are still cooling, very slowly.

The secondary mirror, located at the end of its support structure at a great distance from heat sources, is the coldest mirror, currently at 29.4 Kelvin. The temperature of the 18 segments of the primary mirror varies from 34.4 Kelvin to 54.5 Kelvin.

The advantage of beryllium mirrors is that they do not change shape with temperature in the same way that glass mirrors do at such temperatures, so the temperature range does not affect the telescope alignment process.

Currently, four of the 18 mirror segments are above 50 Kelvin: 52.6, 54.2, 54.4 and 54.5, respectively. These four segments of mirrors emit some mid-infrared light that hits the MIRI detectors.

Since all mirrors are now below 55 Kelvin, MIRI is expected to be sensitive enough to carry out the planned scientific research, but any additional cooling of these mirrors will only improve its performance. Webb’s team hopes the mirrors will cool another 0.5 to 2 Kelvin.

When we point the telescope at an astronomical target, the telescope and the sunshield move together. The angle at which the sunshield faces the sun is called the pointing “position”.

The tiny amount of residual heat that passes through the five-layer sunshield to the primary mirror depends on this position, and since the temperature of the mirror segments changes very slowly, their temperature depends on the position averaged over several days.

During commissioning, Webb spends most of its time pointed at the poles of the ecliptic, which is a comparatively warm position.

During scientific operations starting this summer, the telescope will have a much more even distribution of targets across the sky. It is expected that the average heat load on the warmest segments of the mirrors will decrease slightly, and the mirrors will be cooled a little more.

Later, during commissioning, we plan to test the thermal dependence of the mirrors. We will point the telescope in a hot direction for a few days, and then point it in a cold direction, also for a few days. This will let us know how long it takes for the mirrors to cool or heat up when the observatory is in those positions for a certain amount of time.


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