Artist conception of the James Webb Space Telescope. Credit: NASA GSFC/CIL/Adriana Manrique Gutierrez
The Mid-Infrared Instrument (MIRI) on NASAs James Webb Space Telescope is now cooled by a gaseous helium cryocooler to under 7 kelvins (minus 447 degrees Fahrenheit, or minus 266 degrees Celsius). With the cooler in its final state, the Webb group is operating the MIRI instrument this week as part of final and seventh stage of the telescope alignment.
Now that the instruments are at their operating temperatures, the telescope mirrors will also continue cooling down to their last temperature levels, but they are not rather there. The primary mirror segments and the secondary mirror are made of beryllium (covered with gold). At cryogenic temperatures, beryllium has a long thermal time constant, which indicates that it takes a long time to cool off or to warm up. The primary mirror segments are still cooling, really gradually.
The secondary mirror, hanging out on completion of its support structure a long method from any heat sources, is the coldest mirror, currently at 29.4 kelvins (minus 406.8 degrees F). The 18 main mirror segments range in temperature level from 34.4 kelvins (minus 397.8 degrees F) to 54.5 kelvins (minus 361.6 degrees F). A benefit of beryllium mirrors is that they do not change shape with temperature level the method glass mirrors would at these temperatures, so the temperature range does not impact the telescope positioning procedure.
The main mirror sections and the secondary mirror are made of beryllium (covered with gold). The secondary mirror, hanging out on the end of its assistance structure a long method from any heat sources, is the coldest mirror, currently at 29.4 kelvins (minus 406.8 degrees F). An advantage of beryllium mirrors is that they dont change shape with temperature the way glass mirrors would at these temperature levels, so the temperature variety does not impact the telescope positioning procedure.
The Webb Telescopes sunshield is crucial to enabling Webbs science. This feature explains how the sunshield works, and its implementation. Credit: NASAs Goddard Space Flight
Presently, 4 of the 18 mirror sections are above 50 kelvins: at 52.6, 54.2, 54.4, and 54.5. The Webb group hopes to see the mirrors cool by an extra 0.5 to 2 kelvins.
When we point the telescope at a huge target, the telescope and sunshield move together. The angle that the sunshield presents to the Sun is called the pointing “attitude.” The tiny quantity of residual heat that makes its method through the five-layer sunshield to the main mirror depends on this mindset, and because the mirror sector temperatures change very gradually, their temperature levels depend upon the mindset balanced over several days.
Throughout commissioning, Webb is currently spending most of its time pointed at the ecliptic poles, which is a relatively hot mindset. During science operations, starting this summer, the telescope will have a much more even circulation of pointings over the sky. The average thermal input to the warmest mirror sectors is expected to decrease a bit, and the mirrors will cool a bit more.
Later on in commissioning, we prepare to evaluate the thermal dependence of the mirrors on the attitude. We will point Webb at a hot attitude for a number of days, and point Webb at a cold attitude for several days, in a process called the thermal slew. When the observatory is at these positions for any offered quantity of time, this will notify us how long it takes for the mirrors to cool down or heat up.
Is Webb at its last temperature? The response is: practically!
Written by Jonathan Gardner, Webb deputy senior job scientist, NASAs Goddard Space Flight.
The small amount of residual heat that makes its method through the five-layer sunshield to the primary mirror depends on this attitude, and since the mirror section temperatures change really gradually, their temperatures depend on the mindset balanced over numerous days.
The average thermal input to the hottest mirror sectors is expected to go down a bit, and the mirrors will cool a bit more.
