One of the key components of Herschel is its 3.5m primary mirror, the largest ever launched on an astronomical satellite. Because of its unfeasible size, the mirror was launched warm, at room temperature. Herschel operations require a much colder mirror – as cold as possible – since the sensitivity of the detectors is limited by the background thermal noise coming from the mirror. This is why Herschel has such a big sunshade, to keep the light of the sun and earth from heating up the mirror and other parts of the satellite, and ever since launch the primary mirror has been cooling by radiating its heat into space.
The final temperature the mirror reaches will be one of the determining factors in Herschel’s eventual sensitivity. The design requires it to reach about 80K, that’s 80 degrees above absolute zero, or -193 C. This plot shows how things are progressing:
For the long stable period form day 5 to day 23 the mirror was heated to keep it at a stable temperature while contaminant gasses were allowed to boil off. Since then the mirror has been cooling well and, as you can see, we’re already pretty close to the goal temperature.