Early yesterday morning, the Black Cat cryostat was sealed up, hopefully for the last time this season. This is the third time that we've sealed up and cooled down. Cooling down the entire receiver from room temperature takes at least five days, so it's something that we can't do very often. We decided to warm up the receiver last time because it was getting too warm (where "too warm" means about 360 milli-Kelvin), which reduces the effectiveness of the sensors. We spent the time we had with the receiver warm doing everything we could to make sure it stays as cold as possible, as well as replacing some of the individual sensor elements which were broken or not as effective as they could be. Now we're done, and Black Cat is once again closed.
As you can see in the photograph below, Black Cat is now connected to the (larger, white) optics cryostat. The optics cryostat contains mirrors, lenses, and filters which transfer the light of the cosmic microwave background from the telescope dish down to Black Cat, where the light will eventually encounter the chilly bolometers which do the work of converting light into an electrical signal.
The first step in the cooldown is to pump all of the air out of the cryostats. We need to make sure that as little heat as possible can flow between the warm outside and the cold receiver inside the cryostat. Removing the air takes away a significant path for heat flow, and makes the cryostat much easier to cool. After the air is gone, we use a pulse-tube cooler (http://en.wikipedia.org/wiki/Pulse_tube_refrigerator) to cool the cryostat down to about 50 Kelvin, then two stages of further cooling which use the evaporation of liquid helium to cool the bolometers down to 300 milli-Kelvin.
If all goes well, the cryostats will remain in this cold vacuum (even colder than deep space) until next summer, when we perform more maintenance and upgrades.