It’s been a couple of weeks since our last update and they’ve been busy ones. As of our last update, we’d just started to cool down the BA1 cryostat for the first time at the South Pole. After about 2 weeks of cooling down to its 4K base temperature, we were finally able to cycle our 3 stage Helium Fridge and reach the sub-Kelvin temperatures required by our detectors.
In the weeks that followed, we undertook a calibration campaign to characterize the properties of our detectors and the receiver, some of which can be substantially different at the South Pole where we are shielded from almost all wireless communication and looking at cold sky rather than a room temperature ceiling. For a few weeks, the receiver team was kept busy collecting and analyzing data in order to figure out how well the actual receiver measured up to its design, and how to set things up for optimal operation.
In the meantime, the mount team was also busy. As of the last update, we finished construction on the Bicep Array mount and enclosed it in insulating fabric and foam panels. Once enclosed, we were able to work up inside the mount without all of our bulky ECW (Extreme Cold Weather gear).
After a few weeks of work re-installing the subsystems which had been integrated in Minnesota, we began to hoist receivers into the mount. Although Bicep Array will eventually hold four receivers, we are only deploying a single one for the first year. The remaining three slots will be filled with Keck receivers from the experiment that Bicep Array is replacing. We already know these receivers very well, so we were able to begin hoisting them into the mount as soon as we were ready.
We’ve also spent some time getting our star cameras up and running. Each of the Bicep Array receivers has an independent optical camera attached to it. Every two weeks we take time out of our normal observations and point the telescope at a series of stars. Ideally these stars are centered in all of our star cameras, but in reality the receivers and the harnesses attaching them to the mount will flex as everything moves. This flexure will move the stars slightly off center in the optical cameras and the offset will depend on the orientation of the telescope. We can use these offsets to refine our model of the true pointing of the telescope and track them over time to see how things settle. Setting up these star cameras is no easy matter however. There is no way to know just how closely they’ve been aligned when they are first installed, or whether they are focused correctly. After a long time staring at a gray screen while making very small movements with the mount, we finally saw a vague blur that looked real. And after some focusing adjustments, we saw our first star.
After our on-the-ground calibration of the receiver had finished, it was time for it to be installed in the mount. It was a big moment for everyone here, and no one wanted to be left out. Which was good news, since its heavy, and required several people to push it across the room and up the corridor to the mount. After a few hours of effort the BA1 receiver had been installed and all slots in the mount had been filled. Although it was quite roomy at first, it became significantly harder to move around inside the mount with four receivers and all the electronics installed.
With the cryostats all installed and cold, we had one final challenge ahead of us. Get real, useful data from all the cryostats at the same time. A slightly complex affair that required figuring out how to get each cryostat and its data flow working and stable in the new system. Just a few days ago we were rewarded with time-streams, BICEP Array was officially observing.
We still have about two weeks until the station closes for the winter and most of us have to leave and there’s still a fair bit of work to be done during that time. But with a “first light” of sorts, we took time out to take a group photo in front of the new telescope with some of the amazing people that have helped to bring this project together.