We’ve been quite busy the last two weeks. After successfully testing out the winch system we were almost ready to run a true receiver cryogenic test in an operating mount, but we still needed to put a few more things on. First, we attached the encoder tapes and their read-heads. The encoder tape is a long adhesive backed tape with precision spaced magnetic North and South poles. As the mount turns, the read-head can count the number of poles it passes. We can then feed this information into our control software in order to track the orientation of the mount to extremely high accuracy! Below is a picture of the Azimuth encoder’s read-head (left) and a close-up of the encoder tape as viewed through a magnetic field viewer.
We have a second encoder tape and read head up on our theta axis for accurate control of what we refer to as our “Deck” angle. The elevation axis also has an encoder to measure its location though it is self-contained and we did not have to install it. With the encoder information in the system we can now define our “home” point and reference all our rotations as a number of degrees away from that point, which is a critical step in being able to scan the telescope.
Before we could run a cryostat up in the mount, we also had to install the Helium lines. Each cryostat uses a Pulse Tube cryocooler in order to get the two largest internal stages down to temperatures of 50K (-369 DegreesF) and 4K (-452 Degrees F). These pulse tubes use compressed Helium to extract heat from the internal structures and each one has to be hooked up to a compressor that is detached from the rotating parts of the mount. As a result, we have to run a set of Helium lines from each cryostat, through the Theta axis rotary joint, then through the elevation cable carrier, then down into the Azimuth rotary joint, then down to the floor and into the compressor.
We have 8 sets of lines lines that run the four cryostats, 2 sets of guard channels that help to control leaks within the rotary union, and one nitrogen set. With so many hoses running up into the structure, it’s imperative that we take care in routing them so that they aren’t in our way. To do that, we decided to run all the hoses underneath the various beams of the mount. When they approach one of the rotary joints, we attach them to a set of custom low-profile rigid sections that bridge the connection between the fixed rotary union and the flexible hoses.
These are a really cool set of fixtures, and each one is unique. We have only manufactured enough for one set of lines so far, but the rest are in production. This area is going to look very neat once the full set is installed.
With the encoder and Helium lines installed, we were ready to unload the dummy receiver and load in a real Bicep Array receiver with a pulse tube. Although we’d installed the dummy receiver using the new winch system, we hadn’t actually installed the real thing so we weren’t sure if there were going to be additional quirks in the procedure that we needed to figure out.
In the end, it installed without a hitch. Everything went smoothly, and we were able to secure it to the mount using the harness system in about 2 hours. We then attached a bunch of extra hardware to the front of it which is necessary for the operation of the pulse tube. With everything plugged in, we turned on the pulse tube and were rewarded with it’s characteristic chirping noise. With the cryostat installed and cooling we took the time to shoot some video.
The chirping noise you can here is the pulse tube. High and low pressure Helium gas are alternately pushed through the pulse tube cold head in order to produce the cooling power required to achieve cryogenic temperatures.
We also took a moment and got just about everyone to sit on the mount and shoot a quick video of a partial scan.
Finally, once we had the telescope cooling and scanning…
We threw an open house for the physics department. Quite a number of people attended and the donuts were quickly claimed. For an hour we chatted with other members of the department and anyone else who happened to walk by the high bay. Each of us got to talk about different aspects of the experiment we worked on and talk about the overall project while the telescope scanned in the background. It was great to see so many people interested!
Bicep Array - UMN 