With Fall rolling in, our available time has been rapidly running out. In order to ship to the South Pole, everything must be packed and shipped out of Minnesota by the end of September. Giving all of September over to disassembly and packing meant that August was a month of long weeks as we tried to finish everything that we needed to.
Since we’re in the shipping stage, I’ll be spacing out a couple posts that showcase what we’ve accomplished in the last few weeks.
One item on the list was the forebaffle; so-called because it rides at the front of the receiver and blocks light. Each of the Bicep Array receivers will have one of these large cup-shaped pieces that rides along with it in the mount. Its two halves are rigidly attached to the structure, so from the point of view of the receiver it doesn’t ever move.
The upper lip of the forebaffle just clears the main beam of the receiver, and the inside is coated with a broad band absorbing foam. What this means is that any secondary pickup from outside of our main beam must reflect first off of the absorbing forebaffle which significantly lowers the amplitude of any reflected signal. Two particular sources of potential pickup that the forebaffle intercepts are from the ground and the groundshield. The groundshield (shown below) is a large screen that prevents the telescope from directly seeing the ground which is much warmer than the sky. However, the groundshield itself is also a warmer object and light it reflects can be polarized and should be blocked from directly entering the telescope. Light from the ground can also diffract over the edge of the shield towards the receiver. The forebaffle intercepts these rays and helps to dampen any response to them that the telescope might have.
Having such a large cup-shaped piece presents its own complications. The absorbing foam has to be protected from the weather, and we can’t capture snow inside it since water (and ice) scatters the very light we want to observe. For the first issue, after we cover the inside with the absorbing foam, we add on a thin protective (and smooth) layer on top to keep ice from catching in the porous surface of the absorbing layer. The second issue is solved by wrapping the exterior of the forebaffle in a long winding of heat tape, then multiple insulation layers. During operation, we heat the forebaffle to just a few degrees above the ambient temperature which helps keep ice from sticking to even the smooth surface of the protective layer.
One other item that’s shown in some of the above pictures but that I’ve neglected to talk about so far are the insulation panels.
Many of the photos I’ve been posting are unique, in that the inside of the mount is visible from the outside. However, the vast majority of the structure will poke out of the building where it will be installed. Since we have a lot of electronics and other equipment that ride along inside the blue steel, we can’t let it be open to the cold air. To help fix this, the mount has insulation panels for its top-most and bottom-most faces. These panels are foam-core with an Aluminum skin which keeps them light but effective. In order to make them fit as well as they possibly could, they came without any holes drilled in them. Instead we had to arrange them in-place with clamps then mark and drill the holes before attaching. The top section had one additional complication, the central circular pieces need to be able to rotate without scraping along the octagonal ones just outside, and by their nature the panels aren’t especially flat. After a few days of inserting and removing shims, we were able to get everything to move quite nicely.
I’ll finish here with another video. We’ve tested the stability of our control system a few times, below is an excerpt from one of these tests. A short video of the mount scanning under computer control. It was left this way for about 48 hours which is the length of one of our typical scanning schedules. The motion you see here is typical of what it will be doing constantly for the next few years.VID_20190816_233013 from Mike Crumrine on Vimeo.