Well, it’s been a busy week. The receiver cryostat for our camera arrived a bit late, only last Saturday afternoon, and we’ve all been working hard to install the focal plane and get it cooling for the first (nearly) fully populated SPTpol focal plane tests. Very exciting!
The SPTpol receiver cryostat is almost identical to the cryostat used for the SPT-SZ camera, (the camera that’s been on the telescope since 2007). The biggest visual difference is the color. The old cryostat is red, while the new one is black. While building it up and testing it at Chicago, Brad and Abby named it “Black Cat,” or “El Gato Negro” as the sticker on it shows.
The Receiver team and Black Cat. From left: Liz, Brad, myself, and Abby.
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Let me do my best to walk you through the close up. We started by finishing preparations on the focal plane. Along with the receiver came little shims for the 150 GHz modules. Each has a slightly different height because the silicon wafers in them are all slightly different heights. To make sure everything stays in focus, we shim up the shorter modules to match the height of the tallest module. This meant removing all the modules and re-installing them, but this time plugging in all of the readout boards. Next we installed the filters that go in front of all the feedhorns. These filters define what color of light the detectors are looking at. Without the filters the detectors would see too much light and they’d fail to operate correctly. Finally, radio frequency (RF) light is bad news bears for our system, so we close up the gaps between temperature stages at the front of our camera with annuli of aluminized mylar, which makes an electrically conducting seal that blocks radio light from getting in, much like the metal mesh in the door of your microwave keeps microwave light cooking your food and not your face.
We shimmed the modules to be the same height, and taped over holes in the focal plane where more 90 GHz pixels will be installed in a couple weeks.
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All 18 readout boards installed on the back of the focal plane. They are attached to the detectors with flexible circuit striplines.
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Filters are placed in front of the horns to block out light we don’t want the detectors to see.
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RF light can ruin our readout, so we put aluminized mylar between the different temperature layers.
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John Carlstrom, the project PI, flew home before we could install the camera, but he left a note for us.
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With the camera built up and ready to go, we started installing it in the cryostat, which was a multiple day process. It’s not just a matter of screwing the camera into the cryostat and calling it a day. We also have to connect thermometry to the camera so we know how cold it is, and we have to heat sink each thermal stage of the focal plane to our millikelvin fridge so it actually gets colder. The fridge does the cooling, and we attach the camera to the fridge by three flexible copper heat straps, each at successively warmer temperatures so we don’t blow the detectors out of the water with too much thermal power.
Abby installing the camera in Black Cat.
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Liz and I installed RF boxes for readout cards while Abby attached heat sinking straps to the focal plane.
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We also have to heat sink all of the readout wiring coming out of the back side of the receiver. Each cable gets sunk to multiple temperature stages, each a little warmer than the last as the cable winds its way back out of the cryostat and into the room temperature world. Heat sinking is REALLY tricky. There are 18 cables coming out of the camera. One end of the cable is in contact with the 250 mK cold stage. The other end is at room temperature (300 K). You have to clamp the cables down at incrementally hotter temperatures so the coldest stage stays cold. Well, you can imagine with 18 cables in close proximity it’d be pretty easy for two lengths of cable at different temperatures to accidentally touch and warm up our detectors. Whoops… So Abby, Liz, and I spent a lot of time designing and planning how to attach the cables to the various temperature stages, and then actually doing it to so there were no thermal touches.
Heat sinking bars attached to the back of the focal plane. Liz had the awesome idea of adding bars to the pre-existing heat sinking ring so we can bring the cables out radially. Abby and I designed the parts, and Liz’s advisor Bill machined the new parts for us.
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Liz tying a copper heat strap up with dental floss to keep it from touching another heat strap.
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Once all the readout cables were attached we could actually close Black Cat. The first radiation shield (at 4K) is split into two parts. We put the first part on and taped all of the readout cabling to its under surface. This keeps the part of the cables at 4K from touching the cooler part of the cables less than an inch below them. From there it’s pretty simple: put the other half of the 4K radiation shield on, cover it in aluminized mylar super-insulation, put the 50 K radiation shield on and cover it in super-insulation, and put the 300 K radiation shield on, which has the vacuum seal.
All of the readout cables attached and sunk to the various temperature stages. The thin orange cables get taped up to the cryostat ceiling.
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4 K and 50 K radiation shields on, and super-insulation installed.
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Closing up the external 300 K shield.
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At this point Black Cat was only partially closed up. When we flipped the cryostat over the other side needed some work too. This is the side that has a window that gathers light from the telescope. Liz and Brad had already installed some baffling in the window that Abby and I had previously covered in black goop that absorbs stray light. You can get some nasty reflections in the telescope’s optics chain, and you want to get rid of light that doesn’t actually hit the detectors straight away. Light that doesn’t can bounce around and enter the detectors at a later time and confuse us by looking like it came from a different part of the sky than it actually came from. So we add baffling to control the path of light and stray reflections, and blacken the surfaces in the light’s path to the camera so that stray light gets absorbed. After the baffling we add a lens to focus the light on the focal plane.
A view of the focal plane installed underneath the extra light baffling. We’re looking through Black Cat’s window after the cryostat was flipped over.
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Brad and Liz installing the lens.
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Just like the camera, Black Cat’s window needed some filters and RF shielding, so we installed those next. The RF shielding mylar is really thin and fragile, so installing it is a tricky business. There are lots of screws that hold it in place and one slip of the screwdriver means popping a hole in the mylar and starting over.
Installing a 50 K filter into the system in front of the lens.
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Installing more RF shielding.
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The receiver cryostat is one of two cryostats in the system. The secondary mirror of the telescope is also in a cryostat, called the Optics cryostat. This thing is enormous (about the size of a VW beetle). It accepts light from the main dish of the telescope through one window, then focuses it into the receiver cryostat and onto the camera through another opening. All of this stays under vacuum and stays really cold, so that means we have to mate the Optics cryostat with the receiver cryostat to effectively make one giant cryostat. To do that, we needed to install mating cones on Black Cat. There is a cone for each radiation shield layer: an outer cone at 300 K that also holds vacuum, a slightly smaller nested cone at 50 K, and a still smaller cone at 4 K. With the cones installed, we put one more filter in Black Cat designed to keep hot light from warming up the filters underneath it, and then we were ready to mate the two cryostats.
Installing the mating cones so we can attach Black Cat to the Optics cryostat.
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Mating the cryostats is a bit tricky. There are two quarter-inch dowel pins in the opening of the Optics cryostat and if the two cryostats don’t line up perfectly when you push them together you can bend the dowel pins and get them permanently stuck together. More bad news bears. So, to control the mating process both cryostats are on chain hoists. It takes several people to hoist the corners of the cryostats just so to get them perfectly level, then a couple more people to slowly push the cryostats together and seal them up. Once the screws were tight and the cryostats were detached from the chain hoists, we moved them into position for testing and started pumping down to get them under vacuum. Five days from now we should be cold enough to start taking data. That’s the plan, anyway.
Preparing to mate Black Cat to the huge Optics cryostat.
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Tightening screws after Black Cat was mated to Optics.
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To celebrate the close up and to relax a bit, we all spent some time in the station sauna, then ran out to the Pole in our bathing suits for a picture. It’s really not as bad as it sounds. You’re sweating like crazy in the sauna, which is sitting around 200 degrees, so you can stay outside for several minutes before getting really cold. And yesterday was pretty warm – only -35 F or so with wind chill. We went to the Pole once, but cycled between the sauna and the station entrance outside several times to prolong our sauna session. On the last cycle, Brad, myself, and a grad student from McGill named Tijmen decided to have a little competition to see who could stay outside in our bathing suits the longest. (It was actually my idea and they were willing to go with it). Brad is from Wisconsin and I’m from Michigan, so we were doing just fine, but Tijmen (who is from the Netherlands) was shivering like crazy. The man has some determination, though. After some awesome intimidating stares and several minutes of standing out in the cold as our body hair frosted over, Brad finally gave in to warm up his fingers. I stayed out an extra minute or so but Tijmen and I were both starting to get pretty cold, especially our fingers and toes, so we decided to go back inside as joint winners. Well played, sir!
Me in my bathing suit standing at the geographic South Pole after a session in the sauna. Black Cat is closed!
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Christmas is this weekend, and that means the Race Around the World and Christmas dinner, so stay tuned for another update soon.
Good luck, Black Cat!
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