Here We Go!

It's been nearly a month since I last posted and SPT-3G has made a huge amount of progress in that time.  It's hard to describe all the effort and emotion that has been a part of this past month, but I'll try.   Working on an experiment at the South Pole is an all-out exhausting but exhilarating roller-coaster experience.  Every day is a new and different challenge.  The time we have is short since the last flight out leaves on Feb 15, and everyone has to be on it except our two winterovers.  SPT-3G a big complicated receiver, and not the kind of thing that we can just leave in pieces for them to finish up.

Around 10pm on New Year's Eve, we ended our first operation run of the receiver  and started warming up the cryostat.   A few days later, a new crew of SPT people arrived with the new detector modules.  From that moment on, it was a race against the clock to assemble all 10 wafers and install them into the focal.  It took about 3 days to complete the detector wafer assembly and about 2-3 more to fully assemble the focal plane and finalized the receiver.

The detectors are connected to cables using tiny aluminum wire bonds (they look like hairs and you usually need a microscope to see them).  The first step in assembling the wafers was to electrically check all of these connections.  Luckily, we have an automated setup to make these measurements for us (it's ~6400 measurements per wafer).  Based on these measurements, we go in and remove wire bonds that have become problematic.  The detector wafers traveled all the South Pole in specialized carrying cases, but they still feel all the vibrations and jolts of travel.  This is probably my least favorite step of wafer assembly because you have to have the detectors fully exposed.  But it does allow you to take pictures of the detectors themselves, which you can see below. Each of the little dark spots on the purple background are the individual antennas that bring the light to the detectors.  There are six superconducting detectors that are fed off each antenna, but they are too small to see in this picture.  For scale, the wafer is about 4.5" across.    Once plucking the wire bonds is done, we put the protective back plates back on and start connecting each of the 48 cables on a wafer to the readout electronics.  It's a slow process, but we (Adam, a postdoc from Fermilab and I) took our time and managed to do all 10 wafers with no mistakes.

An exposed SPT-3G detector wafer waiting for plucking of wire bonds.

The number of SPT-3G detector wafers grows!  Under the static bag in the back is a fully assembled module, and its counterparts are prepped and waiting for readout electronics.

More modules, fully assembled now.  You can see the electronics towers and the wiring coming off the top.

Next the wafers get installed into their spots in the millikelvin plate.  It was the first time we had all 10 spots filled and I have to say, it was a beautiful sight. Once the wafers are secured, we spent a couple days dealing with all the readout wiring on the other side.  Each wire has go to the different cryogenic temperature stages in order, or else the instrument won't get cold (and there are three stages).  Weaving the wiring in and around the different parts of the focal plane took about 20 straight hours in the end.  To keep wiring from touching different places we use dental floss (waxed, unflavored).  Sounds weird, but works great because it is strong but has low thermal conductivity.  You can see the result in the picture below where the focal plane is installed in the back of the cryostat.  The lenslets are pointing down the optics tube, and aren't visible in this picture.  Instead, you see the full glory of all the readout electronics.   And then we closed up the back of the cryostat, and that was it.  The SPT-3G receiver was fully assembled for the first time ever, and the last time unless we decide to open it during the next South Pole season.

The lenslet-side view of the focal plane as it sits on a benchtop.  10 modules!!!!

Myself, Chrystian, and Junjia with the fully assembled focal plane.  Chrystian and Junjia are two of the material science fabrication experts that actually made the detector wafers we installed.

Backside view of the focal plane, installed into the cryostat.  Lots of detectors results in lots of electronics and lots of wiring to deal with.

The receiver close-up team, in front of the fully assembled cryostat.  Left to Right: Jason, Zhaodi, Donna, Wendy, Me, Brad, Adam, Daniel.  

Closing up happened around January 10, and then we were in a holding pattern waiting for the receiver to cool down.  It takes about 7 days total to bring the receiver down to 4 degrees Kelvin and then the detectors to 250 millikelvin.  During that time, we got in a little bit of leisure time and rest (another blog post coming soon on this) and prepped plans for operating the receiver.   The receiver got cold right on schedule and we started working on setting up and operating the detectors.  We've never operated this many simulataneously before, so there were some kinks to work out.  There still are some, as we're learning more about our new detectors and electronics, but we've made some steady progress.   A little less than a week later it was time to hoist the receiver into the telescope.  We did a test lift back into December, so we had a general idea of what we were going to do.  But lifting a 2500 pound cryostat up many feet into the air through a space with only a few inches of clearance on side without damaging it is still a big operation.  The lifting itself went truly smoothly.  Attaching the receiver to its mount in the cabin was very challenging because it requires an extremely precise alignment of the cryostat.  But in the end we got it.  In case you are wondering how we hold such a big instrument into the telescope, the answer is some really big bolts.  There are four of them that connect mounting points on one side, and then two stiffener bars that come on the other side.  In the picture below, I'm holding one of the main bolts that holds the cryostat into the cabin. 

Preparing for the lift of the cryostat into the telescope cabin!.  That's an M20 bolt, one of four main bolts that hold SPT-3G in place.

And it's up!  The SPT-3G receiver fully lifted into the telescope cabin.

 

After the cryostat was lifted, the next day we lifted our electronics up and worked on hooking everything up.  That was last Friday.  Since that time we've been working towards operating the detectors and telescope as we push towards first light (but that's another very special post).  

In the meantime, the summer season at the South Pole is starting to wrap up.  I realized I haven't posted a lot of pictures of South Pole itself, and we had some really beautiful lighting the other day.  So here it is, one of the entrances to the Amundsen Scott South Pole station.  This is one of the first things you see when you get off the plane, and its the door I use every day going back and forth to the telescope.   If you look carefully, you'll notice the station is actually elevated above the snow.  Snow drifts are a huge problem and buildings are continually getting buried over time (including SPT and the dark sector lab building it is attached to).  The design of the elevated station helps it resist this build up by directing the wind.  The whole station can also be jacked up higher when needed to increase the lifetime of the building.

South Pole Station.

I also spent some time working on the roof of SPT the other day and took a nice panorama showing the view out to the horizon.   I really love looking out at the great nothingness there, but it does get cold fast when you are on the windy side of the telescope.  I also took a panorama from the room of the building attached to SPT (the dark sector lab).  You can see the MAPO building (home to the Keck array CMB telecscope), elevated station, and Ice Cube lab in the distance.

View of the South Pole area as seen from the room of the dark sector lab.

Horizon view from the roof of SPT.