Building SPT-3G

It's been an extremely busy week, but the new SPT-3G camera is now built and ready for our first set of tests here at pole!   The last of our cargo arrived a little over a week ago, and as soon as we had it out of the crates it was a race to assemble as quickly as possible.  We are only planning on one short test run of the new camera before we have to work on it some more to finalize the configuration for the Antarctic winter.  I work mainly on the focal plane, the structure that houses the detectors and  electronics.
 
First, the two modules I showed in my last post get inserted into the structure seen in the picture below.  You can see the other eight silver hexagons where our other wafers will go.  Right now, these spots are blanked off to prevent light from leaking onto the detectors in the middle in a strange way. 

The SPT-3G focal plane before it was installed into the cryostat.

The rest of the focal plane looks complicated but really is just a set of supports.  Our detectors operate at a temperature of 0.25 Kelvin (about -273 degrees Celsius).  The focal plane structure is designed to help us keep that cold temperature.  We use carbon fiber rods to isolate the ultra-cold stage that the detectors mount on, from pieces that run at other temperatures (between 0.35 to 4 Kelvin).   Finally, there is a sheet of aluminized mylar (looks like tin foil between different metal pieces) that we put into place to prevent radio frequency (RF) noise from getting behind the detectors to the electronics.  The aluminum on the  mylar creates a continuous metal sheet that the RF can't penetrate.  Installing this sheet is one of the most tedious parts of the assembly, as the mylar is delicate and a single pinhole would ruin the effect.  Luckily, Joshua (UChicago grad student who also works on the focal plane assembly) and I have steady hands.

Next we take the entire focal plane and install it into the the cryostat.   The lenslets face towards the rest of the optical elements, so you only see the backside of the focal plane from here on out.  Inside the cryostat are also the two mechanical refrigerators (out of sight in the pictures below) that cool the focal plane down, and some additional electronics (bottom of the picture).

The backside of the SPT-3G focal plane after install.

Me, the focal plane, and the cryostat.

 Once everything was plugged in and checked out, we closed up the cryostat.  There are three layers of shells to put in place.  The inner two (see one below) help shield the cold focal plane from hot objects around it.  The heat radiated from the room temperature outer shell would completely overwhelm the refrigerators and we wouldn't be able to get cold otherwise.

First of the shields on the cryostat covering the view of the focal.

In parallel to the work on the focal plane,  there was another crew of people assembling the optical elements (lenses & filters) that the camera looks out into on the other side of the cryostat.   We ended up finishing up around midnight last Saturday.  The very happy crew is below.

The receiver assembly crew and the completed SPT-3G instrument (that big metal thing in the back).

Now we are waiting for the refrigerators to do their job and cool the cryostat down.  It is a big instrument, so we're estimating it will be done sometime late this weekend.   Once it's there, we'll start turning on and tuning up the electronics and detectors for what will hopefully end with the first light of SPT-3G.

In other news, the first South Pole Overland Traverse (SPOT) arrived yesterdays.  These are a group of seriously awesome tractor drivers that drive from McMurdo to the South Pole to bring us fuel.  It's much more efficient than bringing it on the planes. I don't have any pictures yet, but there are some in posts from past years.   Also, someone has been anonymously sending a member of SPT five-pound bags of gummy candy.  Whoever you are, thank you.  You have now fueled many late nights working at the telescope and played a critical role in this first setup and cooldown. 

A portion of the current combined stash.