The SPTpol focal plane.  The inner seven hex-shaped modules are our 150 GHz array (made by NIST in Boulder, Colorado), and the outer 100 GHz array (made by Argonne National Labs, just outside Chicago).  Every detector has its own individual horn, which couples light to two detectors, each sensitive to two different types of polarization.

Today we just started our second cooldown of SPTpol!  It has been a marathon last two weeks; starting and finishing the first SPTpol cooldown, the re-working the receiver and optics (secondary mirror) cryostats to be in their (hopefully) "final" configuration for the observing season, and then closing them up and starting another cooldown.

A "cooldown" as I refer to the above basically consists of:

1) ~3-5 days of work assembling the innards of both the receiver cryostat (which holds the actual detectors and is cooled to 0.25 K degrees above absolute zero, and is what I usually refer to as the camera for the telescope) and the optics cryostat (the cryostat that holds the secondary mirror, and is cooled to 10 K degrees above absolute zero). 

2) one day of "pumping" on the cryostats, and bringing their pressure down to about 1 / 10 millionth of an atmosphere,

3) ~3-4 days of "cooling" the cryostats, bringing each down to the base temperatures above, and

4) ~2-several days of tests, where we characterize the cryogenic (cooling) performance of the cryostats, and the characteristics of the detectors (e.g., their optical efficiency, noise, sensitivity, etc.)

We began our second cooldown today, so if all goes well, we should be on the telescope by about January 12th!

Abby and Jason assembling the SPTpol focal plane.  In this picture, they are testing the electrical connections to each detector by probing wires on the back of the focal plane pictured above.

Liz, Jason, and Abby putting the focal plane into the SPTpol receiver (the black and gold plated thing below), which will cool the detectors down to 0.25 degrees above absolute zero.

A significant effort in preparation for this cooldown was working on the inside of the optics cryostat pictured above.  The cryostat is about the size of a Volkswagen Beetle and weighs nearly 2,000 pounds.  We are lifting the top half of it so that we can work on its innards, specifically the heat strapping that cools the radiation shields to 10 and 77 K degrees above absolute zero.  The shiny silver material is "super-insulation", which is basically multiple thin layers of mylar with aluminum deposited on its surface, which shields the cooler shields from the hotter exterior shields. 

Me reaching into the radiation shields looking for a lost allen key.

A picture of me working on the underside of the optics cryostat, thats 2000 pounds above my head!

Nils taking a picture of the heat strapping from our pulse-tube cooler to the shields. The pulse-tube cooler is a mechanical cooler, which alternatively pulses low and high pressure helium gas, and cools each of the shields to 10 and 77 K. 

Bill Holzapfel and I re-connecting the pulse-tube heat strapping to the radiation shields. 

A close-up of Bill and my hard work, replacing each one of these screws so that they have double-conical washers and Apeizon-N grease between each interface. 

The SPT team at Pole mating together the optics and receiver cryostats, pictured are Kyle, Liz, Jason, Abby, Tijmen, Bill, and me, Nils is taking the picture.

In parallel to the receiver team's work, another team has been working at night to install a guard-ring to the SPT.   The guard-ring effectively acts like a ground-shield, which reflects detector power to the "cold" sky instead of the "hot" ground, and extends the diameter of the primary by about 1.5 meters.