Journey to the End of the World: SPT-3G

Now, finally time to talk about some of the work that’s been keeping me busy the past 3 weeks!  Each day we walk out to the Dark Sector Laboratory (DSL), the building where the South Pole Telescope is located (about 1km from the station).  You can also find the BICEP telescope, Keck Array, and Icecube Neutrino Observatory in the Dark Sector area of the South Pole, where ‘dark’ simply means that radio use is restricted due to potential interference with the experiments.

The part of the telescope we’ve spent most of our time working on is the SPT-3G camera.  The camera itself is responsible for focussing and imaging the microwave light that’s reflected from the large 10-metre primary mirror (and smaller secondary and tertiary mirrors).  Weighing over a tonne, we first needed to carefully lower the camera down from the optics bench that couples the camera to the telescope’s dish.  Once on the ground, we could finally begin the long process of disassembly and reassembly (twice!).

While it might be many times larger than cameras we’re used to using day-to-day, the SPT-3G camera looks surprisingly like a gigantic DSLR.   This is because the design is actually very similar; a long cylindrical section containing lenses focussing the light, and a rectangular section at the back contains detectors for measuring the light.

These two halves of the camera are contained in whats called the ‘optical cryostat’ and ‘detector cryostat’.  These are essentially huge fridges under vacuum for keeping components of the camera at cryogenic temperatures.  The detectors themselves need to reach nearly -273 °C to operate properly (only half a degree above absolute zero!).  In order to reach these low temperatures, each cryostat is made up of a series of shells that gradually step down the temperature towards the centre.

SPT-3G is designed to measure microwave light in 3 different bands; 90 GHz, 150 GHz, and 220 GHz.  In order to maximise the optical throughput in each band equally, each lens is covered with a special anti-reflective coating.  Over the past year some major advancements have been made in the anti-reflective coating we want to use, requiring us to replace each of the 3 lenses in the camera (including the cryostat window).  At almost a metre in diameter, each lens is made out of heavy alumina.  Even though they look a solid cream colour, each lens is almost completely transparent to the microwaves we want to measure.  They focus the microwave light in much the same way that glass lenses in an optical camera do.

Perhaps the biggest upgrade of the SPT-3G camera this season is the installation of ten new detector wafers.  Each of these detector wafers contain around 270 pixels, with each pixel able to measure microwaves in three different bands and two polarisations.  A lot of my work this year has been spent testing these new detector wafers, providing feedback to the fabrication process to help improve how the detectors operate.  A couple of the detectors we tested in Toronto made the cut and have ended up down here at the South Pole to be installed in the camera this year!

Due to the way we read out the detectors in the camera, each detector wafer needs to be attached to a set of ‘LCs’.  These LCs contain a series of inductors and capacitors that allow us to measure the signal on the resistive detectors in parallel.  Another grad student and I spent a week holed up in our little clean-room at DSL assembling each of the ten detector wafers, using tweezers to carefully place hundreds of tiny 0-80 screws.  Not for the faint of heart, a single dropped screw onto a wafer can potentially wipe out hundreds of detectors (or worse, crack the wafer).

Once we finished assembling each of the ten detector setups, we were ready to start populating them back into the focal plane.  Each LC board is attached to a Niobium-Titantium (NbTi) ‘stripline’, with delicate ultrasonic solder joints connecting each end.  These striplines carry the signals from the many detectors to SQUID amplifiers inside the cryostat.  Assembling the focal plane was a slow and delicate process, where a broken connection in any of the 120 striplines can potentially force us to take everything apart again for repairs.

Finally after routing and heat-sinking all striplines, the camera focal plane was ready to be re-installed inside the detector cryostat.

At this point the optical cryostat had been completely disassembled, and so it was possible to see the tiny white alumina lenselets above each pixel facing out of the detector cryostat to where the large lenses would eventually be placed.

With the help of four chain hoists and an engine jack, the heavy SPT-3G camera was back together again and ready for the second cool-down of the season.  The first step is to put the cryostats under vacuum for 24 hours, removing all the warm gases inside that would prevent the stages from cooling to cryogenic temperatures.  Once a low enough pressure is reached, we switch on the two Helium pulse tubes (special refrigerators that help us get to -223 °C and -269 °C inside the cryostat).  Due to the huge amounts of material that need to be cooled, it takes almost a week for everything inside to reach base temperature.

We have currently been cooling for 4 days, with 3 days to go until we can start testing all the new upgrades we’ve made to the camera.  In the meantime, I’ve finally found time to write these long-overdue blog posts and catch everyone up with some of the exciting work we’ve been doing!

The station is starting to look pretty festive here, gearing up for some big Christmas celebrations.  In my next post I’ll look at what life has been like here at the South Pole, with photos from around the station.

Happy Holidays everyone!

– Matt

 

Journey to the End of the World: the Commute

Hello to everyone from the South Pole!!  After 10 days of delayed flights we finally arrived on November 28th, with plenty of work to catch up on over the past 3 weeks.  It’s been a while since my first post so there’s a fair bit to cover.  First things first, the commute!

What was supposed to be 2 nights in Christchurch quickly turned into 6, with our scheduled flight being cancelled each day due to bad weather at McMurdo station.  We still had to be up at 5am each morning just in case, and as a result I think we visited almost every coffee shop in Christchurch!  Our favourite haunt was a cafe called C1 Espresso, where you can get curly fries delivered straight to you table via pneumatic tubes in the ceiling.  We even managed to squeeze in a couple of quick hikes, before finally the day came that our flight was ready to depart.

By this point there was a huge backlog of people waiting to fly out to Antartica.  To help get everyone onto the ice, we ended up on a 757 flight operated by the Royal New Zealand Air Force.  I was even able to head up to the flight deck for a chat with the pilots during the flight!

McMurdo station is the largest base on the continent, with a population of around 850 during our stay.  Soon after arriving at McMurdo we had to ‘bag drag’, where you check in your luggage for the flight the next day. Unfortunately this leaves you with just your carry-on bag, even if the flight is cancelled.  Not knowing that our expected 1 night at McMurdo would eventually turn into 5, I learnt the hard way that I should have held onto some extra clothes… oops.

As the weather can be so unpredictable, flights usually get delayed in small 2-hour blocks.  This means lots of hanging around and watching the ‘scroll’ (information TV’s around McMurdo), just in case the flight is about to leave.  Quite a few times our check-in time got to within 30 minutes, only to be pushed back at the last minute.  Our stay at McMurdo coincided with American Thanksgiving (no flights over holiday weekends), giving us the chance unwind and do some exploring on trails around the station.

After an long but enjoyable 5 days of hanging around waiting, we shuttled out to the airfield and boarded the LC-130 aircraft that would take us on to the South Pole.  I was excited to finally be on our way again, but will definitely miss the 24-hour hot pizza from the McMurdo galley.

For the LC-130 flight our seating was just red netting along the sides of the plane, but we were free to walk around a peer out the portholes as we crossed the Antarctic continent.  The South Pole is on an ice plateau almost 3km thick, so it was incredible to see huge mountain ranges shrink as they become buried under the ice.

We arrived at the South Pole late on November 28th, with the sun still high in the sky (it never sets during the summer!).  First off we were shown a quick orientation video and given tips by the station doctor, with each tip being DRINK LOTS OF WATER. Thanks to the high altitude and extremely low humidity, dehydration is a major risk while you acclimatise.  Even after drinking multiple litres of water, I spent the first day in bed with a splitting headache that even painkillers couldn’t fix.  While some people fair better than others, it took almost a week before I felt like I wasn’t on the verge of a serious hangover.

Since arriving we’ve been hard at work on upgrading some major components of the SPT-3G camera.  Just this weekend we hit a large milestone in the upgrade and maintenance work, leaving me some spare time to write up a couple of blog posts!

 

Until next time,

– Matt