Graduate students spend a hot summer in Texas preparing the SPIDER CMB Polarimeter for Antarctic campaign
10/15/2022 11:10:52 AM
Elle Shaw and Sho Gibbs are graduate students in Illinois Physics Professor Jeffrey Filippini’s experimental cosmology group. Filippini is the PI for SPIDER, which is a balloon-borne experiment observing the cosmic microwave background – the afterglow of the big bang. Shaw is in her 7th year and works primarily on SPIDER. Gibbs is in his 2nd year and works on SPIDER and its successor, Taurus. They spent the summer preparing to launch SPIDER in Antarctica this December. Shaw shared the following narrative of her experience.
In mid-April our team got a tentative go-ahead from NASA and the National Science Foundation (NSF) that SPIDER might have an Antarctic deployment opportunity if a lot of logistical requirements could be arranged, and if we could meet all the deadlines. We had to immediately start preparing for a summer compatibility campaign at the NASA Columbia Scientific Ballooning Facility (CSBF) in Palestine, TX, and be ready for a shipping date at the end of July so our equipment could reach McMurdo Station in Antarctica by the end of October. But first, we had to get SPIDER to Texas.
SPIDER is an experiment designed to measure the polarization of light at three different millimeter-wave frequencies, probing the cosmic microwave background (CMB) – a signal from the early universe – as well as polarized emission from our own galaxy. Six individual telescopes are housed in one large, shared cryostat that keeps the detectors at cryogenic temperature during operation. The cryostat, electronics, and pointing controls are all supported by a lightweight carbon fiber and aluminum gondola surrounded by sun shields and solar panels. The roughly 6000 lb. SPIDER payload will be lofted to the stratosphere by a NASA Long Duration Balloon, where it will make observations of a large patch of sky in the Southern Hemisphere from above the bulk of Earth’s atmosphere. This flight above Antarctica will last about two weeks while the circumpolar winds keep the payload over the continent, ideally until the cryostat runs out of liquid helium.
The first flight of SPIDER launched on New Year's Day in 2015 from the NASA Long Duration Ballooning Facility at McMurdo Station, Antarctica. It employed six telescopes to observe the CMB, with three telescopes tuned to 90 GHz and three to 150 GHz. SPIDER’s second flight, our current work, will include a third observation frequency of 280 GHz, a frequency difficult to observe from the ground. Data from the 90 and 150 GHz telescopes will provide a complementary data set to that from SPIDER’s first flight, and data from the 280 GHz telescopes will provide important information on polarized Galactic dust emission that contaminates CMB measurements.
The majority of my doctoral research involves building and characterizing the new 280 GHz receivers using a test cryostat in our lab that cools down one telescope at a time. SPIDER’s second flight was delayed for four years because of Antarctic logistical constraints and the COVID-19 pandemic. As of early April 2022, we were still unsure if we would have a final chance to deploy SPIDER for this upcoming Antarctic summer season. I began to hedge my bets (and thesis) around not getting to launch the telescopes I had built and tirelessly tested, and I started to tell my family and friends that I wouldn’t get to go to Antarctica during my PhD as I had been hoping for the past 5 years. As you might imagine, this was disappointing and disheartening.
After our team received word from NASA and the NSF in mid-April, we had to move quickly to get SPIDER to Texas to start the compatibility campaign on time.
During a compatibility campaign, a ballooning experiment must pass the required NASA safety benchmark tests, ensure all systems are compatible with the NASA electronics and communication systems, and prove that all flight systems are operating correctly. For some smaller ballooning experiments, this testing can be done in a week or two, but for an experiment as large and complicated as SPIDER it is a two-month affair. The compatibility campaign is the last opportunity to take data and test equipment before being on the ice in November and serves as a final dress rehearsal of what will happen during deployment. Running the experiment in a new place also forces us to check our inventory and make sure we buy and pack everything we could possibly need. There is no 2-day shipping in Antarctica!
Over the next five weeks, we ran one last characterization test with the cryostat at UIUC to inform design changes for the flight. Team members at Washington University in St. Louis fabricated new sapphire filters based on the tests we did at UIUC, as well as spare plastic vacuum windows, all essential components in the optical chain of the telescopes. The team in Princeton had to disassemble and then pack up the flight cryostat, gondola, electronics, and all other lab equipment into shipping containers and send them off to Texas to arrive by May 30th.
Meanwhile, at the end of May, I flew to Ferrara, Italy on behalf of the SPIDER team and gave a talk on results from SPIDER's first flight. I got to announce our upcoming flight opportunity to a talented group of cosmologists at the From Planck to the Future of CMB conference. Much better than packing duty!
When I returned from Italy, I repacked my bags and flew to Texas at the beginning of June, followed about a week later by Sho. Teammates who had arrived a few days earlier had already begun unpacking and transforming the empty high bay into a functional lab space.
Although we had been keeping the test cryostat we have at UIUC busy with characterization tests, SPIDER's core hardware team, which is spread across several universities, hadn't worked together to run the flight system in almost two years. The summer campaign was the first time our team all worked together, as some graduate students graduated, and a lot of younger team members joined in the last few years. The entire campaign is a true team effort, the many sub-systems of our experiment require expert knowledge from many different people. At the beginning of the campaign this subdivision of labor is critical to getting everything done in time. There are sub-teams working on the telescopes and cryostat, gondola building, and electronics testing. With a limited number of folks that get to deploy, everyone has to collaborate and help where they are needed.
When I arrived, I began inspecting the telescope receivers post-shipping and preparing them for installation into the flight cryostat. After each telescope passed inspections, it got loaded into the cryostat and attached to all the readout, thermometry, and heat sinking cables. After the telescopes were all loaded, we began closing up the cryostat, installing the delicate optical filters, half-wave plate optics, and layers of insulation at intermediate temperature stages along the way. Finally, we installed the outer vacuum shells and windows before we could begin to pump the cryostat down to vacuum pressure and begin cryogen fills to start the cool down from room temperature to 4 Kelvin (-452 F).
During this time, Sho put together the carbon fiber and aluminum gondola, solar panel array, and sun shield frame with another team in an adjacent high bay. The gondola, which supports the weight of the entire payload, is protected by a 25-foot tall sunshield frame. Once it was assembled, the team began installing the flight electronics to the gondola "floor" and routed all the cabling and wires. Keeping the cables and wires organized, accessible, and appropriately constrained without pinching or stressing the cables is a logistical feat in itself!
Meanwhile another group was testing all the electronics in a vacuum chamber and making sure all components will operate at extreme temperatures in a vacuum, simulating the harsh environment our electronics are exposed to at flight altitude above the Earth's atmosphere.
By the beginning of July, we had integrated the gondola and cryostat, the cryostat was down to liquid helium temperatures, and the detectors were down to their base operating temperature of 300mK. We could finally start taking data.
Because of a tight shipping schedule, we only had 6 days with cold detectors before we needed to start warming the cryostat back up. During this time, I took a variety of lab measurements on the detectors and got crucial experience working with the telescopes installed in the flight cryostat. I have a lot of practice "talking" to the detectors directly with the test cryostat setup at UIUC, but the flight system uses more layers of automation and software. So this summer I got to work with the flight code and operating system for the first time.
After we took our last data set, the large sunshield frame and solar array were installed onto the gondola, the NASA engineers installed the Support Information Package communication hub, and we prepared for the compatibility checks. We wheeled the payload in and out of the high bay, testing the solar panels, SPIDER's star cameras, antennas, and the pointing and scanning operations that are crucial for scanning over our intended patch of sky during flight.
Once we passed all the requirements, we shifted packing mode into high gear and began to take apart the whole payload in reverse order. We spent the last week and a half inventorying and arranging our entire lab and experiment into three shipping containers, packing everything carefully to make the long journey south to Antarctica. We managed to meet our shipping deadline on July 23rd and officially ended the summer campaign.
SPIDER is scheduled to launch this austral summer from McMurdo Station, Antarctica. I head to Christchurch, New Zealand in mid-October with the first wave of SPIDER team members, and then we will continue south a few days later after we have been issued our Extreme Cold Weather gear from the U.S. Antarctic Program. Sho and Jeff will make their journeys south in the following weeks, along with the rest of our eighteen team members.
The Antarctic campaign will proceed similarly to the compatibility campaign in Texas, except it will be much colder and the nightshift will also be working in broad daylight! Our goal will be to have the entire payload flight ready by mid-December. Then we will wait for a perfectly clear day for a launch opportunity before mid-January, when the window of good weather starts to close.
I am so excited to finally deploy our experiment, and I am equally anxious about the hundreds of things we need to get right to pull it off. Many challenges this summer could have derailed us, but we came up with creative solutions, and against all odds, we managed meet our deadlines. So, I am trying not to lose too much sleep over the things outside our control. I’m letting myself be excited about this adventure of a lifetime, and I am confident our team will do everything in our power to make this campaign a success!
You can follow along on our journey on our Twitter account @SPIDER_CMB
Sho Gibbs contributed to this article.