A day in the life of SPT (by Wendy Everett, with a little help from Tom Crawford)

Since the telescope camera is remaining the same this year, most of the summer work has been spent on annual maintenance tasks such as changing oil and greasing gears in anticipation of the oncoming winter. But we're also taking advantage of the relatively light work year to take some data, observing a patch of sky we've never yet looked at to find the most massive galaxy clusters in that part of the sky. What does a typical day look like for SPT when it's taking data?

Rather than 24 hours, a day for SPT lasts about 36 hours, which is the length of time our refrigerators can keep the detectors cold. Because SPT is measuring very faint signals (one part in 100,000 or even fainter!), the SPT detectors need to have very little noise due to thermal energy, and we therefore keep them at about 1/4 of a degree above absolute zero Kelvin while we're taking data. Keeping anything that cold involves a complicated layering of refrigerators. In the outer layers, we use pulse tube cryocoolers, which compress and expand gas at different temperatures to remove heat. The inner-most layers involve a recyclable refrigeration system called a sorption fridge, which uses the evaporation of an isotope of Helium, Helium 3. Helium 3 is condensed into a liquid and then a chunk of charcoal acts as a pump to suck up the gas that evaporates. Just like how evaporative cooling helps one's body cool down after exercising, by sucking up the evaporating gas above the liquid Helium 3, the charcoal helps the liquid cools down, and the liquid can be used to cool our detectors.

Once the charcoal pill has absorbed all of the liquid Helium 3, it can no longer suck up any more heat and everything will start warming up. This means we need to recycle the fridge. To do this, we heat the charcoal to expel and liquify the Helium 3, then cool back down and start the pumping process over again. This cycling limits how long SPT can observe for in a single "day."
After we've cooled the fridge, we prepare SPT for new observations. First, we apply voltage to the detectors and tune the amplifiers. Our detectors are essentially small thermometers which measure the amount of heating that results as CMB photons are absorbed. To make our detectors extremely sensitive, we make them out of a material that goes superconducting at low temperatures and then we apply a voltage to keep each detector in the region of its transition between superconducting and normal. This means that very small changes in temperature cause very large changes in the electrical resistance. Next we tune our readout amplifiers so they are operating in their ideal state. These amplifiers consist of SQUIDs: Superconducting Quantum Interference Devices, which include a current loop that is very sensitive to changes in magnetic flux. We then couple the SQUID readout to our detectors using an inductor. The readout is actually a little more complicated: in order to reduce the number of wires that need to go to each detector (since each wire will also allow some heat to travel to our cold detectors and warm them up), we set each detector to "ring" at a different tone so that we can read out multiple detectors with a single SQUID. This process is known as frequency-domain multiplexing.

Now, SPT is set up to start taking data. While we want to prioritize using SPT's day for observing our science fields on the sky, we also need to do a number of calibration measurements to make sure we can most effectively use our data once we've taken it. Therefore, once our detectors are all ready, SPT continues its morning with a noise stare, where the telescope observes a small blank patch of sky for 10 minutes to get a sense for the amount of instrument noise to expect on that day. Next, we perform calibration measurements at a variety of different elevations in the sky. In order to calibrate the relative power each detector observes, we have an external calibration source, which is a glowing filament sitting behind the secondary mirror, which we can shutter off from the optical chain when it's not in use. The brightness of the calibrator source is anchored to an astrophysical source in the sky (of known brightness) once each day, and then we check in with the detectors using just the calibrator several times in the mean time to make sure nothing strange is happening with any of the detector's response. We measure this response to the calibrator at several different elevations, because looking through different amounts of atmosphere means a slightly different heat load on the detectors, which could mean a slightly different responsivity for the detectors (in reality this effect is tiny, and we can mostly ignore it).

The next step in SPT's day is to make a quick observation of one of the astrophysical calibration sources I mentioned above. These include two galactic HII regions, which are large clouds of ionized Hydrogen within our galaxy. They are mainly useful because they have well-studied properties and don't change day-to-day, so we can use them to calibrate how much power each detector sees. We also use these observations to calibrate the pointing of the telescope. We want to know very precisely where the telescope is looking in the sky, but this can be effected by things like gravitational sag of different parts of the telescope when looking at different elevations or temperature gradients causing differential thermal expansion or contraction, so we combine observations of astrophysical sources with measurements from sensors placed all over the physical body of the telescope to help us create a model of where SPT is looking at any time and orientation.

Now SPT is ready to take measurements of the sky! Right now, we're observing four different sky fields at declination -35 degrees. These fields are two hours wide in right ascension (i.e., twelve of them end-to-end would make a horizontal circle around the sky) and ten degrees high (so nine of them stacked on top of one another would reach from the horizon to the zenith). The telescope scans over these fields by starting at the bottom left of the field, scanning across horizontally at about one half of a degree in azimuth per second (so that crossing the field takes about 1 minute---because two hours of right ascension is 30 degrees in azimuth), scanning back across in the other direction, stepping up a small amount in elevation, and repeating until the whole field is covered. This takes about two and a half hours, and we do 10 or 11 of these observations in one "day".

The SPT computers at the South Pole are set up to automatically process these observations as they come in. The end product of this processing is a map of the field. After only a few observations, we can already see bright emissive sources (usually active galactic nuclei emitting synchrotron radiation) and, if we're lucky, some massive, as-yet-unknown galaxy clusters. Stay tuned!