Polarization Calibration at the South Pole

Caption: Tijmen, Amy, and Ryan in front of the SPT calibration source located about 3 km away from SPT. The reflecting panel is about 20 feet tall, covered with aluminum siding that acts as a mirror for microwaves. It is tilted at 30 degrees so that reflections go to 60 degrees elevation, and the squarish hole in the middle contains a chopped, polarized source for us to measure our detector polarization angles.

The last few weeks we have been finishing up calibrating the polarization angles of the SPT detectors. This requires the SPT to re-focus on a source 3 kilometers away from the telescope, and detecting the signal from a ~1 inch square source. The source is a hot filament, behind a plastic lens that directs the light to the telescope. In front of the lens is a rotating polarizing grid, that is modulated at 7.5 Hz, which we lock into while keeping track of the position of the polarizing grid. Using these measurements, we can measure the exact polarization angle of our detectors.

There are a few challenges doing this measurement. First, we can't focus on anything closer than 3 kilometers away, so we had to build the source that far away from the telescope, or about 4 kilometers away from the South Pole Station. Its way out there, and requires about a 15 minute snow-mobile ride just to get there. We have a little warming hut out there to keep us warm, but it definitely puts in perspective the need to stay warm and be careful.

Second, our detectors are designed to look high up in the sky and are saturated when looking at the horizon. To get around this, we built a ~20 foot tall reflector, that reflects the majority of our beam to the sky. In the center of the reflector is our calibration source, which as I said above, is behind a rotating polarizing grid. We have to measure each one of our detectors one at a time, so to measure the polarization angle of every detector on the focal plane of the camera takes about 40 hours of working, uninterrupted measurements.

Third, even with this setup, some of the detectors barely operate correctly. We had to take the measurements in a variety of configurations, and it took a lot of care to get measurements for even ~90% of the focal plane. Anyway, we think we figured out how to do it right, which is important because we will need to repeat these measurements in late December, early January, after we re-configure the SPTpol focal plane.

Last week, we finally took the SPTpol camera and secondary mirror out of the telescope cabin, and opened them up. We will post more pictures and comments on that progress soon, but so far so good!