A Playful Search For Beauty: Working on polcal

South Pole Telescope

One of the main objectives of my trip to pole this year is to help with "polcal" measurements. In this post, I will describe these measurements.

Last season, 2011-12, we completely replaced the detectors on the telescope with a new focal plane. This new focal plane is sensitive to the polarization of the light that we observe. Light is an electromagnetic wave, and the polarization of a wave of light is the direction that the electromagnetic wave is oscillating in. Our polarization-sensitive detectors are organized into pixels, where each pixel has two detectors that are sensitive to orthogonal polarizations. In simple terms, the two detectors in a pixel effectively form a cross.

With this "SPTpol" camera (short for South Pole Telescope polarization), we are studying the polarization patterns of the Cosmic Microwave Background (CMB). This experiment requires us to know the precise orientation of each detector, and therefore what polarization each detector is sensitive to. This is where "polcal" comes in.

"Polcal" is our short-hand phrase for "polarization calibration." The goal is to measure the orientation of polarized light that each detector is sensitive to, to an accuracy of less than a degree. We accomplish this measurement by installing a source that produces light of a known polarization, pointing the telescope at the source, and measuring the response of each of the detectors. The source is set up as follows; a "black-body" source provides the light. As a reminder, the SPT sees light that has a wavelength of approximately a millimeter, which is a much longer wavelength than our eye sees. The black body source is heated up to 750 Kelvin, and radiates light. This light is focused by a plastic lens, then goes through a polarization grid. The grid allows light that is polarized in one direction to pass through, and absorbs all other light with a different polarization angle. This contraption produces a beam of polarized light that we can observe with the telescope.

The final piece is that the polarization grid described above mounted on a rotation stage, and the angle is rotated in steps of 15 degrees. This way we can fully map out the detector response as a function of the polarization angle of the incident light.

Ben working on the polcal source box

Inside the box. From bottom to top is the sense, polarization grind #1, then the rotation stage with the bar-code and grid #2.

Still not working...

We place this source on a stand ~3 km away from the telescope. The telescope is set up to focus on light from the CMB, which is effectively at a distance of infinity. We can adjust the focal length of the telescope some, but 3 km is about as close as we can get. We put the source box on a stand that is surrounded by a reflective pannel, which reflects the sky. The ground radiates too much light (at the frequencies that the telescope can see), and the telescope detectors are optimized to observe the sky; thus the reflector is necessary to keep the detectors working properly.

The reflector

My job has been to install or take down the box at the 3km reflector, make sure everything is working properly, and take measurements.

Driving out to the 3km source. Can you see the reflector?

Looking back at the SPT and the station from the 3km source.

Raising the source box up to the platform on the reflector with Ben and Clarence.

Installing the source box on the reflector platform.

The source box weighs about 150 lbs and it unwieldily. We install in on the platform on the back of the reflector using a chain hoist. This is not so easy when it is -25 deg F outside! It usually takes four people to install the source.

It is pretty cool to be out at the source. The source is 4 km from the station, which is far enough to really feel the isolation of this amazing place.