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SPT Construction
Photo Credit: Steve Martaindale
With the stark South Pole horizon as a backdrop, workers piece together the 10-meter telescope’s
supporting structure. The structure grows out of the control room, where scientists can run the operation in relative comfort.

Telescope recorded first light in February

Getting warm

These sensors will not work like a digital camera, recording visual images. It is actually a bolometer, which is used to measure radiant heat, and was built by researchers at the University of California, Berkeley.

“With carefully chosen filters in front of the bolometers,” Carlstrom said, “you’re looking at a little window of frequency that you want to observe and you’re actually letting the radiation heat up these little elements and measuring their temperatures.”

The measurements are taken as the telescope slowly glides across the sky, quickly looking for miniature spots in the sky that emit radiation which is maybe 10 microKelvin warmer – 10 millionths of a Celsius degree.

“Our sensitivity is such that, in one second, integrating on the spot for one second, our precision, in a relative sense, is a couple of hundred microKelvin,” Carlstrom said. “If you make that measurement over and over, you get to average them. That’s how you get a higher precision. With this experiment, we hope to get below 10 microKelvin for every arc minute spot – know what the temperature is to a few microKelvin over 4,000 square degrees.”

In 2010, he said they plan to begin polarization experiments in which they hope to get to a precision of about 10 nanoKelvin — 10 billionths of a degree. They will be surveying a smaller portion of the sky but will have to spend much more time focusing on that area.

Supporting role

While scientists were piecing together the primary reflector one panel at a time, ironworkers and heavy cranes were putting together the support structure responsible for bringing everything together in this finely tuned dance of accuracy.

“If you just made this whole structure and the backup structure out of steel, “ Carlstrom said, “it would actually, from its own gravitational distortions, not meet the specs.”

A couple of special materials were used to assure the precision requirements needed in such a cold environment. One is a carbon fiber reinforced plastic that is very light yet stiff enough to tilt at different angles without deforming. That was used as the backup structure to hold the primary reflector.

The other key material, Invar, was required to act as a buffer between the main steel structure, which will be affected by temperature changes, and the carbon fiber backup structure, which could be damaged or distorted by those expansions.

“So we have this adapter,” Carlstrom said. “It’s made out of Invar. Invar is a very high-nickel content steel which you can match the coefficient of thermal expansion to that of the carbon fiber. And then you design it so that it takes all of the stress and all the deformation from the main steel structure without transmitting it to [the backup structure and reflector].”

The entire support structure sprouts out of a control room that is linked to the Dark Sector Laboratory. The telescope was designed to maximize the number of moving parts that could be accessed from inside a warm environment, especially anything that would more likely need to be worked on during the winter months of operation.

In fact, at the end of the arm extending from the structure which supports the primary reflector is a receiver cabin that can be lowered onto and accessed through a hatch in the roof of the control room.

Coming together

On Jan. 3, two cranes coordinated to lift the backup structure and primary reflector assembly onto the structure while another crane positioned workers to bolt everything together, giving the South Pole Telescope the appearance of completion.

The plan is to start receiving light through the telescope in early February before the winter season begins and to put the instruments to work as the six-month night sets in.

“To get from nothing to a working instrument at this scale size and doing science experiments in a short time is pretty hard,” said Padin, who will winter over to run the operation. “So we’ll do the bulk of that work during the summer season, when we can bring lots of people in. But, for sure, it’s going to drag on into the winter.”

“The goal is to actually get some real science out,” he said, “so we want to make a start on a galaxy cluster survey.” 

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