Bigger is better
Science goals guide construction of South Pole Telescope and its 10-meter dish
Posted January 21, 2007
When cosmologists — astronomers who study the origin and structure of the entire universe — must try to explain their work to laymen, they sometimes struggle to put the topic into words and images others can relate to.
For instance, why does the South Pole Telescope have to be so large?
John Carlstrom, a senior collaborator on the telescope with a 10-meter-wide dish, explained that the experiment researchers want to do requires “an arc-minute resolution.” At arm’s length, hold up a finger toward the sky and look at it. Imagine a square in the sky roughly the size of your fingernail.
Such a spot would be about one square degree. Mentally, divide that fingernail-sized square into 3,600 squares and one of those infinitesimally small areas is what the South Pole Telescope is designed to look at – that one and about 14.4 million others.
“The other thing [in addition to resolution] is that we have to observe where the microwave background is emitting, and that’s at a millimeter or two,” Carlstrom said, speaking of wavelengths of radiation. “So, diffraction, as light spreads, means that in order to get that resolution, you need about a 10-meter telescope. ... If you just start with your science goal, you end up with a telescope like this.”
If you build it ...For those who think on that level and effortlessly speak the language, deducing the necessary size of the telescope may have been the easy part. What followed were two and a half years of design work and another year and a half to fabricate the parts.
“This is a one-of-a-kind telescope,” said Carlstrom, of the University of Chicago. “You don’t just order it from the catalog.”
The required precision of the telescope, which is nearing completion at Amundsen-Scott South Pole Station and is scheduled to track the skies this austral winter, is intimidating enough, but then there are the challenges presented by its location.
The South Pole is perhaps the best spot on the planet for researching the cosmic microwave background, but there is a price to pay.
One challenge is dealing with extremely cold temperatures. While it is plenty cold during the summer construction (with highs of about negative 15 Celsius), winter temperatures can drop to below negative 70 Celsius. Such temperature fluctuations can lead to expansion and contraction of materials, thus throwing off precise settings.
Even when designers work around those problems, they encounter one more challenge. Every part of the huge structure must fit into an LC-130 aircraft for transport to the Pole.
“Most of these pieces,” Carlstrom said, “you would like, for stiffness and requirements like that, to have as one big weldment, set up in a huge shop, but you can’t do that here.”
So, where one might wish to have one solid piece of construction, there may be two or three joined together. Those joints require special attention or they “will flex like crazy.”
Concerns about flexing and expanding come back to precision viewing of those tiny, dark spots in the sky, and it all begins with the dish.
Get it togetherThe most visible feature is the telescope’s 10-meter-diameter primary reflector. Rising more than 22 meters above the snow, equivalent to a seven-story building, it demands attention in an area where everything else sits much closer to ground level. And attention is what it has received with thousands of adjustments made to an accuracy of a millionth of a meter.
“It looks symmetric,” Carlstrom said of the reflector dish. “It looks like this panel must be the same as that, et cetera, but it’s an offset parabola. No two panels are alike.”
The 218 aluminum panels are carefully machined on all six axes to fit together very tightly. They are etched chemically to achieve a dull finish.
“That’s so if the sun goes in the beam, we don’t get this incredible focus of the sun and burn a hole in things,” he said. But, he added that at the microwave range that it is built to pick up, it is “a beautiful mirror.”
Each panel has eight adjusters. Five are like pistons that move the panel up and down. The other three give it adjustments along the x and y axes and a rotation. They are set by hand to micron accuracy. There are 25,400 microns to an inch.
The scientists put the reflector together, operating underneath a huge white tent to shield them from the wind while they made fine adjustments. Placing the panels and making the initial 1,744 adjustments took the team four weeks.
Photo Credit: Jerry Marty
Two cranes place the SPT dish on the support structure of the South Pole Telescope while a third crane lifts scientists to bolt it into position.
“There’s a bit of a standing joke at the moment that cosmology here involves sledge hammers and saws,” said Steve Padin, SPT project manager, while reflector assembly was under way last month.
“It’s very serious construction at this point, but that’s part of the business, actually. I think that a lot of the students that have been involved in the project have been surprised just how hands-on astronomy can get.”
The primary mirror reflects concentrated light toward the 1-meter secondary mirror, which sits to the side of the parabolic reflector. The secondary mirror is part of the optics cryostat, which is cooled to 10 degrees Kelvin. (Zero degrees Kelvin is absolute zero, the lowest temperature theoretically obtainable.)
The secondary mirror directs the light to the receiver cryostat, where it will be absorbed by tiny detectors on the focal plane, or the camera. This package of instruments is kept at a temperature of 250 milliKelvin, a quarter of a degree above absolute zero.
The detectors fill an area that is basically a circle with a diameter of 180 millimeters, smaller than the average salad plate. There are 960 detectors in that area. Each one consists of a 4-millimeter-diameter mesh that absorbs the microwaves, and is equipped with a sensor only 30 microns across, small enough to hide underneath a human hair.
“So, we’ve gone from 10 meters down to a thousandth of an inch,” Carlstrom said.