Date: October 3, 2011
Title: ALMA Opens Her Eyes
Podcaster: Tania Burchell
Organization: National Radio Astronomy Observatory (NRAO)
Description: The world’s most complex, ground-based astronomy observatory is under construction within a fortress of extinct volcanoes high in the remote Chilean Altiplano. Although she is not yet complete, the Atacama Large Millimeter/submillimeter Array (ALMA) has already begun observing the invisible Universe. What is she seeing? What makes her so complex? And who is building her?
Bio: Tania Burchell is the Public Information Officer for ALMA in North America, working out of the National Radio Astronomy Observatory (NRAO) in Virginia. Her astronomy career spanned the wavelengths and her communications career spanned the Atlantic, landing her on stage, radio and television in the US and UK. She tweets as RadioAstroGal and blogs when she can at radioastrogal.wordpress.com.
Sponsors: This episode of “365 Days of Astronomy” is sponsored by Steve Nerlich from Cheap Astronomy: …helping you conserve more than just angular momentum.
This episode of “365 Days Of Astronomy” has also been brought to you by lifeboat.com.
Hi, my name is Tania Burchell, and I am the ALMA Public Information Officer at the National Radio Astronomy Observatory. I’d like to welcome you to another edition of the 365 Days of Astronomy Podcast. For this podcast, I will take you to a new telescope that just opened to astronomers this weekend…
We go to the desert of northern Chile, home of the world’s most complex ground-based observatory, the Atacama Large Millimeter/submillimeter Array.
We call her by her acronym, ALMA, because, well we’re space scientists, and we just love acronyms. But as a word in Spanish, ALMA means “soul.” It is a name that greatly becomes her, for ALMA’s work is to peer into the deepest, most mysterious, and hidden cores of our Universe to reveal its history and our future.
From the formation of the first galaxies, stars, and planets to the merging of the first complex molecules, the science of ALMA is a vast spectrum of investigation. And this weekend, the as yet unfinished ALMA officially started her journey to peer into the soul of the Universe by beginning her first phase of observing, called Early Science.
Thousands of astronomers from around the world competed for Early Science project time on the new array, and 112 teams got their wish. A testament to the excitement of ALMA coming online is that the ratio of time requested versus available observing time was larger than with any other telescope you could name. Clearly, folks are eager to use this new tool – and she’s still under construction!
So, what makes ALMA so special? Let’s look at her name again.
The first “A” in ALMA stands for “Atacama,” the region in northern Chile that hosts this revolutionary new telescope. The Atacama boasts one of the driest landscapes on Earth, and elders have told us that some villages here have never experienced any rainfall in all of their recorded history. Little rain means few clouds, means clear skies for astronomy.
The Chajnantor Plain where ALMA sits is 16,500 feet above sea level. This puts ALMA higher than any other telescope array on Earth. At this elevation, the temperatures hover around freezing all year round, and the air pressure is half that at sea level. Cold with little air is superb for telescopes like ALMA. Why? Because of the “M” in her name…I’ll get to that in a minute.
The Chajnantor Plain is an extreme location for humans as well as our state-of-the-art telescopes, and ALMA workers are either acclimatized to this altitude or else they wear oxygen tanks while they build ALMA’s infrastructure. More day-to-day activities happen at our Operations Support Facility, a high-tech work encampment at a more comfortable 9,500 feet. Telescopes are assembled and tested here, and after they join the array at the high site, they are controlled from here as well. Some ALMA staff live here, too.
The “L” in ALMA stands for “Large,” because when she is completed in 2013, sixty-six ALMA telescopes will be remotely linked across a nearly 100 square mile section of the Chajnantor Plain, making her the largest telescope of her kind in the world.
The telescopes themselves are marvels of modern engineering. Fifty-four of ALMA’s eventual 66 telescopes are nearly 40 feet across, called the “12-meters.” The other twelve are around 23 feet across and are called the “7-meters.” The smaller ones will crowd into a tight group. Why? Hold that thought, because I’ll come back to it when I get to the last “A.”
The Sun never sets on the ALMA project – it has a global reach unlike any other ground-based scientific endeavor. Of ALMA’s 66 superconducting telescopes, 25 are being delivered by the National Radio Astronomy Observatory, 25 are being delivered by the European Southern Observatory (ESO), and 16 are being delivered by the National Astronomical Observatory of Japan (NAOJ). And other critical components of ALMA arrive daily from all over the world.
The letter “M” in her name stands for millimeter and submillimeter — the wavelengths she gathers from the Cosmos. In the spectrum of radiation we can detect, mm/submm waves fall between infrared radiation and microwave radiation. They fall into the observing band of the radio astronomers.
Historically, collecting, focusing, and imaging mm/submm waves has been very tricky. These waves are so large that mirrors cannot focus them, and their frequencies are too high for off-the-shelf receiver technologies to process. The warmth of a telescope’s own electronics is enough to ruin the weak cosmic mm signals that, by the time they reach us, sputter in at about a billionth of a billionth the power of a cell phone call. And as an added torment, humidity itself broadcasts at these frequencies, turning most skies into a glare of mm/submm light.
Why did radio astronomers even try to overcome these seemingly impossible challenges? Because we are human. At mankind’s inquiring best, we have a fundamental need to look over the next horizon. The mm/submm Universe beckoned as a final piece of the puzzle of understanding the Cosmos.
Back in 1961, Dr. Frank Drake and others at the National Radio Astronomy Observatory ran the world’s first millimeter telescope, a five-footer in Green Bank, WV. The NRAO later erected a large, 36-foot mm-telescope on Kitt Peak, in Tucson, Arizona that discovered numerous molecules out in space, igniting the field of astrochemistry. We ran it until the year 2000 when advancements in technology finally meant that our dream for an array of mm-wave telescopes was to be a reality.
Working with engineers at the Microfabrication Lab at the University of Virginia, we created miniature mixing circuitry that could fit inside a submillimeter telescope’s cryogenic receivers. This new technology finally allowed high frequency signals to pass into a computer at a reasonable rate. With a supercomputer, then, we could process many more of these signals simultaneously, such as from an array of submillimeter-gathering telescopes.
Which leads me to the final “A” in ALMA that stands for “Array.” The ALMA “telescope” is synthesized by computer from the views of her many separate dishes. This technique of “aperture synthesis” was developed by Sir Martin Ryle at Cambridge University in the 1950s. He won the Nobel Prize for this work, because synthesized arrays made it possible, finally, for radio astronomers to see the Universe in the same amount of detail as our optical cousins.
The technique is rather elegant: when two waves are added together, their signal peaks and troughs add or subtract, depending on their location along the wave. The result is an interference pattern whose peaks and troughs are narrower than the original single waves. The narrowness of the peaks equates to finer resolution of the view of the pair.
Telescope arrays like ALMA gather interference waves from every pair of telescopes; and more is better in interferometry — ALMA will have up to 2145 pairings when she is complete. And as the Earth turns, the view changes slightly for the pair, and thus the object they are observing is seen in a different perspective. Each pairing and its changing perspectives are combined to complete a picture of the object being observed. It’s a lot more complicated than that, but suffice it to say: Many pairs make a better picture…interferometry is a win!
To combine those interference pairs into interferometry data for a single ALMA observation, we needed one of the world’s fastest special-purpose supercomputers. Enter the ALMA Correlator, designed and built by the NRAO Technology Center here in Charlottesville, Virginia. It performs 17 quadrillion operations a second to pair up the signals gathered by ALMA’s individual telescopes.
The data pumped out by the Correlator not only include the strength and location of the signal, they also include radio frequency information, which, depending on the application, can be used to relay information on dynamics, chemistry, or distance to galaxies far, far away.
The arrangement of the telescopes in an array is critical to the quality of the synthesized image. A widely spread array, like strong, binocular vision, sees very crisp details, and astronomers complete a picture of a large chunk of the sky by hopping across it in a grid pattern of smaller, high resolution pieces. A closely packed array reduces the detail but increases the field of view and the sensitivity to fainter material, so astronomers can observe a larger, dimmer chunk of the sky with every telescope pointing.
Like her flexible centimeter-wave cousin, the Very Large Array, ALMA has several different array configurations to take advantage of these different styles of observing. Nearly two hundred powered, fiber-linked concrete pads will dot the Chajnantor Plain, ready to receive a telescope. Two 28×28-wheeled monster trucks, called the ALMA Transporters, handle the rearrangements. Under remote control by a specially-trained driver, these gentle giants can ramp a 115-ton ALMA telescope up on to their backs and keep it powered and safe during its desert drive.
Lastly, ALMA is a two-fer-one deal: she is a full-time changeable array of 50, 12-meter ‘scopes plus a full-time, compact array (called the ACA) made up of the 12, 7-meter telescopes with four 12-meter corner sentinels. In fact, the ACA has its own Correlator, designed and built by NAOJ, which allows ALMA to work simultaneously as separate, powerful arrays when needed.
There are so many more marvels to share about ALMA, but I’ve run out of time in my first podcast! I’ll end by saying ALMA is the largest leap in telescope technology since Galileo first aimed a lens on the Universe, and we are looking forward to what she sees now that she has opened her eyes.
Thanks for listening.
End of podcast:
365 Days of Astronomy
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my brother in-law helped design and build the electronics for alma