Date: July 29, 2011

Title: Encore: Location, Location, Location!

Podcaster: Sue Ann Heatherly

Organization: NRAO – National Radio Astronomy Observatory

Link: http://www.nrao.edu/

Description: Why are telescopes located in some of the most remote locations on Earth? Join Dr. Ron Maddalena to find out why optical telescopes are on tops of mountains or even better, in space, and why radio telescopes are down in the valleys.

Bio: Sue Ann Heatherly is the Education Officer at the NRAO Green Bank WV site. She comes to astronomy by way of biology (BA in 1981), and science education (MA in 1985) She visited the Observatory as a teacher in 1987 and knew she’d found Camelot. She has been employed with the NRAO since 1989.

Sponsor: This episode of “365 Days of Astronomy” is sponsored by — NO ONE. We still need sponsors for many days in 2011, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.

Transcript:

This podcast orignally aired on April 23, 2010
http://365daysofastronomy.org/2010/04/23/april-23rd-location-location-location/

SUE ANN HEATHERLY: Welcome to this edition of 365 Days of Astronomy podcasts. My name is Sue Ann Heatherly. I work at the National Radio Astronomy Observatory in Green Bank, West Virginia, and I’ll be your host for this addition.

Today we are joined by Dr. Ron Maddalena, who is an astronomer here at the observatory in Green Bank. And, um, today’s topic is going to be, we’re going to call it location, location, location. We’re going to talk about some of the issues, uh, that you have to worry about when you build radio telescopes, when you build optical telescopes, and why these telescopes are so hard to find: why they’re in the middle of nowhere.

So, Ron, welcome to the program.

RON MADDALENA: Thank you very much.

SUE ANN HEATHERLY: So, first of all, let’s talk a little bit about optical telescopes. Everybody is familiar with these beautiful white domes sitting on top of Kitt Peak, sitting on top of Mauna Kea in Hawaii, and other places like that. Why are optical telescopes on the tops of mountains?

RON MADDALENA: There’s a couple of reasons. For an optical telescope, what you want to do is, you want to get away from light pollution, so you need to be in a place where you are far away from any major city or town. Next thing you want to do is get yourself above most of the weather. And the way you do that is. . .well, since clouds are at low altitudes usually, you can get yourself up by, uh, if you go up to a mile or so high in altitude, then a lot of the weather is below you. And so, the chances for clouds are less when you’re on the top of a mountain. The atmosphere adds. . .uh, is turbulent. So when you look through a telescope – well, you see this at night with your own eyes sometimes – you can actually see stars twinkle. And twinkling stars are bad for optical astronomers. It means that when you look through a, an optical telescope at a twinkling star, it just moves around in the view of your telescope. So what you want to do is, you want to get above the atmosphere to a place where, uh, there is less atmosphere, so less possibility of seeing twinkling stars.

SUE ANN HEATHERLY: Do they call that– You’ve used the term around me before, and I’ve never really known what it meant. I’ve just sort of pretended like I’ve known what it meant. What does “good seeing” mean?

RON MADDALENA: “Good seeing” means, essentially, that the stars are not twinkling; that when you look at an object in the sky, that you, that it’s, that it doesn’t move when you look at it through an optical telescope. When you see a, a star twinkling in the night sky, what you actually are seeing are slight changes in the, uh, direction of the star due to refraction from the Earth’s atmosphere. So “good seeing” are places where the atmosphere is completely stable, and so the path of the light through the Earth’s atmosphere doesn’t bend and twist as it hits pockets of air of different densities or, uh. . .or humidity. So, if the atmosphere is nice and stable, nice and uniform, then you don’t get twinkling, and these are great places to do astron. . .or optical astronomy from.

SUE ANN HEATHERLY: So, the Hubble Space Telescope, why did they put it in space? Was it because of the “seeing”?

RON MADDALENA: Yes. Yes. It’s a small telescope in comparison to what you could build on the ground. But by putting it above the Earth’s atmosphere, you’re able to remove all the twinkling that you would normally see from a ground-based telescope. So. it gets some of the nicest, cleanest views of objects, even though it’s a small telescope. So the next generation, of course is– The Space Telescope is going to be a larger version, um, and it will have all the capabilities of Hubble and then some.

SUE ANN HEATHERLY: So, there’s a telescope called the Keck Telescope that’s up on top of a mountain in Hawaii, right?

RON MADDALENA: That’s right, yes.

SUE ANN HEATHERLY: And, uh, this telescope is still looking through atmosphere, but they have techniques to compensate for twinkling?

RON MADDALENA: Yes. They have– There’s a technique that’s been developed over the last ten years called adaptive optics where, uh, they can monitor what the atmosphere is doing and then remove it, uh, by basically tilting and twisting mirrors that are on the, uh. . .on the telescope itself. So if there is a, uh, a deformation in the, in the, uh, wave coming down from the, through the Earth’s atmosphere, they are able to, um, to, to correct for it, and they do it in a number of ways, one of which is by looking at a nearby star, a bright star, and basically have a little servo system that makes sure that that bright star stays in the same place on your, on your CCD camera, or they shine up a laser into the Earth’s atmosphere. At very high altitudes the, uh, this laser, uh, makes the atmosphere fluoresce, and so you see a perfect star, and then you basically servo these mirrors against that perfect man-made star in the Earth’s atmosphere. And, uh, by, just by twisting, very quickly twisting and tilting mirrors in the optics of the telescope, you are able to remove a lot of the effects of the Earth’s atmosphere. Hubble still has the advantage, though, because it’s still. . .this. . .these techniques take you a large way towards getting better images, but they, they’re still not as good as being above the atmosphere altogether.

SUE ANN HEATHERLY: Now, some telescopes, other than optical telescopes, are also in space. We have x-ray telescopes, we have gamma ray telescopes, we even have, I guess, infrared telescopes.

RON MADDALENA: Right.

SUE ANN HEATHERLY: And why are they in space?

RON MADDALENA: Well, it’s, again, because of the Earth’s atmosphere. Much of the radiation that comes to us from space reaches us, uh, but doesn’t penetrate the Earth’s atmosphere. The Earth’s atmosphere is opaque to ultra violet light, x-rays, gamma rays, and infrared lights. There are only two places in the electromagnetic spectrum where light from, uh, from outside the Earth’s atmosphere, can actually penetrate and, and go down to the, uh, to the surface of the Earth. And that is at optical wavelengths, where people see, and at radio wavelengths. So, most of these other wavelengths – and there is more wavelengths that we can’t see from the Earth’s surface than we can – can only be observed from, uh, telescopes that, uh, that are in orbit around the Earth.

SUE ANN HEAHTERLY: All right. So, folks, we’re getting to the radio astronomy segment of the show now because, of course, we live on the surface of the Earth here in Green Bank, West Virginia, and we have a very large radio telescope here on the surface. What kind of, of effects do we have to worry about with radio telescopes? Why is the Green Bank Telescope located in a valley surrounded by mountains instead of being on top of a mountain, for example?

RON MADDALENA: Okay. For radio telescopes, a lot of the requirements are the same as to where you would, uh, locate a telescope. Uh, you want to be away from civilization, not much because of light pollution, but because of interference from devices that people own; like, microwave ovens, and, uh, cell phones, and Wi-Fi devices: all of these things wreak havoc for radio telescopes, just like having a huge spotlight would have a, a problem for an optical telescope. So, we want to be remote. Uh, so again, the criteria is pretty much the same: we want to be away from civilization.

For radio telescopes, some of them you’ll see on top of mountain tops, some you’ll see in valleys –like the ones in Green Bank – and even you’ll find some in not so rural locations. In this case, it all depends upon the wavelength at which the telescope is observing. At long wavelengths, like we observe from Green Bank, uh, you want to be away from man-made radio interference; that mostly happens at low frequencies, so that’s why we’re in a very rural location. We’re in a mountain valley because the mountains that surround our observatory, we want to use those as natural shields from the towns that live just on the other sides of those hills. So, for us, low frequency means you put ourselves in a rural location inside of a valley. If you’re at high frequencies, then you don’t have such a problem with interference from man-made devices, so you can actually have observatories in cities. My PhD was done with a telescope in the middle of New York City, but it was at such a low frequency that we saw no interference from New York City.

SUE ANN HEATHERLY: And that was also in the Stone Ages, right, before [Laughs]. . .

RON MADDALENA: [Laughs] Well, it’s not. Yeah, long before, uh, many of these Wi-Fi devices, yes. So, it might be different nowadays. But there are still some radio telescopes that do productive work that are very close to, uh, major cities. But at high frequencies, you have to worry about the weather. Optical telescopes you can’t observe through clouds. You can’t observe through rain. The radio telescopes at low frequencies can observe through the, through both rain and clouds, but at high frequencies they have the same problem as optical observations because, uh, the clouds attenuate the signal as it goes through the– So, when we’re observing at high frequencies, we want to place our telescope in a dry location where there is. . .tends to be less cloud cover, and so you sometimes see these kinds of telescopes on top of mountain tops.

SUE ANN HEATHERLY: Why do. . .uh, why do clouds attenuate, uh, radio waves at high frequencies? And that also means at short wavelengths.

RON MADDALENA: Right. Sorry.

SUE ANN HEATHERLY: That’s okay. [Laughs]

RON MADDALENA: [Laughs] Okay. At short wavelengths clouds are mostly made of water molecules, and water molecules have this property that at long wavelengths the clouds are transparent. So if you are looking at a radio. . .through a cloud, through a radio telescope, you would not even see it, if you were observing at long wavelengths. If you are observing at short wavelengths, then the clouds become opaque.

SUE ANN HEATHERLY: Now, here in Green Bank, of course, it is cloudy a lot, it rains a lot, but yet we do observe at higher frequencies sometimes. And I know your work , when you’re not doing science, you’re trying to figure out how to get astronomers their best data – the visiting astronomers that use the telescope – how to optimize their observing time. So, what do we have to take into account here in Green Bank in order to observe at high frequencies or short wavelengths some of the time?

RON MADDALENA: One of my major jobs here is to be able to forecast the weather for radio astronomy. And the GBT, the Green Bank Telescope, has this great suite of receivers that go from long wavelengths to short wavelength. So, if we know that the wave, that the weather is going to be bad for short wavelength observing, then we will schedule the telescope for long wavelength observing. If it’s going to be great for short wavelength observations, then we take advantage of it. So, we have this suite of receivers that are on the telescope for all times, and we switch between them when we know what’s gong to be going on with the weather over the next day or two.

SUE ANN HEATHERLY: Did you ever think when you got your PhD in astronomy that you would become a weather forecaster?

RON MADDALENA: No, not, not directly. When I was doing my thesis work, it was at a high enough frequency where you had to look at the forecast to know whether or not you were going to be observing the next day. I was also taking classes at the time. So in order to be able to sleep, take classes, and observe, I had to know very well what was going on with the weather over the next couple of days. So, no, I didn’t think I would be spending so much time doing it, but I knew that it was part of my job.

SUE ANN HEATHERLY: So, we’re building a new telescope in partnership with European countries, with Asian countries, with Canada, with various other nations, called ALMA, the Atacoma Large Millimeter Array. Why is it located truly in the middle of nowhere up on this desert in Chile?

RON MADDALENA: It is by far one of the most remote places you can get to; one of the driest locations on the earth; it’s at high elevations. And the reason that that site was selected for ALMA was, again, to get above most of the weather. It’s in a rural location, so it gets away from radio frequency interference, and it is. . .and it is observing at such a short wavelength, many times shorter than what the Green Bank Telescope can observe, that it needs a place where it is much, much drier, much, much more above, uh, all the weather that we see at lower elevations.

SUE ANN HEATHERLY: And then, finally, we’ll talk just a moment about future radio telescopes: telescopes that are sort of in the planning stage right now. Here in Green Bank we have talked about it before, on some of the podcasts on this show, about how we reduce the amount of interference around us to keep the, the area here in Green Bank really quiet for the Green Bank Telescope so that astronomers can get good data, and we’ve talked a little bit about the National Radio Quiet Zone that surrounds us, in which we’re able to regulate the placement of fixed commercial transmitters so that, for example, you can’t use your cell phone if you come here to visit us. It won’t work, ‘cause there is no towers nearby for it to talk to. Now, there is, uh, another telescope in the planning stage, and maybe multiple versions of it, called the SKA, and what does that stand for?

RON MADDALENA: That’s the Square Kilometer Array. With any kind of telescope, what matters significantly is the size of the telescope. The larger you make the telescope, the larger the light bucket you build for collecting these weak signals we’re trying to get from these astronomical objects. So the Square Kilometer Array is this dream telescope that astronomers have been thinking about for. .decades, where their. . .the collecting area is that of a square kilometer. This telescope is only going to be working at low enough frequencies where. . .they need a location which is farther away from radio frequency interference, and so they’re going to places like the Outback in Australia, or real, real rural locations in South Africa, as potential sites for this, this huge telescope.

SUE ANN HEATHERLY: Okay, so there you have it folks: location, location, location. Well, Ron, thank you so much for you joining us today. We appreciate it.

RON MADDALENA: Thank you very much, Sue Ann.

SUE ANN HEATHERLY: And , uh, that does it for this edition of 365 Days of Astronomy. Thanks for joining us.

End of podcast:

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