Date: November 20, 2009
Title: MUSTANG Gallops Towards Clusters Last Stand
Podcaster: Sue Ann Heatherly
Organization: NRAO: http://www.gb.nrao.edu/
Description: Join us as Dr. Simon Dicker from the University of Pennsylvania chats about a new tool for the Green Bank Telescope and what it can do. The receiver is called MUSTANG. Although you may not be able to remember what MUSTANG stands for at the end of this podcast, you’ll be amazed at how it allows astronomers to image galaxy clusters in a very unusual way.
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.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the National Radio Astronomy Observatory, celebrating Five Decades of Training Young Scientists through summer programs. Explore the hidden universe in radio at www.nrao.edu.
Transcript:
SUE ANN HEATHERLY: Welcome to this addition of 365 Days of Astronomy podcasts. My name is Sue Ann Heatherly, and I work at the National Radio Astronomy Observatory in Green Bank, West Virginia. And joining me for today’s podcast is Simon Dicker, and he is an astronomer at the University of Pennsylvania. And, uh, Simon, uh, welcome to the program.
SIMON DICKER: Hi, there. Oh, thank you very much.
SUE ANN HEATHERLY: I was looking at your . . .your resume online, because it’s online. If you dig hard enough, you –
SIMON DICKER: Oh.
SUE ANN HEATHERLY: –can find it.
SIMON DICKER: I left it up there?
SUE ANN HEATHERLY: It’s there. It’s not easy to find, but I . . . but I’ve noticed that throughout your whole career, from your PhD days through now, that you spend a lot of your time inventing things and building things for astronomy.
SIMON DICKER: Yeah, a lot of people think that astronomers just look at the stars and stuff. Well, someone has to build the Hubble Space Telescope, someone has to build the Green Bank Telescope, and I guess that’s my main role. I do like to use them, as well, but I’m one of the people who is down in the lab, usually in the basement with no windows, actually building the telescopes. Astronomers get to live on the top floor with windows.
SUE ANN HEATHERLY: Well, we’re going to talk about one of your projects in particular, but I noticed there were three instructing projects on your resume – three, uh, instruments that you’re working on. Tell us what those are.
SIMON DICKER: Well, we have Mustang. I think we’ve got to talk about that one. We have Blast. There’s actually a movie out on that. Uh . . . we have the ACT Telescope, and those are my three most recent projects. All of them have a few things in common. We have sensors that we need to cool down very, very cold, and, uh, they go on telescopes.
SUE ANN HEATHERLY: So, we’re going to talk today about Mustang, because you’re here at the Green Bank facility working on this instrument for the Green Bank Telescope. The, the acronym for it is mustang. Now, I want to see if you can remember what mustang stands for.
SIMON DICKER: It’s the multiplexed . . . um . . . array of TS detectors at 90 GHz.
SUE ANN HEATHERLY: Now, that’s a tortured acronym, if ever I heard one. I don’t even know what you . . .what you just said there. Tell us what this thing does.
SIMON DICKER: Basically, all telescopes need a camera of some sort, so this is just a camera. The Green Bank Telescope, being a radio telescope, mostly has one pixel. A digital camera, you know, will have several million. Typical radio telescopes only have maybe four pixels. Well, we’ve actually built a different sort of camera. This one has a whole sixty-four pixels, and it also operates very differently. In your . . .in your camera you have something called the CCD. This we’ve simply got heat sensors; just if you want to detect light, you can hold your hand up, fill the warmth; basically, that’s what we’re doing. We’re using the telescope to focus the light, the radio light, from the other side of the universe. And we’re focusing it onto our detectors, and they warm up and cool down, and that’s how we detect the light.
SUE ANN HEATHERLY: So, for those of you out there that think sixty-four pixels is not a whole lot to brag about, it’s really a pretty remarkable achievement, as Simon was saying. Our Green Bank Telescope, very often, is a one pixel camera. So, if you want to make an image of a, an area of the sky, you have to move the entire seventeen million pound structure back and forth to measure the intensity of the radiation from different parts of the sky. Now, in this case, we have sixty-four pixels on the sky. What does that do for observing?
SIMON DICKER: Effectively, you could have sixty-four Green Bank Telescopes. It’s a lot easier to build a camera with sixty-four pixels. But one of the most important things is each pixel looks through the same bit of the atmosphere. The atmosphere is a horrible, cloudy thing. We wish we could get rid of it in astronomy. That’s why we build, uh, telescopes in space. But, uh, if you have all of the pixels looking through the same atmosphere, they see the same atmosphere, but they see different astronomy. So, you can just subtract out an average of all the pixels, and you can actually remove a large fraction of the atmosphere that way. And you also get sixty-four times as much data.
SUE ANN HEATHERLY: Is it easier to remove the effects of the atmosphere when you have multiple pixels? Is that what . . . what we’re getting at here?
SIMON DICKER: It’s easier to remove the atmosphere. It’s easier to move . . .remove noise I make in the instrument, because it’s certainly a big cause of my own noise. And, in general, it’s much easier to build up a map.
SUE ANN HEATHERLY: One of the features of Mustang, and the other work that you do for your other instruments, is that parts of this receiver have to be extremely cold.
SIMON DICKER: Yeah. Your hand, um, it can feel the heat from a fire, or something like that. The sort of heat we’re looking at, you wouldn’t even be able to detect it. If you put your hand at the focus, you wouldn’t feel a thing. In order to detect these tiny amounts of power, you have to cool things down to near absolute zero to gain the sensitivity; otherwise, just, you know, random fluctuations and the temperatures of things just drown out the signal we’re looking for.
SUE ANN HEATHERLY: Let me . . . let me see if I get this and, if I don’t, feel free to say “That’s wrong.” So, let’s say that you were going to take the Green Bank Telescope and look at the Orion Nebula with the Mustang instrument on it.
SIMON DICKER: Yeah. We did that.
SUE ANN HEATHERLY: So, if you were going to do that, what would be the difference between looking at the Orion Nebula and power versus looking at blank sky? Is this the difference we’re trying to see?
SIMON DICKER: That is the difference we try to see. Have to remember the calculation off the tip of my head. I think I worked it out that it would be ten to the minus nineteen watts for a very bright source, or . . .that’s almost one billionth of a billionth of a watt. Maybe your standard light bulb – a small one, that is – is about sixty-so watts. So, yeah, it’s a really small amount of power.
SUE ANN HEATHERLY: And how much will that heat up this little detector?
SIMON DICKER: Well, they’re actually superconducting sensors, so there is very little change in temperature. Superconductors are a material that when you get it cold enough it goes to zero resistance, and that’s what makes them incredibly sensitive sensors. To actually get them to power transition temperatures, as we call it, is all the way down; it’s about three hundred milli-Kelvin is what we need to get to.
SUE ANN HEATHERLY: Wait a minute, now. You said three hundred milli-Kelvin is–
SIMON DICKER: Yeah.
SUE ANN HEATHERLY: Is the temperature you want to get your sensor to?
SIMON DICKER: Yes. That’s right. And we do that with a four-stage refrigerator. Yeah.
SUE ANN HEATHERLY: Wow. I didn’t even know that could be done. I mean for the other receivers that we have on our telescope, they’re typically – the coolest parts — are around 15 Kelvins, and you’re talking about milli-Kelvins here.
SIMON DICKER: Yeah. Uh, so, to start off with a bigger refrigeration – a different type, uh, than the ones you have here, but not so dissimilar – it works much the same as any other refrigerator you might have in your house, in that you’ve got a big compressor. If you look around your frig, you will see a motor. In our case, the compressors are really big, on the size of your refrigerator. And then, after that, the gas expands. We use helium gas because it’s the only thing that doesn’t freeze at these temperatures. And as it expands throughout the first two stages of this refrigerator, we cool down to about 3 Kelvin, and it’s a hundred times colder than room temperature. And then we have to go on to some harder things. Liquid helium is one thing. The first two stages condense the liquid helium, and then we use, um, basically, what you just put on your barbeque: regular charcoal. When you get it cold, it absorbs the helium gas and creates a vacuum. When you boil things under vacuum, they get really . . . they boil at much slower temperatures. So, first we do that with ordinary helium, and then we have to use a special isotope of helium. It’s missing one of the neutrons in its core. So, it’s only got three-quarters of the gas of normal helium, and so it boils at an even lower temperature, and that’s how we get to three hundred milli-Kelvin.
SUE ANN HEATHERLY: Wow. Where do you find an isotope of helium? I mean. . .
SIMON DICKER: Um, the atomic bomb program.
SUE ANN HEATHERLY: Oh, my goodness.
SIMON DICKER: Pretty expensive stuff, so that’s why we don’t let it boil up into space. We just, like, let it all evaporate and then we condense it again.
SUE ANN HEATHERLY: The other thing that is really interesting about this receiver – different – is that this receiver operates at a high frequency. You’re actually looking at radio waves that are very short in wavelength. You’ve actually achieved the highest frequency we’ve been able to go on the GBT.
SIMON DICKER: Yeah. We built the camera to do that, and also the folks here have worked very hard to make the surface smooth enough.
SUE ANN HEATHERLY: So, we’ve got this incredible receiver that you’re working on. We’ve got this incredible dish. What does that get us? What are the kinds of science that you’re doing with the Mustang receiver?
SIMON DICKER: Well, astronomers will think of all sorts of uses for it, but what I’m personally most interested in is working at the beginnings of the universe. At the beginnings of the universe, it was very hot. Um, it was the Big Bang. Basically, there’s the big glow leftover from it. When things get hot, they glow. And the light from that glow has been traveling around the universe ever since. We see that as the cosmic microwave background. But sometimes that light goes through a cluster of galaxies on its way to us. And that can change it, it’s . . . the amount of energy it has. When it does that, you can effectively see the shadow of the cluster of galaxies, and you can actually see a lot of the detail. And we have got the finest resolution map of the cluster of galaxies that’s ever been made.
SUE ANN HEATHERLY: In our clusters of galaxies, do they extend . . . I mean, how far back?
SIMON DICKER: That’s a very good question. We’d like to know that. When you look at them in optical light, we found many clusters. You can look at them in x-ray light; you can find many clusters. The trouble is, the further and further away you get, the dimmer and dimmer they get, and the harder it is to actually see them. But there’s many other telescopes, um, that are actually looking for these clusters using this effect, and that . . . as this effect doesn’t, like, decrease, as things get further and further away from you, then you can see the most distant clusters and we’ll be able to find out how they are formed.
SUE ANN HEATHERLY: What is the most distant cluster you guys have been able to see?
SIMON DICKER: Well, we’ve only so far gone off to known ones, because what we’re off to doing is actually seeing the details inside the clusters. So, as things fall in and form– the clusters, you know, started forming right at the beginning – and the biggest objects have collapsed in the universe, and things fell in and there’d be shocks and — they’re not just one blob. You expect to see lots of details, and we’re actually looking to see what those details are like.
SUE ANN HEATHERLY: You can image the details?
SIMON DICKER: The one cluster that we’ve got an image of – we’re planning to look at many more – we can actually see a big shock. Very hard to see with any other effects. And though we have found, like, small signs of it, but you would never believe it if you hadn’t have actually measured it this way.
SUE ANN HEATHERLY: Well, is this the image that I have seen around and about on line? Is it the Sunyaev-Zeldovich effect on this cluster?
SIMON DICKER: It—
SUE ANN HEATHERLY: Have I seen that imagine?
SIMON DICKER: Um, you may have done. Um, the paper has just been submitted, so it’s not actually published yet. Um, but it is available on line.
SUE ANN HEATHERLY: Explain to me what the Sunyaev-Zeldovich effect is, cause that’s what you’re talking about.
SIMON DICKER: So, basically, at the beginnings, we had the Big Bang when the universe formed. Back then, it was very, very hot. You know, everything was basically one glowing mess – very, very smooth, but glowing. Eventually, as the universe expands, it cooled. What happened to that glow? Well, that light is still traveling; it’s still around us. The universe has expanded a lot, so, we now no longer see it as a glow, cause as the universe has expanded, the photons that make up that light have been stretched. Effectively, they’re now very, very cold photons, and, uh, actually their typical size is about three millimeters, or an eighth of an inch.
SUE ANN HEATHERLY: And that’s the wavelength at which Mustang operates.
SIMON DICKER: Yes. And those photons – some of them, in their way traveling, like, towards us – will have passed through a big cluster of galaxies. When they do that, a small fraction of them collide with electrons and other atoms in the galaxy, but electrons most importantly. And when they do that, then they can gain some energy. So, when you look at them at low frequency, you no longer see those photons because they have higher energy. So, when we look at a cluster of galaxies with our instrument, what we actually see is a decrease in the amount of light. It’s a very, very distinct signature.
SUE ANN HEATHERLY: And it has structure. You can see places where you have a greater decrease in the light and places where you have a lesser decrease in the . . .
SIMON DICKER: Yeah. And, uh, normally, when we’ve looked at them in the past, um, people have you know, looked at them, but it’s been hard to actually image it with much detail, cause it is a very, very small effect. As we’ve built more and more sensitive instruments, we’ve been able to look into more and more detail. And now it’s becoming apparent that they’re not just simple blobs. You’ve got . . like, some parts will have a bigger decrease; some parts will have a small decrease, and some of that could be due to the fact that this is a cluster forming. We’ve studied the microwave background a lot, and we’ve learned a lot of how the universe has formed that way. Um, but this is really about understanding the clusters and the formation of structured. You look at the sky today, you see stars, galaxies, all sorts of stuff. Back then, though, there was really nothing. It was just one flat boring place, and, somewhere in between, something changed.
SUE ANN HEATHERLY: Well, I think that sounds like a lot of fun, and you’re a lucky guy to not only be able to do the research, but build the instrument that allows you to do the research.
SIMON DICKER: I think it’s fun.
SUE ANN HEATHERLY: And I thank you so much for joining us today, Simon.
SIMON DICKER: Okay. Thank you very much.
End of podcast:
365 Days of Astronomy
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