Ep. 632: Building Images: Optical vs. Radio

Feb 28, 2022 | Astronomy, Observing, podcast | 1 comment

A recent image from the South African Meerkat telescope blew our minds. It was a high resolution image of the center of the Milky Way, showing delicate filaments and other structures. What was so mind blowing is that this was an image from a radio telescope. Today we’re going to talk about why this was such an accomplishment and what the future holds for radio astronomy.

Download MP3 | Show Notes | Transcript

Show Notes

MeerKAT (SARAO)

New MeerKAT radio image reveals complex heart of the Milky Way (SARAO)

PODCAST: Ep. 16: Across the Electromagnetic Spectrum (Astronomy Cast)

PODCAST: Episode 130: Radio Astronomy (Astronomy Cast)

Visible Light (UCAR)

Radio Waves (UCAR)

HEFT Science Objectives (Caltech)

Active Galactic Nuclei (Swinburne University)

A Multi-Wavelength View of Radio Galaxy Hercules A (NASA)

Basics of Interferometry (Georgia State University)

Fast Fourier Transform (Wolfram Mathworld)

Very Large Array (NRAO)

Green Bank Observatory

Jodrell Bank Discovery Centre

Contact (IMdB)

Murchison Widefield Array

Portal

Stargate SG-1 (IMdB)

Square Kilometre Array

The Hydrogen 21-cm Line (Hyperphysics)

Transcript

Transcriptions provided by GMR Transcription Services

Fraser Cain: Astronomy Cast. Episode 632. Building Images in Optical and Radio. Welcome to Astronomy Cast. Our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me as always is Dr. Pamela Gay, Senior Scientist for the Planetary Science Institute and the director of Cosmo Quest. Hey, Pamela. How are you doing?

Dr. Pamela Gay: It’s a week where I’ve decided that is a question that we probably shouldn’t ask. And it’s way better to simply say, “Are you okay?” To which I can’t answer yes.

Fraser Cain: Yes. Are you okay? Yeah. Yeah, it’s a tough week. I was mentioning before the show, our webmaster on Universe Today is from Ukraine. He’s not living in Ukraine right now, but that’s where he comes from. And a bunch of other people on our team.

Dr. Pamela Gay: Yeah.

Fraser Cain: Similar situation. So, it’s tough. It’s hard and it’s scary. And I’m really hoping for just as much safety and minimal casualties and a peaceful resolution to this whole situation. It really sucks. I hate it.

Dr. Pamela Gay: Yeah. What he said.

Fraser Cain: A recent image from the South African MeerKAT Telescope blew our minds. It was a high resolution image of the center of the Milky Way showing delicate filaments and other structures. What was so mind blowing is that this was an image from a radio telescope. Today, we’re gonna talk about why this was such an accomplishment and what the future holds for radio astronomy. All right. So, did you see this picture?

Dr. Pamela Gay: I did, and I’m actually gonna bring it up so that everyone here can see it in glorious, not quite as blocked out by Fraser, detail.

Fraser Cain: Yep.

Dr. Pamela Gay: This is the heart of our galaxy in a way we’ve never seen it before.

Fraser Cain: Yeah. I’ll try to describe it because a lot of people are getting this as a podcast. And so, I will try to explain it a little bit because I think it’s really important. And if you want to see the picture for yourself and you’re just—Just do a search for MeerKAT like the animal. Milky Way.

Dr. Pamela Gay: And we have show notes.

Fraser Cain: And you’ll get to see an image that’s—And, yeah. We have some links in the show notes. But you’ll see this image that looks like a negative with mostly white with this red core, but there’s some other images out there as well. And it’s sort of weird and cool with a lot of really intricate details. Why is this such an important image?

Dr. Pamela Gay: It’s the first time we’ve been able to see the core of our galaxy at this particular combination of resolution and wave length. And every time we’re able to get a better image of something, it reveals new details. It’s literally just the new details.

Fraser Cain: Yeah. So, if I think back to some really old episodes that we did, we went through the electromagnetic spectrum, we did an episode on radio telescopes. And we talked about how radio telescopes, the way they work isn’t conducive to making pictures. So, can you explain how a radio telescope produces an image?

Dr. Pamela Gay: Well, all telescopes likely produce images pretty much the same way. But the difference comes, and radio wavelengths are really, really big. So, when we talk about optical images, we’re talking about light that is a few hundred nanometers in size. 400 nanometers, 800 nanometers, somewhere in there. And that is significantly smaller than the size of a hair. And with a radio telescope, we’re talking about wavelengths that are measured somewhere in millimeters to meters and kilometres. It just keeps going.

Fraser Cain: Yeah. And, so, sorry. And, so, when you have these nanometer, hundred nanometer, hundreds of nanometers falling on, say a telescope or on your eyeball, you’re getting a lot of them all at the same time.

Dr. Pamela Gay: Yes.

Fraser Cain: And so, you’re seeing a thing. Whether it’s a nebula or an apple or a tree or whatever, you’re seeing all of these photons all at the same time.

Dr. Pamela Gay: And what’s more than that, the resolution that we’re able to see isn’t so much related to just how many photons you get, period, but how many of the wavelengths can fit across your detector. So, if you have a single mirrored optical telescope that’s a meter across and you’re looking at light that is measured in nanometers, which is 10 to the -9, you’re gonna get a lot of wavelengths across your 1-m telescope. A lot of them.

Fraser Cain: Right. Yeah.

Dr. Pamela Gay: But if you’re instead looking at 1cm wavelengths, you’re gonna get a hundred of them. And that many factors of 10 that go into that, from centi to nano, that is the difference in your resolution.

Fraser Cain: And I guess it’s like—I think back to that episode that we did and imagining—I remember the conversation quite vividly because I’ve used versions of it in explaining this as well. With a radio telescope, you’re taking a fairly large sensor and you’re just scanning it across the sky and you’re saying, “Yes, radio. No, radio.” And the strength of the signal. Seven. Three. And you’re moving. And the big modern innovation was to go from just one sensor to maybe four. And I think, you were saying on the Arecibo, they still have a handful, like what? Like 16 or something like that when Arecibo was rolling? Your resolution was terrible. Again, it was just some version of, “Is there radio over there? Yes. There’s radio over there.” Not producing this beautiful subtle image.

Dr. Pamela Gay: Yeah.

Fraser Cain: And if you look at journal articles, you’ll see just these blobs. These quite mediocre looking blobs that I’m sure are filled with all kinds of really fascinating scientific information. But we have a hard time running those on Universe Today. This blob is a super massive black hole gobbling material from the nearby surroundings and blasting out stellar waves. Look at it and all its blobby glory.

Dr. Pamela Gay: Right. And this is where luckily a lot of times, the things that we look at in radio are quite big. So, there has been some use for all these blobby images. Specifically in looking at active galactic nuclei. Active galaxies are little bitty tiny things in optical, where we’re only seeing their star-filled areas. But that black hole feeding in the center is devouring massive amounts of material developing massive magnetic fields. Developing even more massive jets spewing material out of the center of the active galactic nuclei. So, for decades, the fact that we were dealing with really blobby images that were super low resolution using our—Sometimes when we were lucky, kilometre-across telescope to look at centimeter wavelength light, we were fine with that. And we were happy enough. And we did lots of science. But then what was realized, pretty early on, with optical telescopes, if you want to combine the light from optical telescopes, you have to do crazy stuff with fiber optics. So, the light effectively has the same travel distance from object to detector no matter where you put the telescopes. And you are mechanically combining the light from the different telescopes so that you’re dealing with the same wavefront hitting your detector from all of the scopes. This is very hard.

Fraser Cain: It’s like focusing. When you think about an image that’s blurry and then you turn the focus wheel and then it comes in, gets sharper and sharper, and I guess, when you’re using interferometry, there’s some version of that where all the wavefronts align. And you’re like, “Haha. We’re there.” Within 500 nanometers. Whatever you do, don’t jiggle any one of the telescopes because there’s no—You gotta be perfect within this incredible tolerance. And so, what was the big discovery with radio telescopes then?

Dr. Pamela Gay: Well, with radio telescopes, you just record it all on tape and deal with it later. Because with radio data, the wavelengths are so big, we can look at the data and pretty much go, “I see the wavelength.” And combine them by having fairly precise clocks on each of the telescopes. So, instead of trying to mechanically combine light with fiber optics, it’s just a matter of taking all of the recordings and shoving them into a really fancy high-power computer. And out comes your integrated data on the other side. And the really amazing thing. And this is the place where the maths in astrophysics get insane with combining electronics, everything you ever wanted to know about electronics and physics altogether at the same time. They have realized that any one individual telescope has a field of view of beam size on the sky that is related to how big that dish is. How many wavelengths fit across the dish give or take things like shadowing from the detectors and stuff like that that the fancy word is attenuates the beam. It makes it less efficient. Now, that gives you a field of view on the sky that is the same thing as your resolution pretty much if you’re only using that one dish. Now, if instead, you’re combining a whole bunch of different dishes, there is complex maths called a Fast Fourier Transform. Which is beyond the scope of attempting to explain in anything less than 16 weeks.

Fraser Cain: Can we do that? Can we do a 16-week-long episode of Astronomy Cast, where we take you from the beginning to doing your own Fast Fourier Transforms by hand?

Dr. Pamela Gay: No.

Fraser Cain: No.

Dr. Pamela Gay: No.

Fraser Cain: No.

Dr. Pamela Gay: So, there’s this way of combining the—This detector looked at that beam width on the sky, which equates to the specific field of view. And when you combine the data across all of the different dishes, suddenly, you still have that same field of view. But you’re able to resolve out features within that field of view that are the resolution of if you had used a single dish the size of all of those spacings added together. So, if east to west, you spread out your telescopes a hundred kilometres, you now have a resolution east-west on the sky that is related to that hundred kilometres divided by your wavelength instead of the however many meters your dish is divided by the wavelength. It’s a huge improvement and is what allows us to do what is done with telescopes like MeerKAT, where they’re starting to spread things out over continents.

Fraser Cain: Yeah. All right. Well, we’re gonna talk about this some more in a second. But it’s time for a break.

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Fraser Cain: And we’re back. And so, we were leading up to this conversation about MeerKAT. So, how is MeerKAT different from this very traditional big radio dish that we’re so familiar with.

Dr. Pamela Gay: So, it is not all that different. It has an off-axis detector. And this allows it to get a better view with less, I’m gonna say the word noise, which is not scientifically correct, and I’ll explain what I mean. So, normally when you’re using a radio dish, you’re pointed at your object. You get this beautiful peek in your data related to that core beam of sensitivity coming in. But then various things also are like, “Hey. We’re gonna make a mess of your data for you.” And it creates side lobes. And those side lobes, there’re literally blobby bits that come off the sides of your data. Those side lobes get bigger and bigger the more stuff you stick in the center of your telescope. So, when you have a big old, right where everything comes together, the top of the dish detector, it gives you bigger side lobes.

When you instead move your detector off axis like they’ve done with MeerKAT, it knocks down the side lobes. And it allows you to have a cleaner footprint on the sky and less noise and start to get better images. So, they improved how they’re able to see by reducing their side lobes by using this other geometry. And in addition to that, they just build a whole lot of telescopes spread out over a whole lot of space. And all of this together, with added sensitivity, they have cleaner electronics that are able to be more sensitive to what’s in the sky. All of that added together gets the beautiful images we see.

Fraser Cain: Yeah. And they got like what? Sixty-four separate telescopes acting as one.

Dr. Pamela Gay: It’s awesome.

Fraser Cain: Yeah. And they both have the separation, which, as you said, gives you that interferometry. So, really, they’re acting like a telescope that is as big as the left-most telescope across to the right-most telescope. The gap in between, that is the size of the telescope that they act like. But also the combined resolution of all of the—If you add up all of their surface areas, you also get that as well. And so, it makes for a very gigantic, very sensitive telescope. Now, Murchison, which is also gonna be the sister project to MeerKAT, looks like spiders in the desert. What’s going on there?

Dr. Pamela Gay: So, as you look at longer and longer wavelengths of light, your telescopes stop looking like telescopes in a lot of ways. When you look at an optical mirror, there are no flaws you can see with your human eyeballs unless something really bad happened. I mean really bad happened. Now, with a standard telescope like the VLA, like Murchison, for radio, you start to see things that are the size of basically chickenwire. Because your wavelengths are big enough that that kind of deviation doesn’t matter. Now, once you start getting to the long wavelengths, the really long wavelengths looked at by Murchison, they don’t even bother with a dish anymore. They just stick an antenna out there and go, “Hi. We are looking up.”

Fraser Cain: Yeah. Think about your television aerial. I don’t know about you. But when I grew up, we had this giant television aerial poking up the top of our house that would turn. And it was a radio telescope?

Dr. Pamela Gay: Exactly.

Fraser Cain: Right? Yeah. Yeah. And so, now they’ve just got these—And so, same thing. They just put, I don’t know how many there are, but there are lots out there in the desert. Separated and same thing. Each one is collecting radio waves, but they’re also working together like one giant telescope the size of their separation.

Dr. Pamela Gay: And they come in basically small herds of spiders.

Fraser Cain: Right.

Dr. Pamela Gay: It’s these little groups of something that looks like a portal nightmare video game horror story. I read way too much science fiction y’all.

Fraser Cain: Yeah. And you don’t take enough microphotography images of bugs.

Dr. Pamela Gay: No. I don’t. I don’t.

Fraser Cain: There’s your problem. Yeah. Staring into their beautiful little eyes.

Dr. Pamela Gay: Actual bugs, I’m okay with. It’s the robotic ones that worry me.

Fraser Cain: Right. Of course. Yeah. The robot spiders.

Dr. Pamela Gay: Yeah. Yeah.

Fraser Cain: Neck arachnophobia.

Dr. Pamela Gay: It’s Stargate. I blame Stargate.

Fraser Cain: Oh. Is that what it is? Stargate? It’s the nanites?

Dr. Pamela Gay: Yeah.

Fraser Cain: All right.

Dr. Pamela Gay: Exactly.

Fraser Cain: All right. Bring it all together for me, Pamela. We’ve talked about how these images are made. We’ve talked about this incredible image that came out of MeerKAT. How will this come together for the Square Kilometre Array? The merging and expansion of these two telescopes into one continent-spanning mega telescope.

Dr. Pamela Gay: So, it unfortunately is not going to be a telescope that is the size of whatever the distance between South Africa and Australia is. The two parts of the Square Kilometre Array that they’re building on the two different continents are technologically very different because they’re working at a different set of wavelengths. Which is why Murchison in Australia and MeerKAT in South Africa look radically different. So, we’re going to end that up with sprawling across South Africa a series of—You can recognize them as collecting radio signals of telescopes that together will each look at their own tiny patch of sky, but where tiny is defined in uncomfortably large patches of the sky for us optical people. And when working together, each telescope’s beam size becomes the field of view for the entire telescope when the data’s brought together, and they can see the high resolution. And then that whole scanning thing you mentioned, that still happens. They still have to either scan the telescopes across the sky to get a bigger field of view or do a series of snapshots to get a larger field of view. But we know how to do that. And it’s not like we don’t mosaic Hubble images as well. It’s just a different size. So, we’re gonna end up with one continent working at one set of wavelengths.

Another continent working at another set of wavelengths. Each set bringing us into, I’m going to say a redder and redder part of the spectrum and allowing us to create images where when we put side by side the optical and the radio, it’s no longer this highly detailed optical image and a blob of radio light. It’s instead going to be that highly detailed optical image and, in some cases, an even more detailed radio image that are just looking at completely different physics of what’s going on in the object. And once you start being able to combine the physics that creates the radio light, the physics that creates the optical light, at the same resolution, it gets you a whole new way of trying to understand what’s going on. And it’s an amazing era I can’t wait for.

Fraser Cain: Yeah. Just like if you were looking at one of these radio images, what are you seeing? What are the radio waves trying to tell us?

Dr. Pamela Gay: So, radio waves are coming from electrons and magnetic fields. They are coming from very low energy transitions in molecules. That famous 21-cm line, which neither of these telescopes are really gonna be looking at, it’s created just by a spin flip in the hydrogen atom that is very rare. Only occurs in diffuse cold areas. And it’s these kinds of low energy physics that we’re able to see with the radio world. And with optic, we’re looking at hotter things, but not the hottest things. The hottest things come to us from X- Ray. So, what we’re really probing is different energy events, different, well, since electrons and magnetic fields are happily creating all sorts of really cool radio astronomy stuff to look at, we’re looking at physics of magnetic fields. And you can also do all sorts of cool measurements of the dust off of systems and how things are, how the light gets scatters, as it comes through by looking at the polarimetry of—It’s, I can just, I’m gonna stop now. There’s a lot.

Fraser Cain: Yeah, yeah.

Dr. Pamela Gay: It’s cool.

Fraser Cain: Magnetic, magnetic fields.

Dr. Pamela Gay: Yes. And cool stuff. And cool stuff.

Fraser Cain: And very, very cool stuff.

Dr. Pamela Gay: Yeah.

Fraser Cain: Like temperature wise.

Dr. Pamela Gay: Yes.

Fraser Cain: But also esthetically.

Dr. Pamela Gay. Yeah.

Fraser Cain: All right. Very cool. Well, again. I think if you are listening to this podcast, when you get a chance, check the show notes. Look at the image. And I hope this time, when you see it, you’ll be like, “Okay. This is important.” And this is the first time we’re seeing an image, a radio image, with this level of clarity. And it is a harbinger of the future, of a really exciting future, where radio finally gets to stand up and be appreciated for the beauty that the visual astronomers have been receiving for decades.

Dr. Pamela Gay: Finally.

Fraser Cain: Finally. Radio astronomers, you have arrived. All right. Thanks, Pamela.

Dr. Pamela Gay: Thank you, Fraser. And thank you to all of you out there, who support our show through Patreon. This week, we would like to thank Kellianne and David Parker, Gfour184, Gabriel Gauffin, Rachel Fry, Andrew Stephenson, Dustin Ruoff, Planetar, Brent Kreinop, Peter, Sean Matz, Kseniya Panfilenko, Cemanski, Sean Freeman (Blixa the cat), The Mysterious Mark, Joe Wilkinson, Benjamin Davies, Steven Coffey, Glenn McDavid, John Drake, John Oiseth, Roland Warmerdam, Dean, The Air Major, Lew Zealand, Bart Flaherty, Tim Gerrish, Claudia Mastroianni, Brian Kilby, Corinne Dmitruk, Naila, Arcticfox, she gave me pronunciation guides. Thank you. Thank you. Jordan Turner, Leigh Harborne, Mark Phillips, Kathryn Mattson, Bob the boodle cat, Chris Wheelwright, Jason Kardokus, Olivia Bryanne Zank, Ron Thorrsen, PAPA1062, Robert Hundi, Kim Barron, Vitaly, Paul Esposita, Arthur Latz-Hall, Frank Stuart, Ganesh Swaminathan, Bob Zatzke, Connor, Ruben McCarthy, Geoff MccDonald, Iggy Hammick, Wayne Johnson and Rabekkah. Thank you all so much very much.

Fraser Cain. Awesome. Thank you so much.

Dr. Pamela Gay: Bye-bye.

Female Voiceover: Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a creative comments attribution license. So, love it, share it and remix it. But please. Credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website AstronomyCast.com. This episode was brought to you, thanks to our generous patrons on Patreon. If you want to help keep this show going, please consider joining our community at Patreon.com/AstronomyCast. Not only do you help us pay our producers a fair wage, you also get special access to content right in your inbox and invites to online events. We are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.

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1 Comment

  1. Mark

    Meerkat image … Eye of Sauron!!!!

    Reply

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