Ever seen one of these before?
Yeah, it’s a bit bigger than your average radio antenna.
That’s because its job isn’t to direct radio signals to your house. It’s a radio telescope, and its job is to collect as many radio signals as it possibly can—from outer space, not from a radio station.
Radio astronomy is a tricky business. It has its advantages over visible astronomy—it certainly works better for interferometers—but radio signals are so weak, they’re hard to detect and study. Which is why you’ll never see a small radio telescope.
So, how do astronomers manage to collect and study radio emissions from the cosmos?
First, let’s recap. What is radio astronomy, and what do I mean by small radio telescopes being a bad idea?
Well, it’s the same concept from before with visible light telescopes. Bigger is better because we can collect more light, for the same reason a larger bucket will collect more rain.
More light-gathering power increases resolution, a telescope’s ability to see detail. It works the same way with radio astronomy. Build a radio astronomy dish the size of your typical amateur telescope, and you’ll hardly see a thing.
But…wait a second. What are we even talking about here? Aren’t radio waves sound waves? So are we collecting sound from space? But…I thought there was no sound in space?
Correct. Space is a vacuum, so there is no “medium” for sound to travel through. And unlike light, sound requires a medium. No, radio waves aren’t sound waves. Your home radio translates them into sound. They’re part of the electromagnetic spectrum.
Is this the fifth post now in which I’ve mentioned the electromagnetic spectrum?
Yeah…it’s that important.
The electromagnetic spectrum is a spectrum of radiation.
Some are harmful, like ultraviolet rays, x-rays, and gamma rays. (Fortunately, they’re not something you have to worry about constantly, unless you go to the beach—ultraviolet radiation causes sunburns.)
You know infrared radiation as heat. Our planet gives off infrared radiation. So does every person you know—as body heat.
Microwaves are named for their household use, but they actually don’t cook as well as infrared radiation. They work in your household microwave because they’re concentrated, combined together, focused.
Then, carrying even less energy than microwaves, are radio waves. Radio waves are weak and have trouble getting through the Earth’s atmosphere. FM and AM waves have to start from the surface—those wavelengths can’t get here from space.
So if radio waves have so much trouble reaching us in the first place, why do we put so much effort into catching them?
Because they’re the longest wavelength we’ve got to work with. And that means they don’t scatter easily.
Basically, electromagnetic radiation likes to scatter. It comes in through the atmosphere traveling parallel, but then it hits the air and interacts with air particles. Shorter wavelengths scatter more than longer wavelengths.
That makes things easy for radio astronomers. Because the single biggest handicap of visible light astronomy is the expense of constructing large telescope mirrors.
This is a mirror in a reflecting telescope. It’s expensive to make the bigger it gets because it has to be ground perfectly. The universe tends toward chaos—glass doesn’t naturally come this smooth. Astronomers have to make it that way.
Why do mirrors have to be perfect? Because too many imperfections scatters precious visible light. And visible light has a short enough wavelength to scatter easily.
Not radio waves.
A radio astronomer’s advantage is that he or she can construct enormous “mirrors” for radio telescopes without spending lots of money on perfection. Wire mesh is perfect enough to collect radio waves.
But…hold on a second. Why are we trying to reflect radio waves? How will that help us collect them?
Because, as always, we’re trying to bring electromagnetic radiation to a focus. And to do that, we have to make parallel rays cross.
Now, however, we’re not aiming them out an eyepiece. No radio astronomer “looks through” a radio telescope. Radio waves are collected on a dish reflector—like a mirror—and reflected onto an antenna.
In the case of the radio telescope diagramed here, the whole assembly that holds the subreflector is the antenna.
Does the setup here look a bit familiar?
If you have a radio antenna on your house, then it should.
Your household satellite dish and a radio telescope look very much the same. They have a dish reflector. They have a little receiver, that arm-like thing that pokes out and faces the dish. It just looks a little different on radio telescopes.
This receiver is able to collect radio signals reflected off the dish and record them. The recorded signals travel through that little cord on the satellite dish and are relayed to your television.
A radio telescope works the same way, except radio signals are sent from the antenna to an amplifier. This is because, like I mentioned earlier, radio signals are about as weak as you get. They can’t be studied until they’re stronger.
The amplifier then sends the amplified signals to a computer, where the data is analyzed.
How exactly? I mean, there are tons of ways to express and organize data so it can be analyzed.
Well, radio telescopes use false color.
Here’s a false color image I used in an example in another post. The first image shows you the Sombrero Galaxy as it really appears to our eyes. The second image shows the same galaxy, but in wavelengths of infrared.
The pink and blue colors represent longer and shorter wavelengths of infrared. They have nothing to do with the visible light colors of the image—they’re just used as color coding.
Radio wave data is expressed in a similar way. Longer wavelengths of radio waves are generally expressed as redder colors, and shorter wavelengths are generally expressed as bluer colors.
Here’s an example, in the form of an image of Jupiter’s cloud bands.
The visible light image is just a plain old image of Jupiter. It’s a recording of wavelengths of visible light—the colors of the rainbow.
The radio “image,” on the other hand, isn’t really an image as we think of it. It’s a map of where on Jupiter’s surface radio wave emission is more or less intense. Greater intensity is represented with different colors from lesser intensity.
It’s a lot like a weather map, where different colors show you where precipitation (rain) is the most intense. But I would also liken it to a topographical map.
Ever seen one of these?
A topographical map uses lines to show how high above sea level the land is. It’s a way of showing a three-dimensional landscape on a two-dimensional paper.
It’s similar to a radio map in one important way. A topographical map doesn’t tell you anything about what the land looks like—just how high above sea level it is.
A radio map doesn’t tell you anything about what an object looks like, either. What it does tell you is how intense its radio wave emission is.
Anyway, that’s enough on radio telescopes for now. Next up, we’ll talk about some of their weaknesses.