Interferometry


Double-stars-Rayleigh-limit-ST

Imagine you have an image like this. This object is faint and faraway, so you can’t make out much more detail. You know that other stars like it—closer, brighter stars—have looked like this and turned out to be two stars, nestled very close together.

How do you figure out what you’re looking at? How do you increase the resolving power on your telescope so that you can make out more detail?

A telescope’s resolving power is limited by its size. Bigger telescopes can make out more detail on faraway objects—that’s because they can gather more light. But now, we can make telescopes that are so big their size doesn’t limit their resolving power anymore.

The atmosphere does.

We obviously can’t change the atmosphere. So how do we get out of this particular predicament?

First of all, how come bigger telescopes can gather more light?

Rain/Buckets

Same reason a larger bucket will collect more rain than a smaller bucket.

If you can collect more light, you can make out more detail. Have you ever shone a dim flashlight on a cave wall and tried to make out the details? What happened when you switched to a brighter beam?

bright light cave

The region to the right is clearly darker than the region to the left.

But do you notice that it’s also harder to make out details in the darker region? If something small—say, some sort of insect—fluttered over this cave wall, do you think you’d make it out better in the dark region or the brighter region?

Making out more detail is basically the definition of resolution, one of a telescope’s three powers.

So if you want better resolution, you want to gather more light from an object. And to do that, you need a larger telescope.

The problem, though, as I mentioned above, is that telescopes these days can be made so big, their resolution isn’t limited by their size anymore. Because every telescope has to look through the atmosphere, and that’s the true limit on resolution.

Well, not all telescopes…but that’s a story for another day.

The thing is, the atmosphere wiggles. It’s annoying to astronomers, but it’s true.

This is a problem because light has a bad habit of interacting with the atmosphere. And the atmosphere scatters the light. Basically makes each one of those little light waves bounce about like kids in a bouncy house.

And we all know how hard it is to gather up those rascally youngsters.

Light works in about the same way, and that means that there comes a point when you’d need an insanely huge telescope to get better resolution. And by insanely huge, I mean…maybe…oh, 200 meters in diameter?

Yeah, that just doesn’t exist on this world yet.

But…it does. In a way.

Interferometry

This is where interferometry comes in. All you need is a few large telescopes spaced a bit apart from one another, and some kind of computer system to combine all their images into one. And you get the resolution of a much larger telescope.

Interferometry doesn’t solve the problem of the atmosphere scattering the light. It does take us that much closer to better resolution.

Here’s how you do it:

First, you need to build a few very large telescopes. And I mean large. Not quite as large as the interferometer, but maybe…oh…around 8 or so meters in diameter, at the very least.

Technically, you could go for smaller, but what’s the point? 8 meters isn’t really that big as telescopes go, and bigger is always better.

Then you set them all up right near each other, like so.

VLT

The Very Large Telescope (or VLT) is an interferometer. These telescopes can all work individually, but see those light paths connecting them all? That’s the computer doing its work, transforming them into one big interferometer.

Obviously, to work as an interferometer, these telescopes all need to be looking at the exact same area of the sky. Otherwise, they’re not really helping each other resolve more detail on one object, are they?

The key to making interferometry work is to fool the light waves into thinking they were all collected by one big telescope. Which means that they all need to be carefully aimed parallel to one another.

Why? Because light waves go into a telescope parallel and come out parallel. The only reason they stop being parallel is because for a telescope to work, it has to reflect the light so it changes its angle and comes to a focus.

And…that’s a little beyond the scope of this post, but it basically means that a telescope’s job is to get parallel light rays to cross.

reflector

When these rays come out of a telescope, all the ones from the same telescope are parallel. But they might not be parallel to the ones from the other telescopes in the interferometer.

That’s the one weakness of an interferometer. Because we’re getting light from different telescopes that are experiencing different atmospheric turbulence, computer systems have to be very careful about straightening those light rays.

It all means that those light paths up above have to be constructed perfectly. There isn’t much room for error in their direction or in their inside surface. Visible light is easily scattered, after all.

But where interferometry is a delicate business for visual telescopes, it’s much easier for radio telescopes.

Wait a second…radio telescopes? I’ve heard of radio stations, but radio telescopes?

Yeah, they’re a thing, and I’ll talk a lot more about them coming up.

Radio Interferometer

Here’s a radio interferometer. It works better than a visual interferometer, because it gathers radio waves rather than visible light waves—and radio waves are much harder to scatter. Why do you think they’re so easy to transmit via…erm…radio?

And yes, before you ask…radio waves are indeed related to visible light waves. They’re not sound waves. Both radio waves and visible light waves are part of the electromagnetic spectrum.

The electromagnetic spectrum is so important to astronomy that it’s going to come up in a lot of posts in the future.

I try to write these so that you don’t need to have read any previous posts to understand what I’m talking about, but it will help to have read my post on the electromagnetic spectrum.

Here’s the basics: the electromagnetic spectrum is a range of different wavelengths of radiation. Longer wavelengths have less energy, shorter wavelengths have more energy. Radio waves are towards the longer end. Visible light waves are right in the middle.

electromagnetic spectrum

Radio waves are, in fact, the longest wavelength. The wavelength just longer than visible light is infrared, which you know as heat. Just shorter than visible light is ultraviolet, which causes sunburns—even though its heat doesn’t feel different from infrared to you.

And here’s another of radiation’s secrets. The longer the wavelength, the harder it is to scatter.

Which means that radio waves are perfect for interferometry. As the longest wavelength we know of, they’re the best at staying parallel even after penetrating the atmosphere and getting bounced around in the individual telescopes.

I know you’re probably dying to hear more about radio astronomy. Don’t worry—just two posts on cool astronomy tech to go, and we’ll get there!

About Emma

I'm a college student, astronomer, writer, Democrat, global warming activist, hardcore trekkie, daring adventuress, and dreamer. I have four blogs to date. The primary one is Science at Your Doorstep, but I also post to The Sojourns of Star Trek, Common Sense Politics (US), and Toleventures. I guest post to A Momma's View. I write for different reasons. Sometimes because I want to share my process of discovery with others. Sometimes because I think a story is too incredible to keep it to myself. Sometimes just because I want to be heard. Regardless, I hope you enjoy it!
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4 Responses to Interferometry

  1. Simon says:

    I liked this, put simply and effectively ☺️

    Liked by 1 person

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