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W2 Part 3 - How Telescopes Work

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AstroTech : les aspects scientifiques et technologiques des découvertes astronomiques

Université d'Édimbourg

4.7 (309 notes) | 5.7K étudiants inscrits

"How do they know that?" Modern astronomy has made some astonishing discoveries - how stars burn and how black holes form; galaxies from the edge of the universe and killer rocks right next door; where the elements come from and how the expanding universe is accelerating. But how do we know all that? The truth is that astronomy would be impossible without technology, and every advance in astronomy is really an advance in technology. But the technology by itself is not enough. We have to apply it critically with a knowledge of physics to unlock the secrets of the Universe. Each week we will cover a different aspect of Astronomical technology, matching each piece of technology to a highlight science result. We will explain how the technology works, how it has allowed us to collect astronomical data, and, with some basic physics, how we interpret the data to make scientific discoveries. The class will consist of video lectures, weekly quizzes, and discussion forums. Each week there will be five videos, totalling approximately 40 minutes. They will be in a regular pattern - a short introduction, an example science story, an explanation of the key technology area, a look at how the technology is used in practice, and finally a look at what the future may hold.

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4.7 (309 notes)

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NP

Dec 02, 2016

Provides new insights for Dark Energy and Dark Matter and also helps one understand the basic Physics as applied to Astronomy.Really nice course !

GP

Mar 03, 2017

A very interesting course for beginners and a very good first step in pursuing deeper knowledge. Thank you Edinburgh University and Coursera

À partir de la leçon

Telescopes and Stars

How telescopes work, how we use them to make images in different colours, and how we use those measurements to understand how stars work.

W2 Part 1 - Picturing the Universe4:06

W2 Part 2 - The Sun - is every star like it?2:43

W2 Part 3 - How Telescopes Work13:12

W2 Part 4 - We are Stardust7:11

W2 Part 5 - Even Bigger Telescopes?1:22

Enseigné par

Andy Lawrence
Catherine Heymans

Transcription

[BLANK_AUDIO]

So in a moment we'll see how to actually make an image but first of all, the

very first job of a telescope, is simply to collect and concentrate the light.

And we'll see in a moment why that explains why telescopes are so big.

But let's have a little look at how a telescope works.

[SOUND] So very, very simply, we've got a, a big primary mirror, it's got a

hole in the middle of it typical telescope and we'll see why in a second.

Up here we have the so called the

secondary mirror, light is coming down from the sky.

It reflects from the primary mirror onto the secondary mirror and then again

down through the hole in the mirror and comes to a focus here.

That's where we place our detector, for example a CCD camera.

Now, the CCD detector may be quite small, but the amount

of light we get is determined by the size of the mirror, not by the size of the CCD.

To understand why this is important let me draw a different picture.

[SOUND].

Last week we saw how stars are very faint because the light from a star

spreads out as it moves out from the star in concentric spheres.

And so the light is gradually spread over a larger and larger spherical surface.

[SOUND] Until when we're a long way away, here at the Earth where its wave

fronts are almost parallel, and here is

the mirror of our telescope catching the light.

Now, if you have a bigger mirror you catch a larger

fraction Of the light coming from that star, so the amount

of light we get is proportional to the area of our mirror,

which is likewise proportional to the square of the diameter of the mirror.

So a mirror that is twice as large, will catch four times as much light.

Engineers tell us that to make a telescope with twice as big a mirror diameter.

In order to pay for the bigger telescope, the bigger building,

all the supporting infrastructure, it costs about eight times as much.

It goes as the cube of the diameter.

So big telescopes are very expensive.

Also bigger telescopes present more severe engineering problems, both the telescopes

and the mirrors tend to bend more as they get bigger.

Gradually during the course of the 20th century and the 21st century,

we've mastered those engineering problems, so

we can build bigger and bigger telescopes.

Another problem with big mirrors is that they tend to

bend under gravity as you tip them in different directions.

We solved that problem during the 1990s and 2000s.

The secret was to push the mirrors back into shape as

you tip them around, and there's two ways of doing that.

One is to have a segmented mirror made up of many separate sections that

you can move independently, that was what

was done with the Keck Telescope in Hawaii.

The other way is to make a very thin mirror which you put rugs behind

and push at to push it back into shape as you tip it around the sky.

And that was what was done with the very

large telescope built by the European Southern Observatory in Chile.

The result is that we now have telescopes that are eight to ten meters across.

Okay.

So how do we make an image?

A telescope is really just a camera.

Very much like this one here.

So, a camera is fairly simple.

We've got a lens, the light comes through the lens and

is focused onto a detector at the back of the camera, here.

So, a telescope is really just the same.

Now, sometimes when you think of a telescope, you think

of a sailor's telescope, and that's got an eyepiece, as well.

But the reason for that is that the eye is also a camera.

It's a lens and a detector, which is the retina.

And the job of the eyepiece is to straighten out the light

again, so that the eyeball can focus the light on the retina.

But when we're using an astronomical telescope, we

don't look through it, we just record the image.

So it's really much simpler, there is no IPs it's just a camera.

Now with a, an astronomical telescope we use a reflecting mirror not a

lens, that's because giant lenses would be much too thick um,and various

other reasons, but I'm going to draw as if it is a lens,

because it is easier to draw So imagine we have a lens here.

The light is coming in here, and it's brought to a focus here.

So that's the basics of what we're doing.

Now [SOUND], imagine we have light coming from

a different direction in the sky, a different angle.

So here's some other light coming in.

Here, so that's also brought to a focus but at a slightly different spot

on a flat region here that we call the focal plane.

So different regions, different directions on the sky are

mapped onto physically different locations on the focal plane.

So that's where we put our detector, and it records an image.

So if we have some weird shape here, that

weird shape is duplicated in miniature on the detector.

That's what an image is.

Different lenses have a different focal length.

So some might have a short, stubby focal length.

Some may focus a larger distance away.

Now for a difference in direction this will make

a small spread and this makes a larger spread.

So the fineness of scale of the image we get depends on

the focal length of the lens or mirror in a normal astronomical telescope.

So a longer focal length means the light is more spread out.

We get a finer scale image.

So that's why Victorian telescopes, mid-twentieth

century telescopes tended to be long tubes.

So how do we keep our pictures sharp?

There are three different effects that can blur our otherwise perfect pictures.

The first one is diffraction.

Now this is an inescapable physical effect that will

happen with any optical system, any lenses or mirror.

Light waves reflecting from different parts of the mirror

interfere with each other as they arrive at focus.

And leaves you with some net blurring.

Now physics tell us that, that effect is worse for longer wavelengths,

and it's also worse for smaller Mirrors or, or, or lenses.

So if we, for the moment if we just stick

with visible wavelength light of let's say 500 nanometer wavelength.

Then for example let, let's look at the human eye

so the pupil of the eye has about 5 millimeters across.

And that has diffraction blurring of about 25 arcseconds.

Now 25 arcseconds is the perceived size of a human head

at about one kilometer away, so that's the best you can possibly resolve.

If you had a larger telescope, if you had telescope one meter across.

Then in principle, that could give you a blurring

size one tenth of an arc second across, much smaller.

Unfortunately, we don't normally get that lovely tenth arc second resolution,

because of the second factor, which is atmospheric Blurry so as

the light comes down through the atmosphere refraction from different parts

of the atmosphere makes the, the the wave fronts jiggle about.

And the, if you take a very fast movie as

you can show and see in this example movie here.

The image of a star, actually waggles about at

this twinkling is what astronomers refer to as seeing.

Averaged over time, that will make

a blurring that's much worse than diffraction,

and a typical site like Edinboro, it might be several long seconds across.

Even on a good mountaintop observing site it's about one arc second across.

So how do we get round that atmospheric blurring?

Well, there's two ways.

The first way is that we can get rid of the atmosphere.

We can launch our telescope into space, like with the Hubble Space Telescope.

And the Hubble space telescope does,

indeed, get that tenth arcsecond resolution.

And make beautifully sharp pictures.

The other way we can do it is to watch a bright star in the sky, as fast as we can.

And track the image motion that it's making.

And try to correct for that with optics that we

can bend in fast time to counteract that image motion.

That's known as adaptive optics.

[SOUND] So now there's one more type of effect.

Which blurs our pictures and that's optical imperfections.

Now any optical system is never going to be perfect.

Then, to, to get perfect imaging, at all angles for all mirror

sizes for, all temperatures and so on is, is actually physically impossible.

So and, and in the design you have to

compromise and optimize at some chosen set of parameters.

However if you have errors in your

design or there's imperfections in the smoothness of

the mirror or its shape isn't quite right,

then you're only going to make things worse.

So for example the human eye which I said in

principle can give diff, diffraction

limited resolution of twenty five arcseconds.

It suffers from what's known as spherical aberration.

The imaging is imperfect and in fact our eyes

can resolve about one arcminute, or about sixty arcseconds.

Likewise, as some of you may have heard, the

Hubble Space Telescope when it was first launched had an

imperfection in the shape of its mirror which was not

quite right by the order of a millimeter or two.

But the result was that it did not get that [INAUDIBLE] resolution.

Now that was fixed by launching up on the space

shuttle some corrective optics which was put into the light path.

Before the detector.

So that was a very expensive fix but it was worth it.

As you can see in this picture I'm showing you here, it did make

a dramatic difference to the quality of the imaging by the Hubble Space Telescope.

So, how do we get that color information that Katherine referred to?

well, what I have here [SOUND] on this light box, is a

color image of the Orion Nebula where new stars are being formed.

Now, the way that color image like this is actually

made is it's made up of separate components added together.

It's really [SOUND] a blue image, the red image, and the yellow image

and they are added together to make the different color picture.

But as an astronomers, we want these three

images, or similar narrow a waveband images separately.

We want A blue image, we want a red image, we want a yellow image.

And for each one of those, oh, we do that by the

way, we have a filter we place over the top of the detector.

It's not so different, they'll be a blue filter, a red filter, and

a yellow filter; something which restricts the

range of wave lengths hitting the detector.

We take one image at a time, and for each

one of those we then simply have numbers around our detector.

So we can measure the brightness of a given star in the red, in

the, in the blue range, in the red range and in the yellow range.

And the way we get at the physics is to take the ratios of those numbers.

Okay, we take the ratio of the blue to the red and then

there'll be some physical prediction from theory about what those numbers out to be.

For instance, in most normal stars, the ratio blue and

red will tell us the surface temperature of the star.

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