Video Transcripts: Telescopes

Looking at the night sky with just our eyes is limited, as we saw.  We couldn't really answer fundamental questions like is the sun at the center of our solar system, or is the Earth at the center of our solar system, unless we had a tool like the telescope.  So that's what we're going to look at in this video is telescopes, how they're made, how they work, the different types of there are.  Telescopes have changed a lot since Galileo first turned his telescope to the night sky.  They've gotten bigger, they've gotten better, and it's truly amazing what we can discover when we use these telescopes.

Take the first picture in our gallery, for example.  This is what I consider a typical, nice telescope that someone might buy and just have in their backyard.  This telescope is probably about eight inches, or maybe 20 centimeters across the diameter.  And if you look through this telescope, you can easily observe the rings of Saturn, the moons of Jupiter.  The Moon would look spectacular; it would fill your whole view.  You could probably even see some galaxies, although they would look kind of fuzzy; you wouldn't see much detail.  But this telescope is way better than the telescope that Galileo used to discover the phases of Venus.  So even a simple, what we call an amateur telescope, really is ten times, or more, better than Galileo's telescope.  So it's amazing how far we've come.  And when you consider professional telescopes, the ones that are used by astronomers, Galileo, I don't think he could have even imagined how far the technology has come. So let's consider the different types of telescopes that are possible.

The second picture in the gallery, you'll see three main categories for telescopes.  The first is what we call a refracting telescope. This is a case where the light comes in through a primary lens and as the light comes in, you know, a lens is kind of a curved piece of glass, and the light bends and it comes to a focus.  And so that light focuses down through the telescope and in this case, bounces off a small mirror, all that does is just redirects the light upward to where your eye can be.  And then there's another small lens, or maybe a couple of little small lenses that are called the eyepiece.  And what those do is they just kind of re-correct the light so that your eye can see it well. 

In practice, what this is doing is it's making it as though your eye is as big as that primary lens. It's like expanding the size of your eyes. That's essentially what a telescope does. But notice how the vast majority of the telescope is just empty space.  There's really not much inside the body of the telescope.  The key is really these optical elements, these lenses that are in there.  So a refractor uses the principle of refraction, light bending through glass, to cause an image, and so you can see through it. 

Now in contrast to that, we have a reflector.  A reflector has the light pass through the empty tube and then bounce off a large curved mirror. So the light doesn't pass through the mirror, it just bounces off the surface of the mirror.  It reflects. So we call it a reflector. It first bounces off a primary mirror and the light starts to focus down the same way it does in the in the refractor, and then it bounces off a secondary mirror, which also bends the light a little bit, and again to some eyepieces, or lenses, so that your eye can look through and see.  So the different primary difference here is how the light is focused.  In the first case, it's focused by passing through glass. In the second case, it's focused by bouncing off a mirror. 

The third case is a compound telescope, which is basically just saying a combination of the other two.  As the light comes in, it bends a little bit through a very thin piece of glass; it refracts a little bit, so there's some refraction.  And then it bounces off of a mirror so there's some reflection.  It bounces off the secondary mirror and back through the bottom.  

And now what I want you to notice is the advantages of these though.  In the case of a refractor, all the light is focused as it travels down the tube.  It bends in the lens and then keeps going and focuses down.  In the case of the reflector, the light bounces off the mirror, and then bounces back, and then up.  So in a sense, the light is traveling twice through the tube of the body.  What that means is that the tube can be shorter.  In a sense, the light is being focused.  It takes a while for that light to come to a focus, and so if you can let the light travel back and forth in the tube twice, then the tube can be smaller, which can make it a little cheaper.

And you'll notice in the compound telescope, it's actually doing it three times because it bends a little bit through here, bounces off the first mirror, so that's one travel, two travels, and then bounces straight back through the middle, three travels.  So now the tube can be even shorter. And again, that makes it even cheaper, you can make a big telescope that's shorter, so you can make it cheaper.

This highlights the challenge of building telescopes over the years.  The earliest telescopes that were built for astronomy were often refracting telescopes.  It was thought those give the best image quality.  And so the problem is, well, there are several problems with refracting telescopes.  One of the biggest problems is that they have to be very long.  And so the third picture in our gallery shows the largest refracting telescope ever built.  It's actually in Wisconsin, and I had a chance to visit this telescope, it's an amazing sight.  It is a huge telescope, as you can see. And it's so big that there's no way for anyone to actually look through the telescope.  There was a ladder.  The ladder would be so tall it would be dangerous to climb the ladder to look through the telescope so the entire floor of this observatory is an elevator.  And the as they move the telescope, the whole floor goes up or down so that you can set up your camera or you can look through the telescope.

As you can see, it's ridiculously long and the lens at the front of this telescope is about one meter across – so three feet across.  That's a big lens.  But if you want to make telescopes bigger than this, then you'd have to make them longer and longer.   And already this one is kind of ridiculously long. And so there's a reason this is the biggest refracting telescope; it is if you want to make it bigger, you really want to make a reflecting telescope instead.  

So reflecting telescopes have become sort of the state of the art in astronomy, really for, maybe fifty or a hundred years or about.  And they've gotten really, really big.  So this is one of the largest primary mirrors in the world.  And you can see the person in the middle of it working on it polishing it.  So this is an example of a mirror that's going to go into one of the biggest telescopes.  This mirror is about eight meters across, like 24 feet across.  And that represents kind of the state of the art, biggest mirrors that can be made for telescopes.

Now, there's a couple questions here, like, why do you want big telescopes?  Why would you want them bigger and bigger? Well, there's several reasons that you might want bigger and bigger telescopes.  One reason you might want bigger telescopes is simply, objects in the sky are faint. They're far away, they don't give up very much light and so the bigger the telescope is, the more light it can collect.  And so it's like a light bucket that just keeps gathering more and more light and the bigger your bucket is, the more light you can get.  So you can see fainter things. That's one reason. 

Another reason is that there's something, this thing is called resolution. The idea is there's fine details in all of these things in the sky.  And if you want to see those really fine details, the telescope has to be bigger and bigger.  So for example, if you want to see individual stars in another galaxy, those are so tiny from our vantage point, that the only way you can ever see them is by having a really big telescope that can resolve those fine details. 

And the last advantage of having a really big telescope is that you can magnify objects to make them even bigger.  So the amount that you can zoom in and magnify an object in the sky, they're far away, they're small in the sky, you want to make up look bigger.  A big telescope can help you do that.  But by far, the most important reasons for having a big telescope is you collect more light and you can see more detail.  

So you look at this fourth picture in the gallery, which is one of these largest mirrors in the world and you say, “Wow, that's a big mirror, but no big deal, I got mirrors in my house.  I assume it's not very difficult to make a huge mirror.”  It turns out that it is extremely difficult and very expensive to make one of these giant mirrors.  And you might ask why?  Well, the mirror has to have a very precise curvature to it. Very, very precise, because the light has to be focused all down to a single spot.  It’s one thing to curve, like, if you were to make a mirror that's as big as my hand, you want to make it a precise curvature, you could do that.  You can just kind of grind at it, keep polishing it, make it just right.  But how do you make a giant mirror, bigger than several people, and have the whole thing have a very precise curvature.  It turns out it is really hard.  And astronomers have developed an ingenious way for doing this. 

This fourth picture is from the mirror fabrication lab, which is at the University of Arizona. And what they do is they have a giant rotating oven.  And they put all this glass that's going to turn into the mirror into this rotating oven and they spin it, very slowly at first, and they heat the glass, and they spin it faster while the glass is molten.  And then they cool it off and let the glass harden while it's spinning, and as you know, as something is spinning, it kind of like spreads out. And so this causes the glass to spread out in this curved shape, which happens to match the kind of curvature that you need for a telescope mirror.  So once they get that ‘blank’, it's called, like that piece of glass, then they can polish it.  And they polish it for weeks and weeks and months to get it perfectly smooth before they put on this reflective coating.

So it's a very difficult long process.  I once got a chance to visit this place. And when I was there, it didn't have the mirror shiny stuff on it, it was just the glass.  And there are these two guys laying on this thing on their stomachs.  They took off their shoes, they were just laying on their stomachs and they had a little microscopic eyepiece and they were looking on a huge piece of glass, this whole size, they were looking for tiny little cracks in the surface of the glass.  And they're doing this over the course of days and weeks, just going bit by bit looking for tiny, microscopic cracks.  So it's an incredibly difficult process making these very precise mirrors.

I want to finish up here by showing you the last picture in the gallery, which is one of the largest telescopes in the world; the largest observatories in the world.  This is in Chile, on top of the mountains there in the desert and each of these buildings contains one of these eight meter telescopes. These are huge telescopes inside each of these buildings.  And one of the amazing things that we can do with light and with telescopes is we can actually connect multiple telescopes together so that they work together as though they were one telescope.  So these telescopes are each about eight meters, but they can be connected together and act like an even bigger telescope.  And that's sort of the future the frontiers of astronomy.  As we continue to go forward, the way that telescopes will get bigger and bigger is not by making bigger mirrors necessarily, but it's by connecting those mirrors together and creating even bigger telescopes. 

Alright.  Very cool stuff. And we'll see you next time.