 Greetings and welcome to the Introduction to Astronomy. In this lesson, we are going to talk about optical telescopes. So as we start working on instrumentation in astronomy, the first thing we want to look at is an optical telescope. Now optical telescopes are those that look at visible light. So there are different ways to be able to collect visible light from space. And there are other ways that we will look at to detect other types of electromagnetic radiation. So starting to look at optical telescopes here, first of all we want to define what is a telescope. So a telescope itself really, what is this? And a telescope is just a device that is used to observe astronomical objects. And it can gather light and or other types of electromagnetic radiation and brings that light to a focus. So it's a way to kind of enlarge what we enlarge the collecting area, be able to see fainter objects. So it allows us to see things that are fainter than we would be able to see with our eyes. So earlier before the development of the telescope in the early 1600s, everything was done just visually. You had to look at the sky and see what you could see. Now that once we developed a telescope, we had ways of being able to observe fainter objects that we would not be able to see at all. Now let's look at some of those early telescopes here. And we have the invention of the telescope. So the telescope was actually invented in 1608, but it was not invented by Galileo. Galileo is often credited or given credit for the invention of the telescope. And we see some of his telescopes here imaged. But Galileo heard of this invention and then made his own telescope. What Galileo did do was he was the first that we know of to observe the sky with the telescope and to record his observations. So we actually have records of Galileo's observations that have been, that exist to this day. So even what he observed over 400 years ago, we can still see. And he used very small telescopes like these, things that were about a half an inch in diameter. So while they're a lot longer here, the lenses were only a half an inch. But the half an inch compared to what you can see with your eye, the half inch is much larger than the pupil of your eye, which is the part that is actually collecting the light. So this allowed Galileo to see things that would otherwise have been invisible. Now let's look at some types of telescopes. And we have two primary types of optical telescopes. And those are the refracting telescope, which uses lenses to bring things to a focus. So what you'd have is a lens here, and the light would come through that, and you would have light traveling through the lens and then would get bent and brought to a focus. So the light coming in here would get bent and would be brought to a focus here and allows you to gather more light than you would otherwise be able to see. A reflecting telescope, on the other hand, uses a mirror. So we'd have a curved surface of a mirror here, look at a nice big mirror, and we'd do the same kind of thing. We would then be able to have light come in, reflect off this mirror, and be brought to a focus. So right here, all that light is brought to a focus from all over the mirror, and we now are able to collect light from much larger areas. So instead of just a little bit of light that can come through our eye, we can actually see light that is from a much larger section, and we'll see some of these telescopes have diameters, not just the half an inch to an inch that Galileo used, but actually meters and tens of meters across now. So let's look at the different types of a foci that we can use for these telescopes. And we have several different types here. We're going to look primarily at optical, sorry, at reflecting telescopes in this case. And here we would see is that in a reflecting telescope, we have a couple of different ones that are often used. There is a prime focus where you don't need anything additional. The light comes in, strikes the mirror, and comes up to the focal point. So you could put some kind of device or detector there to be able to collect the light. Now you might think that's relatively inefficient, especially for a small telescope. If you put your head there to try to view, then you're blocking out some of the light. Now for very large telescopes, you can do this because when you have a telescope that is 10 or 12 meters across, then blocking out a little bit of the light really isn't a big deal. If you're looking at a few inch telescope, then blocking out that light is a big deal. You're going to be blocking a big portion of the light. Now the second one to look at is what we call the casagrain focus. And this takes a secondary mirror. So the light bounces off the mirror just as it did here and goes back and is heading towards a focus at some point here. But instead, we put a secondary mirror to block that light, bounce it back, and through a hole in the primary mirror and bring it to a focus right behind the mirror. Now that's convenient because you can then view the object. You're putting your eye right here. You're not blocking any additional light. You are blocking a small amount, but you can put a relatively small secondary mirror here to reflect that light backwards. Now the third type is a Newtonian focus which also uses a secondary mirror, but instead of putting it straight in, it's angled and then the light comes off to the side. So it would depend on the type of viewing and the type of telescope that you wanted to do. Either one of these could be more convenient or less convenient, depending on exactly how the telescope is working and what you wanted to do with it. But a telescope would be designed generally with one or the other of these. Now telescopes have certain powers that they use, that they attain, and we want to look at these three because these are very important. We have the light gathering power of a telescope. This depends on the square of the size of the primary lens or mirror. So a larger telescope is better. A larger telescope will have a larger light gathering power. The bigger the telescope, the fainter objects will then be able to see. So the light gathering power allows you to see things that are fainter. The resolving power is the second power of a telescope. It also depends on the size of the primary lens or mirror. And a larger lens or mirror will have a smaller resolution, but a smaller resolving power is good. So smaller resolution means better resolving power. So, again, you need a large telescope will allow you to see more detail. It will allow you to see objects that are closer together. So they would allow you then to see more detail. Objects that are closer together would be able to be resolved in a larger telescope that could not be seen in a smaller telescope. Now finally we have what we call the magnifying power. This is the least important of the powers of a telescope. It depends on the focal length of the primary mirror lens and the focal length of the eyepiece. That means that you can change the magnifying power by changing the eyepiece. But it also is not as important because magnifying does not change the light gathering power or the resolving power of the telescope. So when you magnify something that's faint, you're spreading out the light even more and a faint object would not even be able to be seen if you magnify it too much because you're not getting any more light. You're just spreading out that light into a larger image. Resolving power also does not improve with magnifying power. When you increase the magnifying power, you still are not able to see any more detail. So the magnifying power is generally the least important of a power of a telescope. And if you see a telescope that you're looking at that is specifying its magnifying power, it might not be the one you want to look at. You really, if you're going to buy yourself a telescope, you want to look for the biggest telescope that you can afford and that you would reasonably use. So that means that in a telescope that is easy to set up and put down that will not take you a lot of time, so you'll actually use it. Now let's look at some of the limitations of a telescope. So we have some of them here. And one of the primary limitations, for example, of a refracting telescope, we'll look here first, these are refracting telescopes for the first couple here. And one of those is chromatic aberration, which means that different color are brought to different foci. They do not focus at the same place. And that is shown in the sketch here. Essentially, the lens, the top and bottom of the lens act as a prism. And a prism will bend different colors of light by different amounts. So the blue light gets bent more and is brought to a focus here. The red light is bent a little bit less and is brought to a focus here. And that means, depending on where you put your eye or your detector, you will then only be able to see some of the light brought to a focus, not all of it. So you're not able to focus all of the light in any kind of refracting telescope properly. Now there are some methods around this where you can put multiple lenses together to be able to minimize but not eliminate this. There's also a limited size to a telescope. And this really refers to refracting as well. It's easier to build a much bigger reflecting telescope because you need to be able to support the telescope and move it. In a refracting telescope, you can only support the lens around the edges. You can't support it from behind because that would block the light path. In a mirror, you can support it from behind, which is why we're able to build much larger reflecting telescopes than refracting telescopes. Now one of the biggest limitations for all telescopes is the seeing. Now seeing is just the Earth's atmosphere. So when we talk about seeing, that is the blurring effect of the turbulence in the atmosphere. So waves come in. They're all nice and straight and smooth, having traveled through empty space. But they come through the atmosphere and now they're all disturbed. So the wavelengths you see down here are not nice and parallel anymore. They're all spread out and wiggly, and that means it's much harder to get a good clear image. If you have seen twinkling of stars, if you've ever watched a star, twinkle, that is caused by this turbulent atmosphere. If you go to the International Space Station or go to the moon and look at the stars, you will not see them twinkle at all. It is only an effect of the Earth's atmosphere. So the only reason stars twinkle is because of the Earth's atmosphere. But the seeing is a blurring effect, and this limits the resolution of any telescope here on Earth. Now how can we get rid of that? Well, we can minimize it by some ways, and one of those is what we call active or adaptive optics. So these are ways that we can build much thinner mirrors or segmented mirrors, multiple mirrors, little mirrors put together that are controlled by computer. And we can then deform the shape to account for the atmospheric turbulence and eliminate that. And the image here shows that we're not shooting anything at the center of the galaxy here, but we are shooting a laser up into the atmosphere and essentially creating an artificial star. We can then observe that star, and we know its properties, so we can then deform our mirror. So we send that information of those observations back to the computer, which then deforms the mirror to make the image of this artificial star correct, and therefore anything else being observed through the atmosphere in the same direction at that time will also be corrected, and we can get resolution significantly better than we could otherwise get here on Earth. So making these thinner mirrors and being able to deform them and having computer control is very important for being able to increase our resolution. So let's look at some modern astronomical telescopes here. We have, first of all, let's look at the Hale Telescope on Mount Palomar, one of the oldest and for a long time the largest telescope. This is a 200 inch mirror or about a 5 meter telescope that exists. So the 200 inch mirror and for a number of decades, this was the largest telescope in existence. You can see the telescope portion up here and some of the control mechanism that was used to be able to steer it. So this is one of the largest telescopes and is still in use today. About a 5 meter telescope is getting to be relatively, we're starting to get ones that are much larger than this though. So if we look now, we also have what we call the very large telescope, which was completed in 2000 and has a set of four 8 meter mirrors. So significantly larger here and you can see each of those is in a separate dome here. And this is actually in the southern hemisphere down in Chile. So there are four telescopes here that can be used either together or individually to be able to observe astronomical objects. Now if you recall the fainter objects we can see depend on the size of the telescope. So therefore getting a telescope that is a little bit larger than the Hale telescope now means we can see fainter objects. So the very large telescope would be able to see fainter objects than the Hale. We also have what we call the large binocular telescope, which was completed in 2004 and has two 8.4 meter mirrors. And you can kind of see how it gets its name here. It acts as a pair, almost as a pair of binoculars, two telescopes there that can stare out at space. And again working together that gives it a much larger range of what it's able to be able to be seen. So this has two of 8.4 meter mirrors, even a little bit bigger than the very large telescope. Now another large telescope breaking the 10 meter barrier is the Grand Telescope Canarias, which was completed in 2007 and includes a 10.4 meter mirror. So now we are more than twice as big as the Hale telescope, and that means it could theoretically get twice the resolution, see twice as much detail, and could gather four times as much light or see things that are four times fainter than the Hale telescope possibly could. So much larger telescopes and other large telescopes are being planned. So one we'll look at here is the European Extremely Large Telescope, which is projected to be completed in 2025 and is going to have nearly a 40 meter mirror. So a very, very large mirror and why we call it extremely large because it is. And that will have essentially nearly eight times the resolving power of the Hale telescope, the 200 inch that dominated the telescope sizes for a number of decades. And if it's about eight times larger, it will have 64 times the light gathering power and able to see things several magnitudes fainter, about four magnitudes fainter than the Hale telescope. So where do we want to build a telescope? If we're going to put these telescopes, where are we going to build them? Well, we have to look at a couple of things when we're talking about where to put a telescope. One, we want to look at the weather. We want areas that are going to have clear weather. Putting it in an area that is often cloudy, often rainy, often snowy is not going to be efficient because there's going to be lots of days where you simply cannot observe. So we want things that have clear weather. We want to avoid water vapor, which is absorbing some of the light, especially the infrared. So if we want to observe in the infrared, we can do that for some of that from the surface of the Earth as long as we avoid water vapor, which means getting up high in the atmosphere. So we also need dark skies. We don't want to observe near a city. A hundred years ago, a lot of large telescopes were built near cities. So outside of Chicago, outside of Los Angeles, we had telescopes. And it was not such a big deal then. But putting telescopes in those areas now is not a good thing. So we tend to put them out in the deserts. So things like Arizona, the desert southwest is a very good place for telescopes because we get relatively dark skies. We get good weather most of the time, and we're up above most of the water vapor. We also put a lot of telescopes out in Hawaii. It may not sound good for water vapor there, but when you're up high on the mountains of Hawaii, you really do not get a lot of, you're only above most of the water in the atmosphere. And when you're above the atmosphere, you also get better seeing. The less atmosphere you have to look at, the less adjustments you have to make to be able to see clearly. So where do we put the telescopes now? Again, Hawaii is a big one. Southern Arizona is a good one, and some of the desert mountains in Chile are some of the primary ones where we have telescopes today. So let's finish up our section here with our summary. First of all, what is a telescope? Well, they're used to gather light from distant objects and bring it to a focus. The three powers of a telescope were light gathering power, resolving power, and magnifying power. And if you recall, magnifying power was the least important of these. Modern telescopes were able to make them much thinner and much larger than we were previously able to as technology has improved. So that concludes our lecture on optical telescopes. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.