Telescopes
- 2. • • • • • •
Notas do Editor
- Here are the most important things about telescopes. Again, we will come back to this, so don’t worry about memorizing everything right now.
- Let’s start with s short video about telescopes, give you a taste of what they can do…
- The telescope has been around for about 400 years now. Galileo actually stole the idea for the design of the telescope, but he was the first to point it upward, rather than across into some neighbors window, or more likely, for purposes of battle.The person who was credited with the actual invention of the telescope was the Dutch optician, Hans Lippershey, in 1608. He put two eye pieces together and noticed that objects appeared closer. Glasses for reading had been around for a few hundred years in Italy, so he just made lenses like the kind in glasses. He built a tube for his new device, which he called “the looker.”
- Before Galileo began observing the sky through a looker, astronomers observed the sky just with their naked eyes. They could make detailed observations of the movement of the stars, and they did incredible things, documenting this movement. Here is an observatory used by the Mayans, called El Caracol, also known as "The Observatory," at Chichen Itza, Mexico. The Mayans, as well as the Chinese, the Gaelic, the Persians, made great strides in recording and predicting the things they saw in the sky. This was science, but it had its limits. When it came to the mechanisms behind these movements, they could only guess, make hypotheses, but there was no way to test these hypotheses. After the invention of the telescope, all of a sudden, hypotheses about what was making the sky do the things it did could actually be tested! For example, Copernicus had proposed that the Earth went around the Sun, rather than the other way around, but the Catholic Church, which was the dominant power in Europe, thought that no, we are the center of the Universe, so everything must go around us. Galileo looked through his telescope and saw many things, including moons around Jupiter. Now how could there be something going around Jupiter if everything goes around the Earth? All of a sudden, there was evidence to support the claim that Copernicus had made.
- Nowadays, we have all kinds of telescopes, but they all basically do the same thing. They collect light. Because they are so good at collecting light, we see things in space brighter, in more detail, and sometimes magnified. We put cameras on the end of the telescopes nowadays, and those cameras can collect light for a long time, so that instead of getting an image like this.
- We get an image like this. Not too shabby.
- The telescope that Galileo built is called a refractor, and it looked a lot like this telescope (point out refractor). Well, it was brass, and old looking, but it basically worked the same. In this picture is a refractor, one of the largest in the world, that is just East of San Jose, at Lick Observatory. It has 36 inch lenses, that were made by opticians in the 1870s. That’s one of the lenses right there. That lens is still inside the refractor today. I highly encourage you to visit Lick Observatory if you ever get the chance. This is a beautiful telescope, and James Lick himself, who was a crazy guy, is actually buried at the base of the telescope!
- This slide shows you a schematic of a telescope. These are light rays, waves, like what we have been discussing. When the light leaves the object, lets say it is a star, they come in through the objective lens at the top of the telescope, which collects as much light as possible, and then refocuses it at a point, called the focal point; the distance to this point is called the focal length. When you look through the telescope, the image you are looking at is at this point.The lens at the bottom of the telescope is the eyepiece lens; it is smaller and has a short focal length. It re-straightens the light rays making them parallel once again. Because the light is more concentrated, objects are brighter as seen through a telescope. There are two focal lengths, the focal length of the objective lens and the focal length of the eyepiece lens. The distance between the two lenses (d) is just equal to the sum of the focal lenses (fobj + feye). If you are making a telescope like this, you need to know the focal length of each of the lenses you have chosen. Then you add them together, and put them in a tube about that length, at either end. Tadah, you have a refracting telescope!The ratio of the focal lengths gives you the magnification of the telescope. M= fobj/feye. So if the focal length of the objective lense is 1 meter, and the focal length of the eyepiece is 1cm, how much will the telescope magnify? (100 times).The curved surfaces of the lens cause light rays to emerge from the lens in different directions than they had before entering the lens. If the lens is shaped correctly, parallel light rays start converging once they enter the glass. Because the light that refracts through the lens is bent in the opposite direction, the image of the object that you would see through the eyepiece would be upside down and flipped. Astronomers don’t really care whether objects in space are upside down because in space there is no preferred up or down, also most objects in space are symmetric so it doesn’t really matter whether you see them upside down or not. Interestingly, the reason why you are able to see is because everyone has lenses in their eyeballs. The lens in your eye is curved the same way that the lens is in the telescope and acts to collect light and bring it to a focus on your retina. My question is that if the image is suppose to be upside down, why do we see everything right side up?
- So a few decades after Galileo put two lenses together in a tube, Sir Isaac Newton invented a new kind of telescope. His telescope had mirrors in a tube, instead of lenses in a tube. Because of this, we call it a reflector. This telescope is a reflector right here, it has mirrors inside. This telescope in the picture is also at Lick Observatory, but it was built in the 70s, not the 1800s. You can see a small mirror at the top, but there is also a huge mirror, 100 inches across, at the bottom.
- Reflecting telescopes are very simple. Light comes in through the top, and then bounces off a curved mirror at the bottom. It’s kind of like when you look into a spoon, and the light is bent weird. It then focuses the light it collects up to the top again. This length here is the focal length of the mirror. Again, the magnification is determined by the focal length of the primary mirror, divided by the focal length of the eyepiece you use.In the good old days, like 50 years ago and beyond, astronomers used to sit right there at that spot, in a tiny cage, all night long, in order to collect the virgin light without bouncing it off any more mirrors! They would sit there, and it was usually freezing cold, for entire nights. There are lots of stories at Lick about when they used to use the cage, and astronomers would have to pee, but they couldn’t get down because they were in the middle of an observation, and stuff like that. Or once someone turned on the lights when an astronomer was observing, and ruined a photo he had been taking for two hours, and the astronomer had a really hot temper, so he called for the telescope to lower down so he could get out of the cage and beat the guy up, but every time he called it, the other guy, who didn’t want to get punched in the face, would press the button on the bottom, and move it back up, so the guy had to stay up in the cage until he calmed down.
- Nowadays, astronomers either put an automated camera at the prime focus, or they use one of these other kinds of arrangements. The easiest focus is the Newtonian, which is what Newton used. The light comes in, gets focused by the big primary mirror onto a little mirror called the secondary mirror. Then it comes out the side where there is an eyepiece. I have built this kind of telescope, but I couldn’t bring it to show you guys, because it is over six feet tall, and so it doesn’t fit in my apartment. So it’s in a garage in Los Angeles. The second picture here we talked about, with the astronomer in the cage. The third picture is what is going on inside this telescope, its called a Cassegrain focus. The light is bounced right back toward a hole in the middle of the mirror, and there is an eyepiece there. The last one is very helpful if you have an enormous piece of equipment, one that doesn’t fit on the telescope, and needs its own room.
- There are two big reasons why astronomers prefer reflectors to refractors. First of all, refractors suffer from something called chromatic aberration. Different kinds of light have different wavelengths, remember, so they behave a little differently when they move through the lens. Therefore the focal length for each color is a little different. This makes images a little off, especially when you are working with big telescopes. So you have to buy these extra correcting lenses, and this gets expensive and complicated. There is another reason too.This is the Lick refractor again on the right. It’s about as big as you can make a refractor. A mirror is not see through, so you can support a big mirror underneath. But lenses are transparent, so you have to hold them up on the side. You just can’t support a very big lens the way you can support a very big mirror. So the biggest telescopes in the world, tend to be reflectors, with mirrors.
- Pictures like the ones you’ll see in your text were taken from telescopes like these here. These are some of the biggest telescopes in the world, called Keck. They are located on a remote mountaintop in Hawaii. The mirrors in these guys are really enormous.
- This is the one of the10 meter across Keck mirrors, with a human for scale. This isn’t even the biggest mirror in the world, that belongs to the Gran Telescopio Canarias in Spain. It’s 10.4 meters. Astronomers fight over this, when they hear someone is building a 30 meter telescope, they say oh I’m going to build a 31 meter telescope. True story. And true story about them building a thirty meter telescope too, it’s going to be HUGE.
- Remember from the lesson on light, that there are a lot of different kinds of light, dictated by the wavelength of the electromagnetic radiation. So we can use special telescopes to take collect light from various wavelengths, even the ones we cannot see with our own eyes. These pictures are of the same object, a dead star. This particular star actually died in an enormous supernova explosion in 1054, and was observed by ancient Chinese astronomers. Now, if you look at it through a telescope, it looks like a fuzzy cloud. With a nice telescope, and a fancy camera, it looks like the picture on the left, which was taken in the visible light wavelengths. The second was taken in radio waves by the National Radio Astronomy Observatory, and the third was taken in X-rays, by the Chandra X-ray observatory, a telescope in space. Look at what you can see in the last picture! You can see the neutron star, the incredibly dense core of the dead star, spinning incredibly fast, so fast that it is spewing material out at its magnetic poles. It is so powerful, that the light it gives off is very energetic, in the X-ray range. We cannot see the star, because it is surrounded by all this debris. But Chandra can see right through that stuff. So not only can telescopes help us see further, they can help us see things that would otherwise be invisible, at wavelengths that we can’t see!
- Chandra is in space…why would we put telescopes in space? A couple of reasons, but they are all related to the same thing: our atmosphere (it’s very pesky).
- First of all, our atmosphere blocks out most of the light that comes to us from space! This is a good thing for us, since it blocks out high energy wavelengths like x-rays that are harmful, and also a lot of UV. But it kind of sucks for astronomers who want to observe light in those wavelengths. So we put telescopes in space, above the atmosphere. This is why infrared telescopes, UV telescopes, and X-ray telescopes are almost all in space. Radio telescopes and visible telescopes are the only ones on the ground.
- Radio waves are not really affected by our atmosphere, so we can put radio telescopes on Earth. Radio waves are very big, and low energy, so we need very big telescopes, like this one. This is Arecibo, a huge radio telescope in Puerto Rico. It’s huge! 305 meters in diameter.
- How many people have hear of the Hubble Space Telescope? It is a visible light telescope. Visible light can get through our atmosphere, as we just saw. It is one of the most famous telescopes in the world. But why would Hubble be in space?
- Our atmosphere might let visible light through, but it doesn’t leave it alone. There is a lot of air moving around up there, and this can actually distort the starlight. Air changes light, the same way a lens does. Think about when you look through a water glass at a table, how distorted things look. The atmosphere is distorting starlight like crazy, and because the air is in motion, and always changing, that distortion is changing second to second. This is why stars twinkle. If you were in space, above our pesky atmosphere, the stars would not twinkle. It’s very pretty, but it’s very annoying for astronomers. So they put telescopes like Hubble in space.
- When we do put visible telescopes on the ground, they need to avoid atmospheric distortion, and they also need to avoid light pollution. This is my home city at night. I think its kind of pretty, but its definitely not a good place to look for stars. You have to get very far from people to get pristine skies. So to get super clear skies, with very little air movements, and far away from cities, astronomers like to put telescopes on mountaintops, where the air is still, and in remote places. Being in a desert is also a good thing, because there is less weather to move the air around, and cause clouds.
- CerroTololo Inter-American Observatory is located 50 miles from La Serena, Chile in the Andes
- Lick in the snow
- Keck on Mauna Kea, with Subaru on the left. Subaru is Japanese for a groups of stars we call the Plieades, present on every Subaru car. Also known as the 7 sisters.
- Sphinx Observatory in the Alps.