WEBVTT 00:00:03.000 --> 00:00:07.000 I’ve talked a lot about observing the night sky with your eyes; just simply going out 00:00:07.009 --> 00:00:11.370 and seeing what you can see. It’s pretty amazing what you can learn just by doing that, 00:00:11.370 --> 00:00:14.469 and of course that’s all we humans could do for thousands of years. 00:00:14.469 --> 00:00:17.680 But now we can do better. We can use telescopes. 00:00:17.680 --> 00:00:21.930 The first person to invent the telescope is lost to history; despite “common knowledge,” 00:00:21.930 --> 00:00:25.930 Galileo did not invent them. He wasn’t even the first person to point one at the sky, 00:00:25.930 --> 00:00:30.260 or the first person to publish results! But he was a loud and persistent voice over the 00:00:30.260 --> 00:00:34.789 years, and his amazing string of discoveries using his crude instrument landed him firmly 00:00:34.789 --> 00:00:38.249 in the history books. Aggressive self-marketing sometimes pays off. 00:00:48.380 --> 00:00:52.480 You might think the purpose of a telescope is to magnify small objects so we can see 00:00:52.489 --> 00:00:56.199 them better. That’s how a lot of telescopes are marketed, but to be honest that’s not 00:00:56.199 --> 00:01:00.869 exactly the case. If you want to be really general, the purpose of a telescope is to 00:01:00.869 --> 00:01:06.439 make things easier to see: To make the invisible visible, and to make the things already visible 00:01:06.439 --> 00:01:07.810 visible more clearly. 00:01:07.810 --> 00:01:12.729 A telescope works by gathering light. Think of it like a bucket in the rain: The bigger 00:01:12.729 --> 00:01:16.479 the bucket, the more rain you collect. If your bucket is big enough, you’ll get plenty 00:01:16.479 --> 00:01:18.630 of water even when it’s only sprinkling out. 00:01:18.630 --> 00:01:23.090 In the case of a telescope, the “bucket” is an optical device like a lens or a mirror 00:01:23.090 --> 00:01:27.950 that collects light. We call this device the objective, and the bigger the objective, the 00:01:27.950 --> 00:01:32.109 more light it collects. Look at your eyes… well, that’s tough, so let’s think about 00:01:32.109 --> 00:01:35.809 our eyes for a moment. They also work as light buckets, but they only collect light through 00:01:35.809 --> 00:01:40.389 our pupils, which even under the best of circumstances are less than a centimeter across; 00:01:40.389 --> 00:01:42.040 a very tiny bucket indeed. 00:01:42.049 --> 00:01:46.390 But we can do better. To extend the analogy, a telescope is like a bucket with a funnel 00:01:46.390 --> 00:01:50.619 at the bottom. All that light that it collects is then concentrated, focused, and sent into 00:01:50.619 --> 00:01:53.759 your eye. It turns a trickle of light into a torrent. 00:01:53.759 --> 00:01:58.079 The amount of light it collects depends on the area of the objective. That means if you 00:01:58.079 --> 00:02:02.350 double the diameter of the collector, you’d collect four times as much light, because 00:02:02.350 --> 00:02:06.780 the area of the collector goes up as the square of the radius. Make a bucket 10 times wider, 00:02:06.780 --> 00:02:11.250 and you collect 100 times as much light! Clearly, as telescopes get bigger their ability to 00:02:11.250 --> 00:02:14.010 show us faint objects increases enormously. 00:02:14.010 --> 00:02:18.879 In fact that was one of Galileo’s first and most important discoveries: Stars that 00:02:18.879 --> 00:02:23.069 were invisible to the naked eye were easily seen through his telescope, even though it 00:02:23.069 --> 00:02:27.299 only had a lens a few centimeters across. Those faint stars didn’t emit enough light 00:02:27.299 --> 00:02:31.699 for his eyes to see them, but when he increased his collecting area with a telescope, 00:02:31.700 --> 00:02:33.020 they popped into visibility. 00:02:33.030 --> 00:02:37.470 The primary way telescopes work is to change the direction light from an object is traveling. 00:02:37.470 --> 00:02:41.640 I can see a star with my eye because light from that star is sent in my direction, into 00:02:41.640 --> 00:02:46.569 my eye. But most of that light misses my eye, falling to the ground all around me. The telescope 00:02:46.569 --> 00:02:50.000 collects that light, bounces it around, and then channels it into my eye. 00:02:50.000 --> 00:02:53.269 When the very first telescopes were built, this changing of the direction of light was 00:02:53.269 --> 00:02:57.989 done using lenses. When light goes from one medium to another – say, from going through 00:02:57.989 --> 00:03:03.110 air to going through water or glass – it changes direction slightly. You see this all 00:03:03.110 --> 00:03:07.680 the time; a spoon sitting in a glass of water looks bent or broken. The spoon is doing just 00:03:07.680 --> 00:03:12.349 fine, but the light you see from it is getting bent, distorting the image. This bending is 00:03:12.349 --> 00:03:13.730 called refraction. 00:03:13.730 --> 00:03:18.290 The way light bends depends on what’s bending it (like water or glass) and the shape of 00:03:18.290 --> 00:03:22.420 the object doing the bending. It so happens that if you grind a piece of glass into a 00:03:22.420 --> 00:03:27.530 lens shape, it bends -- or refracts -- the incoming light in a cone, focusing it into 00:03:27.530 --> 00:03:29.860 a single spot. It’s a light funnel! 00:03:29.860 --> 00:03:34.040 This refraction has a couple of interesting results. For one thing, the light from the 00:03:34.040 --> 00:03:38.030 top of a distant object is bent down, and the light from the bottom is bent up. When 00:03:38.030 --> 00:03:42.189 this light comes to a focus, it means you see the object upside-down! It also flips 00:03:42.189 --> 00:03:46.610 left and right, which can be a little disconcerting, and takes getting used to when you’re using 00:03:46.610 --> 00:03:47.769 a refracting telescope. 00:03:47.769 --> 00:03:51.860 For another thing, the lens can magnify the image. That’s again because the light is 00:03:51.860 --> 00:03:56.569 bent, and the image created of object observed can appear larger than the object does by 00:03:56.569 --> 00:03:59.950 eye. It depends on a lot of factors including the shape of the lens, the distance to the 00:03:59.950 --> 00:04:05.220 object, and how far away the lens is, but in the end what you get is an image that looks bigger. 00:04:05.220 --> 00:04:09.740 That has obvious advantages; a planet like Jupiter is too far away to see as anything 00:04:09.750 --> 00:04:13.989 other than a dot to the eye, but a telescope makes it appear bigger, and details can then 00:04:13.989 --> 00:04:19.360 be seen. When Galileo and other early astronomers pointed their telescopes at the sky, multitudes 00:04:19.360 --> 00:04:24.360 were revealed: Craters on the Moon, the phases of Venus, Jupiter’s moons, the rings of 00:04:24.360 --> 00:04:28.910 Saturn, and so much more. The Universe itself came into focus. 00:04:28.910 --> 00:04:33.350 When astronomers talk about using a telescope to make details more clear, they use a term 00:04:33.350 --> 00:04:37.920 called resolution. This is the ability to separate two objects that are very close together. 00:04:37.920 --> 00:04:41.630 You’re familiar with this; when you’re driving on a road at night a distant car coming 00:04:41.630 --> 00:04:47.630 toward you appears as a single light. When it gets closer, the light separates out — resolves 00:04:47.630 --> 00:04:48.980 — into two headlights. 00:04:48.980 --> 00:04:53.970 A telescope increases resolution, making it easier to, say, split two stars that are close 00:04:53.970 --> 00:04:57.820 together, or to see details on the Moon’s surface. The resolution depends in part on 00:04:57.820 --> 00:05:01.580 the size of the objective; in general the bigger the telescope objective 00:05:01.580 --> 00:05:03.100 the better your resolution is. 00:05:03.100 --> 00:05:08.060 Resolution is more useful than magnification when talking telescopes. Fundamentally, there 00:05:08.060 --> 00:05:12.320 is a limit to how well your telescope resolves two objects, but there’s no limit to how 00:05:12.320 --> 00:05:17.280 much you can magnify the image. If you magnify the image beyond what the telescope can actually 00:05:17.280 --> 00:05:18.980 resolve, you just get mush. 00:05:18.980 --> 00:05:23.640 Refracting telescopes are great, but they suffer from a big problem: Big lenses are 00:05:23.640 --> 00:05:28.910 hard to make. They get thin near the edge, and break easily. Also, different colors of 00:05:28.910 --> 00:05:33.530 light bend by different amounts as they pass through the lens, so you might focus a red 00:05:33.530 --> 00:05:36.160 star, say, and a blue one will still look fuzzy. 00:05:36.160 --> 00:05:41.160 No less a mind than Isaac Newton figured a way around this: Use mirrors. Mirrors also 00:05:41.160 --> 00:05:45.260 change the direction light travels, and if you used a curved mirror you can also bring 00:05:45.260 --> 00:05:49.570 light rays to a focus. Telescopes that use mirrors are called reflectors. 00:05:49.570 --> 00:05:53.970 The advantages of reflectors are huge: You only have to polish one side of a mirror, 00:05:53.970 --> 00:05:58.650 where a lens has two sides. Also a mirror can be supported along its back, so they can 00:05:58.650 --> 00:06:03.030 be manufactured much larger more easily and for less money. Although there have been many 00:06:03.030 --> 00:06:07.080 improvements made over the centuries, most big modern telescopes at their heart are based 00:06:07.080 --> 00:06:11.990 on the Newtonian design, and in fact no large professional-grade telescopes made today have 00:06:11.990 --> 00:06:15.730 a lens as their objective. Nowadays, it’s all done with mirrors. 00:06:15.730 --> 00:06:18.580 And that brings us to this week’s aptly named Focus On. 00:06:18.580 --> 00:06:22.340 The most common question I’m asked (besides, “Hey, who does your hair?”) is, “Hey, 00:06:22.340 --> 00:06:24.170 Phil, kind of telescope should I buy?” 00:06:24.170 --> 00:06:28.220 It’s a legitimate question, but it’s very difficult to answer. Imagine someone walked 00:06:28.220 --> 00:06:32.370 up to you and asked, “What kind of car should I buy?” That’s impossible to answer without 00:06:32.370 --> 00:06:33.710 a lot more information. 00:06:33.710 --> 00:06:38.250 Same for telescopes. Do you want to look at the Moon and planets, or fainter, more difficult 00:06:38.250 --> 00:06:42.870 to spot galaxies? Are you really devoted to this, or is it more of a pastime? Is this 00:06:42.870 --> 00:06:44.440 for a child or an adult? 00:06:44.440 --> 00:06:48.310 These questions are critical. Most small ‘scopes are refractors, which are good for looking 00:06:48.310 --> 00:06:51.910 at detail on the Moon and planets (they tend to magnify the image more than reflectors 00:06:51.910 --> 00:06:55.930 do). But they’re tricky to use because they flip the image left and right and up and down. 00:06:55.930 --> 00:06:59.710 Bigger ‘scopes are good for fainter objects, but are more expensive, and can be difficult 00:06:59.710 --> 00:07:04.360 to set up and use. I hate hearing about a ‘scope that just collects dust because it was bought in haste. 00:07:04.360 --> 00:07:09.440 So here’s what I recommend: Find an observatory, planetarium, or local astronomy club. They’re 00:07:09.440 --> 00:07:13.660 likely to have star parties, public observing events, where you can look at and through 00:07:13.660 --> 00:07:17.690 different kinds of telescopes. Their owners are almost universally thrilled to talk about 00:07:17.690 --> 00:07:21.460 them — as an astronomer, I can assure that the problem with astronomers isn’t getting 00:07:21.460 --> 00:07:26.840 them to talk, it’s shutting them up — so you’ll get lots of great first-hand advice and experience. 00:07:26.840 --> 00:07:31.660 Also, I usually recommend getting binoculars before a telescope. They’re easy to use, 00:07:31.660 --> 00:07:35.960 fun to use, easy to carry around, and you can get good ones for less money and still 00:07:35.960 --> 00:07:40.030 see some nice things. Even if you decide not to get more into astronomy as a hobby, they 00:07:40.030 --> 00:07:43.750 can also be used during the day on hikes and for bird watching. I have a couple of pair 00:07:43.750 --> 00:07:45.520 of binoculars and I use them all the time. 00:07:45.520 --> 00:07:50.030 There’s a third aspect to telescopes that’s very important, beyond resolution and making 00:07:50.030 --> 00:07:55.020 faint things easier to see. They can literally show us objects outside of the range of colors 00:07:55.020 --> 00:07:56.280 our eyes can see. 00:07:56.280 --> 00:08:00.900 In the year 1800, William Herschel discovered infrared light, a kind of light invisible 00:08:00.900 --> 00:08:05.940 to our eyes. In the time since we’ve learned of other forms of invisible light: radio, 00:08:05.940 --> 00:08:10.430 microwave, ultraviolet, X-rays, and gamma rays. Astronomical objects can be observed 00:08:10.430 --> 00:08:14.170 in all these flavors of light, if we have telescopes that are designed to detect these 00:08:14.170 --> 00:08:17.930 flavors of light. Radio waves pass right around “normal” telescopes, ones that we use 00:08:17.930 --> 00:08:22.060 to observe visible light. X-rays and gamma rays pass right through them as if they aren’t even there. 00:08:22.060 --> 00:08:27.120 But we’re smart, we humans. We learned that giant metal dishes can and will bend radio 00:08:27.130 --> 00:08:31.690 waves, and can be formed just like gigantic Newtonian mirrored telescopes. In fact, different 00:08:31.690 --> 00:08:36.259 forms of light need different kinds of telescopes, and once we figured out how, we’ve built 00:08:36.259 --> 00:08:39.930 ‘em. We can now detect cosmic phenomena across the entire spectrum of light, from 00:08:39.930 --> 00:08:44.639 radio waves to gamma rays, and have even built unconventional telescopes that detect subatomic 00:08:44.640 --> 00:08:49.000 particles from space as well, such as neutrinos and cosmic rays. Because of this, we have 00:08:49.000 --> 00:08:52.220 learned far more about the Universe than Galileo could have imagined. 00:08:52.220 --> 00:08:56.460 And we’re in the midst of another revolution, too. The actual biophysics is complicated, 00:08:56.460 --> 00:09:01.259 but in a sense our eyes act like movie cameras, taking pictures at a frame rate of about 14 00:09:01.259 --> 00:09:04.639 images per second. That’s a short amount of time. Photographs, though, can take far 00:09:04.639 --> 00:09:09.649 longer exposures, allowing the light to build up, allowing us to see much fainter objects. 00:09:09.649 --> 00:09:14.529 The first photographs taken through a telescope were done in the 1800s. This has led to innumerable 00:09:14.529 --> 00:09:19.519 discoveries; for example, in the 20th century giant telescopes with giant cameras revealed 00:09:19.519 --> 00:09:24.589 details in distant galaxies that led to our understanding that the Universe is expanding, 00:09:24.589 --> 00:09:27.800 a critically important concept that we’ll dive into later in the series. 00:09:27.800 --> 00:09:32.459 And now we have digital detectors, similar to the ones in your phone camera, but far 00:09:32.459 --> 00:09:37.790 larger and far more sensitive. They can be dozens of times more light-sensitive than film, able 00:09:37.790 --> 00:09:42.339 to detect in minutes objects that would’ve taken hours or more to see using film. These 00:09:42.339 --> 00:09:47.029 digital cameras can also be designed to detect ultraviolet light, infrared, and more. We 00:09:47.029 --> 00:09:51.860 can store vast amounts of that data easily on computers, and use those computers to analyze 00:09:51.860 --> 00:09:56.009 that huge ocean of information, performing tasks too tedious for humans. Most asteroids 00:09:56.009 --> 00:09:59.720 and comets are discovered using autonomous software, for example, looking for moving 00:09:59.720 --> 00:10:04.259 objects among the tens or hundreds of thousands of fixed stars in digital images. 00:10:04.260 --> 00:10:08.960 This has also ushered in the era of remote astronomy; a telescope can be on a distant mountain 00:10:08.960 --> 00:10:12.960 and programmed to scan the sky automatically. It also means we can loft telescopes into 00:10:12.960 --> 00:10:17.639 space, above the sea of air in our atmosphere that blurs and distorts distant, faint objects. 00:10:17.639 --> 00:10:22.389 We can visit other worlds and send the pictures and data back home, or put observatories like 00:10:22.389 --> 00:10:27.660 the Hubble Space Telescope into orbit around the Earth and have it peer into the vast depths of the Universe. 00:10:27.660 --> 00:10:32.180 I would argue that the past century has seen a revolution in astronomy every bit as important 00:10:32.189 --> 00:10:36.339 as the invention of the telescope in the first place. In the early 17th century the entire 00:10:36.339 --> 00:10:40.779 sky was new, and everywhere you pointed a telescope there was some treasure to behold. 00:10:40.779 --> 00:10:45.819 But with our huge telescopes and incredibly sensitive digital eyes now, that’s still true. 00:10:45.820 --> 00:10:50.319 We learn more about the Universe every day, just as we learn that there’s more to learn 00:10:50.319 --> 00:10:55.240 every day, too. That’s one of the best parts of being an astronomer; the Universe is like 00:10:55.240 --> 00:11:00.029 a jigsaw puzzle with an infinite number of pieces. The fun never ends. 00:11:00.029 --> 00:11:05.480 And remember: Even with all the wonders revealed by telescopes, your eyes are still pretty 00:11:05.480 --> 00:11:10.009 good instruments, too. You don’t need big fancy equipment to see the sky. The important 00:11:10.009 --> 00:11:14.240 thing is to go outside. Look up! That’s fun too. 00:11:14.240 --> 00:11:18.749 Today you learned that telescopes do two things: Increase our ability to resolve details, and 00:11:18.749 --> 00:11:23.949 collect light so we can see fainter objects. There are two main flavors of telescope: Refractors, 00:11:23.949 --> 00:11:27.790 which use a lens, and reflectors, which use a mirror. There are also telescopes that are 00:11:27.790 --> 00:11:31.699 used to look at light our eyes can’t see, and with the invention of film, and later 00:11:31.699 --> 00:11:35.790 electronic detectors, we have been able to probe the Universe to amazing depths. 00:11:35.790 --> 00:11:40.129 Crash Course is produced in association with PBS Digital Studios. This episode was written 00:11:40.129 --> 00:11:44.819 by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant is Dr. 00:11:44.819 --> 00:11:48.899 Michelle Thaller. It was co-directed by Nicholas Jenkins and Michael Aranda, and the graphics 00:11:48.900 --> 00:11:50.000 team is Thought Café.