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Today Phil explains how telescopes work and offers up some astronomical shopping advice.
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Transcript Provided by YouTube:
00:03
I’ve talked a lot about observing the night sky with your eyes; just simply going out
00:07
and seeing what you can see. It’s pretty amazing what you can learn just by doing that,
00:11
and of course that’s all we humans could do for thousands of years.
00:14
But now we can do better. We can use telescopes.
00:17
The first person to invent the telescope is lost to history; despite “common knowledge,”
00:21
Galileo did not invent them. He wasn’t even the first person to point one at the sky,
00:25
or the first person to publish results! But he was a loud and persistent voice over the
00:30
years, and his amazing string of discoveries using his crude instrument landed him firmly
00:34
in the history books. Aggressive self-marketing sometimes pays off.
00:48
You might think the purpose of a telescope is to magnify small objects so we can see
00:52
them better. That’s how a lot of telescopes are marketed, but to be honest that’s not
00:56
exactly the case. If you want to be really general, the purpose of a telescope is to
01:00
make things easier to see: To make the invisible visible, and to make the things already visible
01:06
visible more clearly.
01:07
A telescope works by gathering light. Think of it like a bucket in the rain: The bigger
01:12
the bucket, the more rain you collect. If your bucket is big enough, you’ll get plenty
01:16
of water even when it’s only sprinkling out.
01:18
In the case of a telescope, the “bucket” is an optical device like a lens or a mirror
01:23
that collects light. We call this device the objective, and the bigger the objective, the
01:27
more light it collects. Look at your eyes… well, that’s tough, so let’s think about
01:32
our eyes for a moment. They also work as light buckets, but they only collect light through
01:35
our pupils, which even under the best of circumstances are less than a centimeter across;
01:40
a very tiny bucket indeed.
01:42
But we can do better. To extend the analogy, a telescope is like a bucket with a funnel
01:46
at the bottom. All that light that it collects is then concentrated, focused, and sent into
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your eye. It turns a trickle of light into a torrent.
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The amount of light it collects depends on the area of the objective. That means if you
01:58
double the diameter of the collector, you’d collect four times as much light, because
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the area of the collector goes up as the square of the radius. Make a bucket 10 times wider,
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and you collect 100 times as much light! Clearly, as telescopes get bigger their ability to
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show us faint objects increases enormously.
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In fact that was one of Galileo’s first and most important discoveries: Stars that
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were invisible to the naked eye were easily seen through his telescope, even though it
02:23
only had a lens a few centimeters across. Those faint stars didn’t emit enough light
02:27
for his eyes to see them, but when he increased his collecting area with a telescope,
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they popped into visibility.
02:33
The primary way telescopes work is to change the direction light from an object is traveling.
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I can see a star with my eye because light from that star is sent in my direction, into
02:41
my eye. But most of that light misses my eye, falling to the ground all around me. The telescope
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collects that light, bounces it around, and then channels it into my eye.
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When the very first telescopes were built, this changing of the direction of light was
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done using lenses. When light goes from one medium to another – say, from going through
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air to going through water or glass – it changes direction slightly. You see this all
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the time; a spoon sitting in a glass of water looks bent or broken. The spoon is doing just
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fine, but the light you see from it is getting bent, distorting the image. This bending is
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called refraction.
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The way light bends depends on what’s bending it (like water or glass) and the shape of
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the object doing the bending. It so happens that if you grind a piece of glass into a
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lens shape, it bends — or refracts — the incoming light in a cone, focusing it into
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a single spot. It’s a light funnel!
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This refraction has a couple of interesting results. For one thing, the light from the
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top of a distant object is bent down, and the light from the bottom is bent up. When
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this light comes to a focus, it means you see the object upside-down! It also flips
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left and right, which can be a little disconcerting, and takes getting used to when you’re using
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a refracting telescope.
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For another thing, the lens can magnify the image. That’s again because the light is
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bent, and the image created of object observed can appear larger than the object does by
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eye. It depends on a lot of factors including the shape of the lens, the distance to the
03:59
object, and how far away the lens is, but in the end what you get is an image that looks bigger.
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That has obvious advantages; a planet like Jupiter is too far away to see as anything
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other than a dot to the eye, but a telescope makes it appear bigger, and details can then
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be seen. When Galileo and other early astronomers pointed their telescopes at the sky, multitudes
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were revealed: Craters on the Moon, the phases of Venus, Jupiter’s moons, the rings of
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Saturn, and so much more. The Universe itself came into focus.
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When astronomers talk about using a telescope to make details more clear, they use a term
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called resolution. This is the ability to separate two objects that are very close together.
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You’re familiar with this; when you’re driving on a road at night a distant car coming
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toward you appears as a single light. When it gets closer, the light separates out — resolves
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— into two headlights.
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A telescope increases resolution, making it easier to, say, split two stars that are close
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together, or to see details on the Moon’s surface. The resolution depends in part on
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the size of the objective; in general the bigger the telescope objective
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the better your resolution is.
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Resolution is more useful than magnification when talking telescopes. Fundamentally, there
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is a limit to how well your telescope resolves two objects, but there’s no limit to how
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much you can magnify the image. If you magnify the image beyond what the telescope can actually
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resolve, you just get mush.
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Refracting telescopes are great, but they suffer from a big problem: Big lenses are
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hard to make. They get thin near the edge, and break easily. Also, different colors of
05:28
light bend by different amounts as they pass through the lens, so you might focus a red
05:33
star, say, and a blue one will still look fuzzy.
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No less a mind than Isaac Newton figured a way around this: Use mirrors. Mirrors also
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change the direction light travels, and if you used a curved mirror you can also bring
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light rays to a focus. Telescopes that use mirrors are called reflectors.
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The advantages of reflectors are huge: You only have to polish one side of a mirror,
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where a lens has two sides. Also a mirror can be supported along its back, so they can
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be manufactured much larger more easily and for less money. Although there have been many
06:03
improvements made over the centuries, most big modern telescopes at their heart are based
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on the Newtonian design, and in fact no large professional-grade telescopes made today have
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a lens as their objective. Nowadays, it’s all done with mirrors.
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And that brings us to this week’s aptly named Focus On.
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The most common question I’m asked (besides, “Hey, who does your hair?”) is, “Hey,
06:22
Phil, kind of telescope should I buy?”
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It’s a legitimate question, but it’s very difficult to answer. Imagine someone walked
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up to you and asked, “What kind of car should I buy?” That’s impossible to answer without
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a lot more information.
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Same for telescopes. Do you want to look at the Moon and planets, or fainter, more difficult
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to spot galaxies? Are you really devoted to this, or is it more of a pastime? Is this
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for a child or an adult?
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These questions are critical. Most small ‘scopes are refractors, which are good for looking
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at detail on the Moon and planets (they tend to magnify the image more than reflectors
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do). But they’re tricky to use because they flip the image left and right and up and down.
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Bigger ‘scopes are good for fainter objects, but are more expensive, and can be difficult
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to set up and use. I hate hearing about a ‘scope that just collects dust because it was bought in haste.
07:04
So here’s what I recommend: Find an observatory, planetarium, or local astronomy club. They’re
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likely to have star parties, public observing events, where you can look at and through
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different kinds of telescopes. Their owners are almost universally thrilled to talk about
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them — as an astronomer, I can assure that the problem with astronomers isn’t getting
07:21
them to talk, it’s shutting them up — so you’ll get lots of great first-hand advice and experience.
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Also, I usually recommend getting binoculars before a telescope. They’re easy to use,
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fun to use, easy to carry around, and you can get good ones for less money and still
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see some nice things. Even if you decide not to get more into astronomy as a hobby, they
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can also be used during the day on hikes and for bird watching. I have a couple of pair
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of binoculars and I use them all the time.
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There’s a third aspect to telescopes that’s very important, beyond resolution and making
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faint things easier to see. They can literally show us objects outside of the range of colors
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our eyes can see.
07:56
In the year 1800, William Herschel discovered infrared light, a kind of light invisible
08:00
to our eyes. In the time since we’ve learned of other forms of invisible light: radio,
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microwave, ultraviolet, X-rays, and gamma rays. Astronomical objects can be observed
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in all these flavors of light, if we have telescopes that are designed to detect these
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flavors of light. Radio waves pass right around “normal” telescopes, ones that we use
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to observe visible light. X-rays and gamma rays pass right through them as if they aren’t even there.
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But we’re smart, we humans. We learned that giant metal dishes can and will bend radio
08:27
waves, and can be formed just like gigantic Newtonian mirrored telescopes. In fact, different
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forms of light need different kinds of telescopes, and once we figured out how, we’ve built
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‘em. We can now detect cosmic phenomena across the entire spectrum of light, from
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radio waves to gamma rays, and have even built unconventional telescopes that detect subatomic
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particles from space as well, such as neutrinos and cosmic rays. Because of this, we have
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learned far more about the Universe than Galileo could have imagined.
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And we’re in the midst of another revolution, too. The actual biophysics is complicated,
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but in a sense our eyes act like movie cameras, taking pictures at a frame rate of about 14
09:01
images per second. That’s a short amount of time. Photographs, though, can take far
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longer exposures, allowing the light to build up, allowing us to see much fainter objects.
09:09
The first photographs taken through a telescope were done in the 1800s. This has led to innumerable
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discoveries; for example, in the 20th century giant telescopes with giant cameras revealed
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details in distant galaxies that led to our understanding that the Universe is expanding,
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a critically important concept that we’ll dive into later in the series.
09:27
And now we have digital detectors, similar to the ones in your phone camera, but far
09:32
larger and far more sensitive. They can be dozens of times more light-sensitive than film, able
09:37
to detect in minutes objects that would’ve taken hours or more to see using film. These
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digital cameras can also be designed to detect ultraviolet light, infrared, and more. We
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can store vast amounts of that data easily on computers, and use those computers to analyze
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that huge ocean of information, performing tasks too tedious for humans. Most asteroids
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and comets are discovered using autonomous software, for example, looking for moving
09:59
objects among the tens or hundreds of thousands of fixed stars in digital images.
10:04
This has also ushered in the era of remote astronomy; a telescope can be on a distant mountain
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and programmed to scan the sky automatically. It also means we can loft telescopes into
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space, above the sea of air in our atmosphere that blurs and distorts distant, faint objects.
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We can visit other worlds and send the pictures and data back home, or put observatories like
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the Hubble Space Telescope into orbit around the Earth and have it peer into the vast depths of the Universe.
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I would argue that the past century has seen a revolution in astronomy every bit as important
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as the invention of the telescope in the first place. In the early 17th century the entire
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sky was new, and everywhere you pointed a telescope there was some treasure to behold.
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But with our huge telescopes and incredibly sensitive digital eyes now, that’s still true.
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We learn more about the Universe every day, just as we learn that there’s more to learn
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every day, too. That’s one of the best parts of being an astronomer; the Universe is like
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a jigsaw puzzle with an infinite number of pieces. The fun never ends.
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And remember: Even with all the wonders revealed by telescopes, your eyes are still pretty
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good instruments, too. You don’t need big fancy equipment to see the sky. The important
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thing is to go outside. Look up! That’s fun too.
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Today you learned that telescopes do two things: Increase our ability to resolve details, and
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collect light so we can see fainter objects. There are two main flavors of telescope: Refractors,
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which use a lens, and reflectors, which use a mirror. There are also telescopes that are
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used to look at light our eyes can’t see, and with the invention of film, and later
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electronic detectors, we have been able to probe the Universe to amazing depths.
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Crash Course is produced in association with PBS Digital Studios. This episode was written
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by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant is Dr.
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Michelle Thaller. It was co-directed by Nicholas Jenkins and Michael Aranda, and the graphics
11:48
team is Thought Café.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video.