TechStuff Looks through a Telescope

From the physics of light to ingenious uses of a mirror, we look at the history of telescopes and how they work.

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Episode Transcript

Speaker 1 (00:04):
Welcome to tech Stuff, a production of I Heart Radios
How Stuff Works. Hey there, and welcome to tech Stuff.
I'm your host, Jonathan Strickland. I'm an executive producer with
How Stuff Works in My Heart Radio and I love
all things tech and way back in one a very

(00:26):
not good film titled Robin Hood Prince of Thieves debut
and don't at me. That movie is trash. I loved
it as a kid, but it is garbage. It starred
Kevin Costner as Robin of Locksley, better known as the
outlaw Robin Hood, and the film is set in eleven
ninety four and Robin had a lot of problems. The

(00:49):
Sheriff Nottingham was on his back, his beloved made Marian
gets kidnapped, and his accent kept going in and out,
so you can imagine the stress he was under. But
he also had some advance diages. Now, one of those
was his ingenious companion a Zeem played by Morgan Freeman.
And in one scene, a Zeem helps Robin get a
better look at some adversaries using a telescope. Now that

(01:14):
scene is meant to establish that a Zeem's homeland often
viewed as a backward and savage place through the eyes
of England, and by extension, the West in general is
actually home to great learning and innovation. And that part
was true. I mean, the Middle East has a long
history of phenomenal achievements, but inventing a telescope in the

(01:36):
late twelfth century is not among them. That was historically inaccurate,
probably the least of the historical inaccuracies in that film.
But still I wanted to start with this because this
was just one of the many fictions and fallacies in
The Robin Hood. But I figured it was a fun
place to start off this episode about the telescope. We're
gonna look at where it actually did come from and

(01:58):
how basic telescopes were. I guess you could say it's
the focus of this episode. H m hm. And So
while I now will leave the film Robin Hood Prince
of Thieves behind, just remember that everything I do, I
do it for you. Now, As is the case with
much of technology, it's not really possible for me to

(02:21):
tell you who invented the first telescope. I can tell
you that the person most folks credit with inventing the
telescope was the German Dutch inventor Hans Lipperty. He applied
for a patent for an invention he called the geiker
or kaiker. It's kind of hard for me to pronounce

(02:43):
because I don't speak Dutch, but it means looker in Dutch.
We'll come back to it. For this invention to be
possible at all, the first thing that has to happen
is that humans needed to learn how to make glass.
Now we don't have a record of that actually happened,
but our best guess is that glassmaking became an actual

(03:04):
thing around four thousand years ago in Mesa Potamia, as
the B. Fifty two s would say, a region called Ptolemace,
which is in now modern day Israel, was particularly known
for this, having sand that was suitable for glassmaking, and
early glassmakers would mix sand, soda, and lime which could

(03:27):
then be heated in a furnace to create molten glass.
To make a solid glass object, this mixture of sand, soda,
and lime would first be put into an open mold,
and the mold would be placed in the furnace, which
would be heated up enough for the mixture to become
molten glass. It would fill up this mold. They'd take

(03:48):
the mold out and allow it to cool. Now, if
you want to make a container something that could hold
stuff like a vase or a perfume bottle, the glassmakers
used a process called core forming. And yeah, I realized,
I'm already getting a little far away from talking about
telescopes and lenses. But I also think this process is
super neat, so I want to explain it briefly. First,

(04:10):
the glassmaker would determine what the interior shape of this
object was going to be, so, for talking about a bottle,
whether it's going to be tall and narrow, or whether
it was going to be a wide jug, something along
those lines. Then they would create the core out of
a mixture of clay, sand, water, and uh poop or

(04:31):
dung if you prefer, and then they would shape that
into the rough form they wanted. Before they would insert
a metal rod into one end of it, the essentially
the end that would be in the open part of
the container. They would then allow this core to dry.
After it dried, the glassmaker would use tools to further
refine the shape of the core, trimming it, filing it down,

(04:55):
that kind of thing. Once finalized, Once it's in that
final shape, the glassmaker would heat up a mixture of sand, lime,
and soda in a crucible in a furnace, creating a
molten glass inside that crucible. Then they would insert this
core into that molten glass, hold down the metal rod.

(05:16):
They would slowly twirl the core within the molten glass,
getting a full coating on the core, sort of like
coating a candy apple. The glassmaker would then remove this
let cool a little bit, and use some other tools
like pincers for example, to shape the glass while it
was still a pliable and then after it had cooled
down a bit, they might reheat it a little to

(05:37):
to soften the glass, maybe add different colors of glass
as decorations on top of it. You could twirl a
line of molten glass on top of another layer, have
contrasting colors and decorated that way. You might want to
add things like handles to say a pot. Glassmakers would
then change the color of the glass, by the way,

(05:59):
by by mixing in metal oxides, because different metals would
produce different colors. The whole process is super neat to watch.
There's actually a lot of videos on YouTube about this process,
so if it sounds interesting to you should really check
it out because it's pretty neat to see how these
ancient glassmakers would make this stuff. Anyway, glass was incredibly
useful and it was much sought after, and these early

(06:21):
examples I'm talking about we're really interesting. But the glass
they produced were not at all suitable for creating any
sort of lens. Those would have to wait for a
couple of thousand years. But the foundation for it came
from a pretty simple observation. Water has a magnifying effect,

(06:43):
and humans in the ancient world noticed this and wondered,
is there a way we could replicate this, where we
could create a way to magnify stuff without having to
use water. This led to ancient Egyptians and Mesopotamians experimenting
with polished crystals, usually using courts around seven fifty b C.

(07:04):
One such lens, the Nimrod lens, was made sometime around
then in ancient Assyria. Smartie pants Greek and Roman philosophers
began to hypothesize about what was actually going on with
these materials, What was creating this magnification effect, how did
it really how did it really work? They made some
progress over the centuries and sussing things out, but the

(07:27):
fall of the Roman Empire would set the world back
more than a step or two. A lot of learning
was lost, a lot of progress was halted. One place
that continued the academic exploration of what was going on
in the world of optics was in the Middle East,
and this is probably where the Robin Hood crew got
their idea for including a telescope in their screenplay. A

(07:49):
few influential mathematicians and writers in the Middle East published
their thoughts on what was going on, and they got
the basics pretty much right. So what is going on? Well,
we have to remember that vision is all about light,
our perception of light. We see stuff because light from

(08:09):
some source has reflected off of stuff. The light passes
through the lens of our eyes, and the lens directs
light to the retina. You can think of the lens
as a method of bending light toward a point. In
this case, the lens of our eyes bends light so
that it hits our retina, which in turn then sends

(08:30):
signals to our brains, and that interprets the information it
receives in such a way that we experience vision. So
what we see is a filtered representation of what is
actually out there according to the light that we're able
to perceive. There's stuff well outside the visible spectrum. You know,
there's infrared light, there's ultraviolet light, and then beyond that's

(08:53):
out there too, but we can't see it without the
aid of technology. And even when we do use technology,
what we're really looking at is a conversion of those
types of light into something we can actually perceive within
the visible spectrum. A lens is a transparent material with
at least one curved surface, and the curved surface redirects light.

(09:15):
This is called refraction. The lens bends the light rays
and changes the direction of travel. So in a vacuum,
light will travel in a straight line, but the path
of light changes as it moves through different materials, particularly
as it transitions from one material to another. So when
we talk about the speed of light, we typically are

(09:35):
talking about light as it travels through a vacuum, because
then the speed of light is consistent, it does not
change it, and it's also the fastest stuff that we
know about in the universe. So when light moves through
a different material, transparent material, it slows down a bit
compared to how fast it travels through a vacuum. So

(09:59):
we can divide n is into two very broad categories,
convex lenses and concave lenses. A convex or positive lens
bulges outward. This causes incoming rays of light to converge
on one another, concentrating on a focal point behind the lens.
So you could use stuff like this to concentrate light

(10:22):
into a point and then use that to start a fire,
for example with a magnifying glass. Telescopes also use these
sort of lenses as their object lens. Will talk about
that in a second, so or objective lens. I should say, So,
if you think of this in a in a sense
of an illustration, and you have a convex lens, remember

(10:44):
it bulges out on either side. In this simple example,
you would have parallel rays of light coming in from
the outside hitting that convex lens, and then they would
all start to tilt inwards of each other, converging to
a point further out from that lens. And the point
where they actually do converge is the focal point for

(11:07):
that lens. We'll get back to that. Then you've got
concave or diverging lenses. The surface of a concave lens
bends inward. It's like a bowl, it bends inside. So
when parallel rays of light hit a concave lens, of
lights coming from outside traveling in those straight lines hits

(11:27):
the concave lens, they then bend away from each other.
They move further out from each other. So a projector
might use a concave lens to spread rays out across
a larger surface, like a movie screen. Now that's not
to say all lenses are either convex or concave. You
can make lenses with elements of each or other parts

(11:49):
that These are called compound lenses. So it can get
pretty complicated. But we're gonna really focus on there. It
is again focus We're gonna focus on the simpler versions. Now.
Early lenses like the Nimrod lens were made from quartz
crystal and ground down and polished to create a magnification effect.
This effect was not particularly strong, but it did show

(12:11):
that it was possible to manufacture refracting lenses. This led
to more research and hypothesizing. In the eleventh century, Arabic
scholars were writing about the early signs of optics, carrying
on the tradition begun by the Greek and Roman philosophers.
By the thirteenth century Italian inventors had figured out how
to grind lenses suitable for use as spectacles. Now, these

(12:34):
were essentially a pair of magnifying glasses that one would
wear a hold up to your eyes. And there were
a lot of different stories about who invented eyeglasses, though
many of these lack any substantiating evidence, and a few
have been uncovered as being outright hoaxes. Why is that, well,
because often it's a matter of local pride to lay
claim to an inventor of a transformative technology, and then

(12:59):
you can so that, oh, your village or town, or
city or country was their home, and therefore you are
all elevated in relation to that. Something that surprised me
when I looked into all this was that inventors created
the microscope before they created the telescope. I had always
assumed the opposite was true. Hans Libacy, whom I mentioned

(13:23):
earlier as the person most people credit as the quote
unquote inventor of the telescope, may also have invented the microscope,
but others say that honor should go to Hans and
Zacharias Jansen. They were a father son team of spectacle
makers who happened to live in the very same town
as Liberacy. So whomever invented the darned thing appears to

(13:45):
have been living in Holland, specifically in Middleburg. I guess
stuff was always sort of fuzzy there and they just
really needed a closer look whomever was responsible. The earliest
records we have for a microscope date back to the
fteen nineties. The microscope used a pair or sometimes more,
of magnifying lenses, and they weren't super powerful microscopes. They

(14:08):
were only capable of around three to nine times magnification.
Skipping ahead a few decades to Liberty in his patent application,
at least one version of his story involves him discovering
the potential for a telescope essentially by chance. Supposedly, according
to the story, Lipper she got an order from a
customer to make two lenses. One lens was going to

(14:30):
be convex, thus a convergent and magnifying lens. The other
was to be slightly concave or divergent. So he makes
the two lenses as requested, and the customer comes in,
picks up the two lenses, holds one of them close
to his eye, one further away from his eye and
looks through them, and then happily pays for the order

(14:51):
and leaves. Then, according to this story Lippor, she decides,
what the heck was that about? I gotta make a
pair of lenses to find out why that was what
does that mean? So he goes. He makes essentially a
copy of what he had already made for this customer,
just to see what the heck this is all about,
holds up the concave lens close to his eye the

(15:13):
convex lens further away, and then is astonished to find
out that through this combination he's able to view an
image of a church across town as if it were
right in front of him. It has magnified the image significantly,
and thus, according to this possibly apocryphal story, the telescope
was born. He sent a notice to the States General

(15:35):
of the Netherlands for this patent, and it would have
extended a patent for thirty years. He actually offered up
a couple of different options. He said, well, if you
don't want to do that, you could give me an
annual pension from the government, and in return, if you
do this, I'll promise I will not sell this telescope
invention to foreign powers, and thus the Netherlands will have

(15:58):
a superiority in that regard. Uh Zachary Jensen, the aforementioned
son in that father son duo that worked on the microscope,
would claim that he invented the telescope. And there was
a third inventor, Jacob Matthias, who disputed Liberty's claim as
inventor as well. So ultimately the government of the Netherlands said,

(16:19):
we can't give anyone a patent on this. It's widely
known already people are already making these, so there's no
way of knowing who owns the rights to this. However,
they did give Liberty and a reward of nine hundred florins,
which I am told is indeed a princely some in
those days. When I come back, I'll explain more about

(16:40):
the physics of light within a simple telescope before we
continue our journey towards how modern telescopes work today. But
first let's take a quick break. Now, one thing I
didn't really cover in that first section of the podcast

(17:01):
is why the heck do we need telescopes? I mean,
what is it about our vision that has this limiting
factor in the first place. Why are smaller objects or
objects that are much further away or both, why are
they hard to see? Well, remember when I said that
when we see something, what's actually happening is that the
lens of our eye is directing light reflected off that

(17:22):
object and sending it to our retina. Well, you can
think of the retina as being kind of like a sensor,
and it's picking up that light, and smaller objects or
stuff that's further away take up less space on that sensor,
So that's part of it. It's just it's it's taking
up a tinier amount of space on the retina, so

(17:43):
we're getting less information to our brains. Also, our eyes
are gathering lots of light reflected off of lots of surfaces,
and the light coming from a small, distant object can
be dwarfed by the light coming from everything else, and
eventually the object is too small or too far away
for any light reflected off of it to be read,
just stirred by our retinas. It's not that the light
isn't getting to us, but it's so small compared to

(18:04):
everything else that we can't register it. We can't recognize it,
so to see it more clearly, we would need a
lens that could take the light reflected off that object
and then spread that light across more of the surface
of the retina, and telescopes do that. And there are
a couple of different ways we can achieve this with
optical telescopes. One is through the lenses I've mentioned already,

(18:28):
that would be called a refracting telescope, and the other
is through mirrors, which will get too later, and those
are reflecting telescopes. So let's start off with refracting telescopes.
A simple refracting telescope uses a pair of lenses, like
what Lipacy discovered back in six eight. The objective lens
collects light from distant objects to a point of focus

(18:52):
that's within the telescope itself. So again, now imagine you've
got a lens at the end of a tube and
the peril rays are coming in and they hit this
this convex lens, the objective lens, and because it's a
convex lens, it bends the lights. So now the rays
are now converging into the tube to a focal point,

(19:14):
so they're bending inwards with relation to the tube within
the body of the telescope. Itself. Uh. Now with a
modern day telescope that's not a Galileean telescope, which I'll
get to in a second. Uh, those rays would hit
a focal point and they don't just stop. They're right.
It's not like the rays of light all converge into

(19:37):
a point of space and then just create a point
of light. Those rays will continue on in a straight line.
So now they're diverging from one another. They keep on
going until they hit something and they reflect off of it.
So the objective lens faces out into the world, and
the other lens is the eyepiece or ocular lens, and

(19:59):
that magnifies that light from within the telescope and spread
it out so that that light takes up more of
the space on your retina. So these diverging rays hit
that second lens. That second lens then bends the light
in a way that directs it towards the eye of
the person using the telescope in a in a parallel fashion,
So it returns the light to a parallel alignment. So

(20:23):
what you view through that eye piece is a virtual
representation of the real thing that the objective lens has
in focus. And this virtual object is much closer to
your eye than the real object is, so you get
the effective magnification lipatis. Early telescopes can magnify stuff to
about three times their relative size to the viewer's perspective,
so not incredible, but an improvement. One interesting point about

(20:47):
this type of telescope if we were talking specifically about
using convex lenses on both the objective lens and the
ocular lens, the eye piece lens or the optical lens,
if you prefer if you're using both of those as
convex lenses, and you're you've got the focal point inside
the telescope itself, the second lenses behind that focal point.

(21:09):
As the light converges and then diverges within the telescope,
the image flips upside down. So if you draw this out,
it all makes sense. The light rays are coming from
the outside world, right, and the light rays that are
on the top if you think of it in respect
of the telescope, the top of the telescope part, and
you're looking at a cross section of it, they get

(21:33):
bent so that they aimed downward relative to the telescope.
The light rays coming from the bottom side of the lens,
for example, then get bent upward with respect to the telescope,
and then they continue on their journey. They hit that
focal point and they keep going in a straight line. Uh.
And so the light that was at the bottom of

(21:53):
the objective lenses at the top of the optical lens
or the the ocular lens. The light that was the
top of the objective lens is at the bottom of
the I piece lens. So that's why if you were
looking through such a telescope, the object you were looking
at would be upside down. Uh. If we use such
a telescope to look at a celestial body, that's not

(22:15):
a big deal because top and bottom in space is
largely unimportant. If we wanted to use it in a
terrestrial sense, like you wanted to use the telescope to
look at stuff around you on Earth, Like let's say
that you have a spyglass and you're a pirate, then
looking at a distant object might be a bit of
a surprise because it would suddenly be flipped upside down.
Modern telescopes use stuff like prisms and mirrors to correct

(22:38):
for that vertical inversion. They're called erectors. But what about
old telescopes before we figured that out? We're all those
pirates we see in romanticized movies looking at stuff upside
down the whole time. Well no, so, while my description
of objective lenses and eyepieces and all that sort of

(22:58):
stuff is accurate. From modern refracting telescopes, the type used
by astronomers and seafarers from around oh say, sixteen ten
to about sixteen seventy or so followed the Galilean method.
Galileo began using telescopes for astronomical observations not long after
Liberacy's work became widely known. So like by six ten, Galileo,

(23:23):
like Libacy, used a convex lens as the objective lens,
and a concave or diverging or negative lens as the
eye piece lens. So like coming in through the objective
lens would bend inward towards a focal point, the diverging
lens would reverse the direction of the bend before the
rays could hit the user's eye, so the top and

(23:44):
bottom wouldn't switch, everything would still be in the proper alignment.
As the Institute and Museum of the History of Science
puts it, quote, the eye piece is situated in front
of the focal point of the objective at a distance
from the focal point equal to the focal length of
the eye piece end quote. That gets a little confusing,

(24:06):
but if you were to draw it out, it makes
a lot of sense. You would have the convex objective
lens at the front of the telescope. Lights coming from
outside world in parallel rays. It hits that lens and
it bends inward, just as we've been talking about all
the way up through this podcast, they start to converge
on a focal point that's behind the lens. However, before

(24:28):
it gets to the point where all those rays have
converged into a single point of space, those rays hit
the eye piece lens, the concave lens, So instead of
all converging on a focal point, they first hit this
concave lens, which then bends the light again, and then

(24:49):
the concave lens causes the rays to diverge, returning to
a parallel arrangement. The The early inventors learned that there
was a precise art to getting the distance correct between
these two lenses. You couldn't just have them one in
front of the other and everything works out perfectly. To
really get it right, you needed to take the absolute

(25:12):
value of the focal length of each lens and then
calculate the difference between them. The difference represented the distance
between the two lenses you would need to produce the
magnification effect you wanted. So for a Galileean telescope, the
distance between the objective lens and the I piece or
optical lens is equal to the algebraic sum of the

(25:34):
two lenses focal lengths. That is, the distance between the
lens and its focal point. So concave lenses actually have
a negative focal point. The focal point of a concave
lens is in front of the lens, not behind the lens.
It's a little counterintuitive. So you add a positive value,

(25:56):
which is the convex lenses focal point. The focal point
for a convex lens is behind it. Then you add
the negative value that's the concave lenses focal point, and
then you get the difference essentially, because it would have
been the same as if you subtracted a positive sum
from another positive sum. The result is how far apart
those two lenses should be to produce the magnification effect. Now,

(26:18):
the amount of that magnification is also dependent upon the
focal length of the two lenses. Specifically, it depends upon
the ratio between the focal length of the objective lens
and then of the I piece lens. So you take
the objective lens focal length, you divide it by the
focal length of the optical lens. And later telescopes ones

(26:39):
that would use two convex lenses, rather than adding those
two focal lenks together, you would subtract the focal length
of the optical lens from the focal length of the
objective lens. Now, remember that in that case, the focal
length rh lens is a positive value. That's the only
reason that we had to add the two figures with
the Galilean telescope, because one of the values was negative.

(27:00):
The greater the diameter of the objective lens, the one
that's facing out to the world, the more light it
can collect. That seems pretty obvious, right. The bigger the lens,
the more light it's going to be able to redirect
inward to the telescope. By the way, the reason why
telescopes even have a tube there are a couple of reasons.
One is to keep out dust and other things that

(27:23):
could obscure the lenses, but another is it helps block
out any light that you don't want to come and
hit your eye. You want to really focus on whatever
object you're looking at. So the greater the diameter or
aperture of the objective, the more light it can collect.
Generally speaking, the more light it collects, the brighter the

(27:44):
distant image will actually be. And the greater the magnification
of your telescope, which again depends upon the relationship between
the focal length of lenses, the less field of view
you would end up having. So the objective lens die
ameter was what determines how much light comes in. It
does not necessarily determine how much magnification you get. That

(28:08):
is based more on the relationship between that lens and
the I piece lens. But then the amount of magnification
you get determines how much field of view you have.
If it's a greater amount of magnification, you're going to
see less of the night sky in the view of
your telescope. Now, there are practical limits that you hit
using lenses, because the bigger the lens, the more light

(28:31):
it can collect. But it also means that those lens
lenses have have more mass. That means the telescopes themselves
get heavier as a result. Moreover, if a lens is
too heavy, the weight can actually affect the shape of
the lens. It can warp it. And since the lens
shape determines where the light is going to go, that's

(28:51):
a bad thing. If you've designed a lens to direct
light in a very specific way, and then the lens
warps under its own weight, the light's not gonna go
where you plan, and so you start to reach practical
limits of what you can do using refracting telescopes. The
largest refracting telescope objective lens that's still in use today
is installed at the Yorkey's Observatory in Wisconsin. The lens

(29:14):
on that telescope measures one meter across or or a
little more than three feet. In other words, it weighs
around twenty six tons. That's how heavy glass gets when
you're looking at this, because remember it's a convex lens,
it bulges out. So it's not just that it's a
flat sheet of glass. It's not flat, it's it's curved.

(29:35):
So this is obviously a little heavier than what you
would use in the backyard telescope also I said still
in use, but technically the Yerkeys Observatory has been closed
to the public since the spring of two thousand eighteen,
when the University of Chicago announced it was seeking a
party to purchase this observatory and telescope and essentially take
it off university hands, which has not yet happened as

(29:57):
the recording of this episode. An other practical limitation of
refracting telescopes is that the lens must be in really
good shape right, so scratches, smudges, dust, all that can
make it difficult for light to pass through the lens.
And there's also the issue with lost light. Some of
the light hitting the lens doesn't pass through the lens,

(30:19):
it will reflect off the lens, and we see this
in our daily lives. If you look at a window
and you see your reflection and the window is transparent,
then that reflection is proof that some of the light
hitting that window is not passing through the glass. Instead
it's bouncing off the glass. The same thing is true
for telescope lenses, and the thicker and larger the lens,

(30:40):
the more light is going to be lost due to reflection.
Another limitation is called chromatic aberration, which sounds like a
monster from Dungeons and Dragons. But this all has to
do with the fact that light is made up of
many wavelengths, which we perceive as different colors. Those different
wavelengths have different focal lengths. The focal length of blue

(31:01):
light is different than the focal length for red light,
and these two wavelengths are pretty far apart on the spectrum,
which I'm sure you remember if you remember Roy g BV. Now,
what this means to us using telescopes is that the
different colors of light will not quite line up when
creating the image of the thing we're trying to look at.

(31:23):
The effect isn't enormous, but it's enough to create a
fringe of color around images, sort of like a rainbow
halo effect almost. And adding in other lenses and various
combinations can correct for chromatic aberration. But adding more lenses
means telescopes get way more expensive, delicate, and heavy. There's
a different approach that doesn't rely on lenses at all,

(31:45):
and those are reflecting telescopes, and I'll explain more about
those in just a second. Before we figured out how
to you lots of combinations of lenses and prisms to
correct for a chromatic aberration and other limitations of refracting telescopes.

(32:07):
There was another smarty pants who came up with a
different solution. That smarty pants would be Sir Isaac Newton,
who when not dodging falling apples or inventing calculus. And yeah,
I know he wasn't the only one to invent calculus.
He was coming up with nifty ways to improve telescopes,
and he did this around the sixteen seventies. Newton's solution,

(32:28):
which had previously been suggested by folks like Galileo, was
to rely upon a curved mirror rather than a lens
to gather light. The mirror would sit at the base
of the telescope. So again, if you think of the
telescope as a tube, then the mirror would be at
the bottom of the tube. The top of the tube

(32:50):
would be open, open to the night sky. The curved mirror,
a parabolic mirror, would reflect light so that all the
parallel rays come into the telescope would hit the mirror
and then reflect off on a converging pathway. So similar
in execution. If you can think of it that way,

(33:10):
and maybe not execution. Similar in effect to how a
convex lens bends light to converge on a focal point,
the parabolic mirror would reflect light to converge on a
focal point inside the telescope. However, Newton mounted a second
mirror sitting just ahead of where the focal point would be,

(33:30):
so in between the parabolic mirror and that mirror's focal point.
This the secondary mirror would reflect light coming from the
objective mirror at around a ninety degree angle toward an eyepiece,
which would provide magnification of the virtual image produced there.
So the light coming in from the main mirror bounces

(33:52):
off a second mirror and then you can see that light. Otherwise,
the parabolic mirror would just reflect light back out of
the open into the telescope. That would do you no good.
The only way to look into it would be to
put your head in the telescope, and then you're blocking
the light that's coming into it. So the secondary mirror
was to redirect light so you could actually see what

(34:12):
this telescope was observing. So, instead of an objective lens
to capture and bend light, Newton's telescope had an objective
mirror like a refracting telescope, the amount of light captured
is dependent upon the size of the objective component, but
a mirror's thickness doesn't have to change as you increase

(34:33):
its diameter. It doesn't bulge out, so you can make
a really thin, really large parabolic mirror. By contrast, the
refracting lens would get thicker as you increase the diameter
in order to get the proper refracting properties. So the
switch to a reflecting mirror meant you could construct much

(34:53):
larger telescopes without having to worry about dealing with really heavy,
very delicate lenses. Even a really big reflecting telescope could
be mounted on a sturdy support structure and the mirror
would retain its parabolic shape compared to those glass lenses
that would eventually warp from the weight of the lens itself.

(35:13):
And because the light was bouncing off mirrors rather than
passing through lenses, Newton didn't have to worry about chromatic aberration. However,
reflecting telescopes had their own sets of limitations. Early on,
a big limitation was focal length. The reflecting telescopes were
limited having a relatively short focal length, and since focal

(35:35):
length is tied to magnification, that meant reflecting telescopes were
largely limited and how much magnification you could get out
of them. This would later be addressed with innovations and
telescope design, but it was a bit of a limitation
in Newton's time. Also, while the telescope has had a
relatively short focal point, they also had a relatively large
field of view, so you can see more of the

(35:55):
night sky in the view using a reflecting telescope than
a comparatively similar refracting telescope. Another small limitation was the
reflecting mirror mounted above the objective mirror, you know, the
one that's in between the objective mirror and its focal point. Well,
it would block a little bit of the light coming
into the telescope. It wouldn't block any of your view

(36:18):
because essentially every point of the mirror would have a
full version of the image that was coming into the telescope,
So you were getting a full view, but you were
blocking some of the light coming into the telescope, so
the image would be a little more dim than otherwise
would be. So the bigger this reflecting mirror was the

(36:40):
more light it would block, and the dem or the
resulting image would be. The curved mirror also meant that
objects along the perimeter of the field of view would
be slightly warped. So anything in the center of your
view would be pretty accurate, but the closer you got
to the edge of your you, the more warped it

(37:01):
would get, and you would get these elongated images. So
if you're looking at a star, it might look more
like a tear drop or a comment. So that was
a little bit of a of a setback, or at
least a drawback, I should say. So, yeah, these telescopes
can get pretty big. The biggest in operation right now
is the Grund telescope Eo Canarius in La Palma, Spain,

(37:26):
as a diameter of ten point four meters or thirty
four point two feet. Now, remember, the largest refracting telescope
has an objective lens diameter of one meter, So this
reflecting telescope has an objective mirror ten times that diameter.
That's huge. Now, the mirror is also not a single

(37:49):
unbroken surface. It's not one ten point four meter across mirror.
It's actually made up of thirty six hexagonal mirrors that
fit together snugly, kind of like a puzzle piece. But
there's an even larger reflecting telescope that's currently in development.
It's called the European Extremely Large Telescope. Seems pretty self explanatory.

(38:11):
It's gonna have a reflecting objective mirror that measures approximately
forty meters in diameter, according to the European Southern Observatory.
The telescope will correct for atmospheric distortions, which is one
of the problems that we have just using telescopes here
on Earth. It's the fact that we have this pesky
atmosphere that gets in the way sometimes. Uh. The atmosphere

(38:31):
is why stars appear to twinkle when we look at them,
so that can be a problem when you're trying to
magnify all of that stuff. But this one's supposed to
correct for that. It's also supposed to be able to
collect thirteen times more light than any other optical telescope
we have here on Earth, and again, according to the
e s O, provide images that are sixteen times more

(38:52):
sharp than the Hubble Space telescope was able to. The
plan is to have this telescope ready to make observations
starting in twenty twenty five. Speaking of the Hubble, it
is itself a reflecting telescope. Specifically, it's a type of
reflecting telescope called a Cassegrain reflector, which uses a pair
of curved mirrors. The objective mirror is that concave parabolic

(39:15):
mirror design that I talked about just a moment ago,
but mounted above that, instead of a mirror that reflects
that image ninety degrees, it's actually a mirror facing the
first one, and this one is a convex mirror, so
it bulges outward, not curves inward. The parabolic mirror reflects
incoming light toward a focal point, and mounted ahead of

(39:38):
that focal point is this convex mirror, which then reflects
light back down the telescope in a converging point, and
the main parabolic mirror at the base of the telescope
has a small hole in the center that allows light
to pass through. The idea for the Hubble and other
space telescopes was that by putting telescopes in orbit and

(39:58):
thus outside of our atmosp fear, we could get an
unimpeded look at distant celestial bodies. You wouldn't have to
worry about atmospheric distortion or light pollution from terrestrial sources. Unfortunately,
after the Hubble Telescope had already launched into orbit, it
became clear that the objective mirror wasn't shaped correctly. It

(40:20):
was just slightly too flat by the order of a
couple of micrometers, so a very small error, but enough
to be catastrophic. That was enough to introduce spherical aberration,
which translates to people like you and me, as the
telescope was returning fuzzy images and it was supposed to
be super sharp, gorgeous images of the the galaxies around us. Now.

(40:43):
Eventually astronomers were able to come up with a solution,
though it would mean sending astronauts back up to the
Hubble Space Telescope to install a couple of additional mirrors
to correct for that issue, and in the process they
had to also remove some of the instrumentation that was
intended to get there other types of cosmological data. This
is what we would call a very expensive boo boo.

(41:06):
The James Webb Space Telescope, which is scheduled to launch
in twenty one, is of a similar design, but we'll
be exploring the universe. By collecting infrared light, which is
outside the visible spectrum, it will look at light that
is four times fainter than what current telescopes can detect,
and that means it can detect light from very distant sources.

(41:28):
And in space, you can think of distance and time
as being very closely related because it takes time for
light to travel distances. Now, light moves wicked fast. It's
the fastest stuff in the universe as far as we
can tell, but even so, it still takes time to
get from point A to point B. So when we
look up at stars, the light we're seeing from stars

(41:50):
might have taken a journey that lasted millions of years,
so we're effectively looking into the long distant past of
those celestial bodies. We're not seeing the star as it
is today. We're seeing the star as it was when
that light left the star, possibly millions of years ago.
And the James Webb is going to collect light from
further away than we've ever managed to do up to now,

(42:14):
meaning we'll be looking much further back into the past
of the universe than we've ever been capable of doing,
which is pretty darn cool. Now there's a lot I
didn't cover in this episode. For one thing, I stuck
with optical telescopes, but there are other kinds like radio telescopes.
For another, I didn't really talk about stuff like the erectors,

(42:34):
which are those devices that are meant to reverse that
vertical flipping thing that I talked about with refracting telescopes
if they're using two convex lenses. But I figured this
was a good overview into the super interesting piece of
technology that has at its heart very few components. But
those components have to be precisely designed, constructed, and placed

(42:56):
in relation to one another. So it's a real testament
to human ingenuity and also how sometimes the most impressive
technologies are not necessarily the most complicated when you get
down to it. If you guys have suggestions for future
episodes of tech Stuff, let me know. Send me an email.
The addresses tech Stuff at how stuparks dot com. Drop

(43:18):
on by our website that's text Stuff podcast dot com.
You're gonna find an archive of all of our past episodes.
You can do a search find out if the topic
you have in mind has already been covered. If not,
let me know. You can also find links to where
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Over there, so you can drop me a line there,
and don't forget. We also have a link to our
online store, where every purchase you make goes to help

(43:39):
the show, and we greatly appreciate it, and I'll talk
to you again really soon. Text Stuff is a production
of I Heart Radio's How Stuff Works. For more podcasts
from i heeart Radio, visit the I heart Radio app,
Apple Podcasts, or wherever you listen to your favorite shows.

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Eight