Full transcript
0:00so far we're focusing on visible
0:02telescopes telescopes that let us see
0:05what the human eye can see except again
0:09we're collecting more light also we're
0:12gonna see later on that these allow us
0:14to really work with infrared light as
0:15well we're gonna talk about the various
0:17different parts of the spectrum as we go
0:18but for now we're focusing on what is
0:21commonly thought of as visible
0:22telescopes and the thing is as we said
0:26we want to gather as much light as
0:27possible so we need them being very very
0:29large and that means we're building
0:32progressively larger and larger
0:33telescopes and if you actually go to
0:35table 6.1 you can see some of the ones
0:37that are being designed and if you're
0:40like me you're probably gonna get a
0:41laugh out of the names of some of the
0:43existing ones I'm the ones being made
0:45one of my favorites of course is the
0:47Very Large Telescope which is a large
0:51telescope arguably a very large
0:53telescope of course it's to be outdone
0:55soon by things like the giant Magellan
0:59telescope there's of course also the
1:02what is it the South Southern African
1:04Large Telescope and of course the
1:08European extremely large telescope but I
1:12also have a fondness for the good old 30
1:14meter telescope because you can probably
1:17guess it's going to be 30 meters all
1:21naming creativity aside these telescopes
1:23are getting larger and larger and larger
1:25but most of them actually aren't like
1:28this ones you see here I mean this gives
1:31you a scale just how huge some of these
1:33mirrors are but many of the newer ones
1:37actually aren't just one mirror it's
1:40hard to make one big surface and what we
1:44can do is actually use things like this
1:45Keck telescope here the Keck telescopes
1:48are actually two telescopes but you can
1:51see what we have is this good old
1:53hexagon patterns the telescope's in the
1:57heck cases isn't one big mirror it's a
2:00set of many interlocking mirrors but
2:03they all act together as one that's the
2:06important thing sure it's not one large
2:08mirror like in this case but it's in
2:10essence the same they operate together
2:13as one large mirror so they are like one
2:17large mirror and you might be asking
2:18yourself well why not just have one
2:20large mirror then for one thing is
2:22easier to design many small mirrors then
2:25have one big beautiful perfectly
2:27polished one but also we can take
2:31advantage of what's called active
2:33control we are actively controlling
2:37these mirrors because even when we're
2:41looking at these mirrors we talked about
2:43them being pretty heavy and sure we can
2:45support them at the base but they can
2:48still cause sagging there can still be a
2:51warping or because of their weight but
2:54by actually controlling them altogether
2:56we can actually make sure to try and
2:59account for that this active control
3:01allows us to make sure we're getting the
3:03clean perfect images we should and again
3:07it's because these interlocking mirrors
3:09are all acting as one in this case the
3:13Keck telescope we have 36 mini little
3:16mirrors but it's acting as the
3:18equivalent of one large mirror so it's
3:21still de for that we'd still define the
3:23aperture to be 10 meters even though
3:26it's made up of many small mirrors
3:28acting together as one now another
3:34important thing to consider is we're
3:35trying to gather as much light as
3:37possible we're looking at object
3:39gathering as much light to get a nice
3:41clear beautiful image but remember earth
3:45is rotating earth is rotating as we're
3:49looking at this object so if we're
3:51looking at it for a long enough time if
3:53the telescope was just stationary we
3:57would lose sight of it and it would blur
3:58the image because of the relative motion
4:01so what we actually have to do is time
4:04these telescopes to rotate with the
4:06rotation of the earth and that's why we
4:08have these huge setups like in this
4:09image this telescope will actually
4:12rotate to say tracking the same object
4:17still looking at the same object and
4:20it's time to rotate with the rotation of
4:23the earth just to make sure we don't
4:24blur the image otherwise
4:26something that you might not have
4:27thought about but we're looking at these
4:29images sometimes for so long these
4:31objects are so long that we have to
4:34actually make the telescope track and
4:36move with it or we lose sight now
4:44another property we can consider we
4:45start off with light gathering power
4:47again really fancy way of how much light
4:50is gathered at any given time how big of
4:52our bucket is but we can also talk about
4:55resolution the resolution of a telescope
4:58is very important idea it's how much we
5:02can resolve different objects and this
5:06image is purposely a blurry one because
5:08we want to be able to distinguish
5:10between two things
5:12that's what resolution is distinguishing
5:15between two small objects the smallest
5:17objects we can see in an image and say
5:20those are two separate things
5:21so in this blurry image over here
5:25the resolution is very poor something
5:28over here is kind of a big blur that's a
5:32blur one big blur right there arguably
5:35one big huge blur all together but you
5:37could probably say this is an object
5:39blur however when you go over to this
5:42one that it has been fixed and what talk
5:45how is fixed in a little bit but you can
5:46see this image is much clearer but the
5:49resolution is better because we can
5:51actually distinguish between two objects
5:53that are close together better
5:56resolution means we can distinguish two
5:59objects rather than seeing one big blur
6:01we can actually see they're two separate
6:04blurs I mean might not sound the most
6:06exciting but it's important to be able
6:08to distinguish hey this is actually two
6:09different things so resolution the
6:13better the resolution the smaller the
6:15details we can see resolution is really
6:19how big the angle can be we want a small
6:22number for resolution generally because
6:24we want to be able to distinguish
6:25between two objects that are very close
6:27together resolution talks about how
6:30close two objects can be and still be
6:32determined as two separate things from
6:34the image again this is a very clear two
6:36different cases where the resolution is
6:39very poor
6:40but as resolution gets better you would
6:41notice finer and finer details and this
6:45resolution will also depend on telescope
6:47size so once again bigger is better
6:49bigger telescope resolution is better
6:51but we're also gonna see it depends on
6:54the wavelength studied we're gonna see
6:57and actually I'm gonna define it now the
6:59wavelength the larger the wavelength the
7:02worse the resolution so when we study
7:05different parts of the spectrum say
7:07radio waves which remember have huge
7:09wavelength we're gonna see that's much
7:10harder because the resolution tends to
7:14be much worse because the wavelengths
7:16are much longer in the radial part of
7:18the spectrum and this is related to
7:20diffraction remember diffraction the
7:23bending of light diffraction is the
7:27bending of light when it hits a gap
7:29actually I don't think we've defined
7:30this before fraction is the bending of
7:32light when you hit a gap well the
7:35telescope aperture the opening at the
7:37end of it
7:38is a gap when the light gets in there it
7:40can start to bend which will degrade the
7:43image we get the larger the wavelength
7:45the bigger the diffraction the bigger
7:47the bending at that gap we're gonna get
7:49and so you get poor resolution poorer
7:51images when the wavelengths are larger
7:54of course when we're talking about
7:56images in that case when we're looking
7:58at radio we're gonna see it's not a
7:59visible image it's something we can
8:03create an image from using digital means
8:06but we'll talk about that a sec