So Paul, you've shown that we can use the Hubble Space Telescope to get down to
about a tenth of an arcsecond in resolution. Which is great, compared to what we get at
Siding Spring. But we're going to have to probably do better, and it strikes me that we have
here on Earth ten metre telescopes. But we're going to have to fix the problem
of the atmosphere to move forward.
Yeah, we're kind of stuck in a bind. If we're on the ground, we have
big telescopes so that the diffraction limit is very small, but we're mucked up by the atmosphere.
If we're in space... you can't put ten metre telescopes in space
the nose cones of rockets aren't big enough. So you're stuck with a diffraction limit.
So wouldn't it be nice if there was some way to cancel out the effects of the atmospheric seeing.
And of course there is... kind of. This is a diffraction limit pattern with a laser as you'd get from a space mission.
Right. So that's what you would see if you don't have an atmosphere, and we
want to be able to try to get that pattern in through our big telescopes here on Earth.
Yep. So to understand this we have to go from the ray picture of a telescope
to a wave picture. So here's a ray picture of how a telescope works.
You get the light coming in here, the rays. They bounce off the primary mirror
(And they're coming in from infinity so they're parallel) Yes.
They hit the surface of our mirror.
Yep.
And then they're focussed on the secondary mirror. Not quite focussed, but they're directed there.
And then that secondary mirror curves them and puts them down through the hole in the
back of our telescope, and so this is what we call a cassegrain telescope, so that we
can have our imager here and take a picture.
Yes. Now let's think of it from a wave point of view.
We've at once got these infinite parallel waves coming in, and some of them
enter the telescope and some just miss. And they come down here and they hit the
primary mirror and the primary mirror is shaped so that the path for every wave coming from infinity
should bring it in phase to the focal plane at the same place. So the outer parts have
further to travel so they get intercepted first. So you get a series of more circular
waves coming down to the secondary, and that turns into curving slightly the other way
they come down here and come perfectly into focus. (Ok)
So if all is well it should turn a perfectly plain parallel wave here into...
...into a perfectly plain parallel wave again.
All focussed and brought to a... every point from here to here should have exactly the
same path length to add up to exactly one point over there.
Yep.
That's the principle. What does atmospheric seeing do?
Well if it's cold air, the higher refractive index slows light down.
Right.
Relative to the speed in normal temperature air. So that means the
wavefronts are going to be lagging behind here. Whereas hot air, the air will go a bit
faster than on average, and so it will move forward. So you've now got a distorted wavefront
coming into the telescope, and so we add that all up in phase, it'll give you...
if Brian steps over a bit, you get a sort of pattern like this that we talked about before
(it gives us a mess, is what it gives us) A mess. Yes, speaking technically.
So what can we do? Well, here's a movie from Gemini showing the concept of adaptive optics
how you correct this. So you get the light coming in
off the primary up to the secondary and down through here
and on the back of the telescope you have the instrument support structure which
can rotate to cancel out the movement of the sky as seen from the telescope, and one arm
of that is the adaptive optics system. It's called ALTAIR on Gemini.
So again the light coming in from the top...
So the light enters our instrument through the telescope, it comes
through the hole in the back of the mirror (Here comes the light)
and it's brought into this instrument.
It bounces off a mirror, and this is a dichroic. We talked about them a bit before.
This sends the infrared light this way where it gives us an image, a rather
blurry image because of the atmospheric seeing, whereas the blue light goes over there.
So that could be transferred here, and so that light here is... we could look at it in detail.
So here come the wavefronts, and as you can see they're distorted because of atmospheric
seeing. So they're not flat pancakes... this is the march of the pancakes, and they're
pretty horrible looking pancakes. What we have down here is a wavefront sensor that
measures the exact distortion, and then makes this mirror bend an equal opposite way.
Ah. So now, the pancakes are flattened and we can make sure we keep on keeping those pancakes
flattened, so when those nice wavefronts which are all lined up hit our detector
our star doesn't look blurry, it looks like that wonderful pattern.
Of course you have to change the shape of this mirror hundreds of times a second
because the atmosphere is always moving, and that's one of the big challenges of this.
You have to be able to measure with exquisite precision the distortions and measure it a hundred times a second.
And so that means that you can't just do this on some faint galaxy.
You're going to have a lot of information in the form of a lot of light to actually see what the atmosphere is doing.
When trying to direct image(?) exoplanets you're normally lucky, because you typically have a very bright
star right there, next to your planet. But in general it's a problem because what you
need is a bright star nearby, just so much like that even after you've diced it up in all
the different ways, you can still measure the distortion with great precision.
So what a number of telescopes now do is actually create an artificial bright star just where
they want it. And the way they do this is what's called a Laser Guide Star System
and once again here's the example at Gemini North. So there's the laser bolted onto the side of the telescope.
And this is a sodium laser, and a sodium laser is kind of interesting
because our atmosphere has sodium in it. It has a little layer way up in the sky that
has sodium atoms in it, and you can excite those atoms and literally make a beacon in the sky with this system.
Yes, we'll see that here. So the light goes up, so it's transferred
to the top of the telescope and then bounced off into the sky from the top.
You'll see that in a second, here.
So we point up to the sky... (There we go, as the light goes up)
We take a laser beam, and it goes very dramatically into space.
And it comes up 50 kilometres, 70 kilometres... and we now hit a sodium layer. This is actually deposited
by meteorites primarily. You get meteorites and meteorite dust that lands in the upper
atmosphere and they deposit this layer of sodium. It's altitude varies but maybe about
90 kilometres up, and we look at that from the ground, so we change our orientation
it looks like a glowing orange dot. This is the same colour as sodium street lights, low pressure
sodium street lights. And that's giving us a bright artificial star exactly where we want.
So as the light comes back down through the atmosphere and back down to our telescope
we can get the light coming in and we can also look at that sodium artificial star
and use that to measure the distortion and correct for it.
Kind of interesting. It gets twice the effect, it goes up and down. So it sort of magnifies the effect and presumably
that has to be corrected for but can make it a bit easier to figure out what's going on with the atmosphere.
So you get the dual of the lightsabers here at Mauna Kea
you can see the laser coming from Gemini and also from the cape(?) telescopes in the background.
And presumably it's curved, not... only because of the optics of our picture that we're taking.
Yeah, I think this was taken with a fish-eye lens. It's quite difficult to operate these things.
They have to be absolutely clear that there are no aircraft around. So they have
lots of students up there whose job it is to look at one quarter of the sky and have
a big red button to say "turn it off now, there's an aeroplane coming over!"
They also have to notify US space command ahead of time. We're going to look at these targets at these times...
please tell us if we shouldn't.
We wouldn't want to blind a downward looking device.
So that's adaptive optics.