So Paul, this is absolutely amazing. It's like something out of Star Wars.
You go through, you have these huge telescopes, and then you put these little contact lenses in the forms
of a mirror, which you vibrate around at fifteen hundred times a second. To go through and
take the ripples out of the atmosphere, to make stars go from twinkling masses and blobs
to being sharp as pins. I mean honestly, 20 years ago when I was student, I remember
people talking about this and I was like "that's crazy, and it's never going to happen."
It's now got applications outside of astronomy. In fact originally a lot of it came out of military research.
If you remember the... talking about Star Wars, there was the Star Wars program back in
Ronald Raegan's time which was supposed to use lasers to shoot
down incoming ballistic missiles. The trouble with that is, you're trying
to fire a laser up through the atmosphere to hit a missile, which means you're pumping your kilowatts
or megawatts of power through the atmosphere, and the atmosphere of course has dust grains and
things which will get hot from the laser beam going through it. So you get a nice column of extremely
hot air just where you're looking, and of course hot air has a lower refractive index
and acts as a diverging lens. (Ah)
So the missle comes in and you focus your powerful laser at it
but instead of getting a missile melted, you get it slightly warmed up by a defocussed lens.
So they did a lot of the early research. They had to be able to bend their mirrors
to counteract that, and then it was declassified. I was actually at one of the talks when it was
first declassified, and astronomers have now used it. But it's now getting applications elsewhere.
Just last week a former student of mine who did this class about adaptive
optics in astronomy, and is now doing research on using it for medical imaging.
The idea being to actually look inside cells in the body or through tissues, and of course the
tissues are all different refractive indices, but you can once again bend your mirror to
cancel that out, and get sharp images looking inside cells and through body tissues.
So once again, you know, the little things we do here in astronomy to push the boundaries
end up being useful. So that's a good story. But you know, it's not a complete panacea.
We do have some issues where we can't use this under all circumstances.
Yes, I guess one big issue is at the moment we can only make this work at infrared wavelengths
not at visible wavelengths. The trouble is that... well the benefit of infrared is that hot and cold
air has only a very small amount of difference in a refractive index at these wavelengths.
So you're not trying to create very big effects. And also the changes are relatively slow
only a hundred times a second or so. Whereas if you want to work at visible light wavelengths
you're looking for much bigger shifts. You have to boot move the mirror by far more
and it has to be much faster, thousands of times a second.
At the moment we can't do it at optical wavelengths.
Right, so it's not impossible to do it but it's a little, it's technologically
beyond the scope of what we can do right now. And don't we have a problem that the atmosphere
unfortunately the little puddles of turbulence in the atmosphere are like that big, and our
telescopes are that big, and so we can only, that means correct a tiny little piece of sky?
Yes, of course we're looking through one part of the sky at our target, and we
can correct what's happening there. But let's say we wanted to take a picture of a big part
of the sky. Light going to a different angle will be looking through different parts of
the atmosphere. And so it's really hard to correct... you can correct near our guide
star on a laser, but trying to correct somewhere else... so we tend to get a very sharp image
of a very small part of the sky. That's fine for looking for new planets, but if for example
you wanted to map galaxies with this, it's such a small part we can't even map a whole galaxy.
But here at Mount Stromlo where they built an imager for the Gemini telescope which
has five lasers, and I know the whole point of that system is so that we can look at a
much bigger field of view. So even technology can help that out as well.
Yes, so the idea is by bouncing five lasers up, you can actually have a three dimensional map of the atmosphere
not just the one long line of sight, but a whole bunch and actually see when these blobs
are there moving back and forth across it.
So it's called multiple conjugate adaptive optics, a bit of a mouthful.
But it's now working and getting some spectacular results over wide fields.
And I've seen some of the images that we're using. They're getting to
essentially, I think, 90% of the way of what's theoretically possible where you have quantum
wave effects, which end up limiting you. So we're really close to really getting all the way there, this way.
Yes. I mean another traditional problem with adaptive optics is you've got
the light spread out, and you want to gather it altogether, and most adaptive optic systems
gather some of the light together, but leave us a halo of light that's very spread out.
So what's called the Strehl ratio, it's telling you what fraction of the light is gathered
in the middle and what fraction is further out. And to do better at this you need to
deformable mirrors with more actuators. The early ones might only have a few actuators
so they could take out the big blobs of hot and cold air but they can't take out the small ones.
But they're getting better and better all the time. They're now talking about ones
that have millions of actuators, so they can really start taking out all the really fine
variations in the atmosphere. The trouble with this is, to be able to take them out
you have to be able to measure them.
Right.
So you need to be able to measure say two thousand, three thousand
components of distortion, and for that you need a really bright star.
Or a really bright laser, presumably.
So the other thing they're doing is getting more and more powerful lasers.
So they can just pump so much light out there, that when it comes
down they can measure the really fine details of the distortion.
Right. Ok, and I've seen some of the new deformable mirrors aren't little ones in the lab. They're these giant
ones which are actually the secondary mirror of the telescope itself. I mean, they're this big
and they're huge and tiny little wafer thin pieces of glass with all these little magnets
behind them, and that go at fifteen hundred times a second.
A truly remarkable piece of equipment.
And advancing very fast. (Yep)
But I guess the real question is, can we see anything with these things?
So what we're going to talk about next time is
what have we learned so far from all these adaptive optic systems?
Can we actually directly see planets around other stars?