There's a large amount of research in adaptive optics here at ANU, and it's a great pleasure
to welcome Céline D'Orgeville and Francois Rigaut who are some of our local experts.
Céline works on the lasers that are used in adaptive optics, and Francois works on
the adaptive optic systems themselves.
So Francois, you are, I guess despite your young age one of the grandfathers of the field of adaptive optics.
From my memory, you were part of the first program to put adaptive optics
on a telescope which we are allowed to know about.
I believe there were ones that the military did before, but what was it like
working back in 1989 on that first system?
Oh, it was very very exciting.
I was actually lucky enough to do my PhD, and it was the beginning of the second PhD ever on adaptive
optics for astronomy so, on the first adaptive optics system for astronomy, ever. So it started
in 1989, it was in Europe in France actually. At the time we also had a collaboration with
ESO, and at the time we didn't know if it was going to work of course. So we assembled
everything in the lab, and then we went to the telescope in Haute-Provence in the
south of France at the 1.5 metre. And then we turn the system on and it was working and
we could use it, you know. The image size from maybe typically one arcsecond or one point
five arcseconds, down to like point four, which was the diffraction minutes.
So how were those systems different than the ones we use today?
They were much simpler. They were the same if you want... essentially it's the same, and in this system what you do is
that you analyse the deformation of light, then you correct it with some kind of device.
But nowadays the systems are like a hundred times more potent. Because there are many
more of these little actuators that you use to correct the light, and they will also install
on telescopes that are typically eight to ten metres instead of the one point five metre at the time.
So back in 1989, did you have powerful lasers to make your own little beacons
(No) or did you just use stars?
Yes, exactly. At the very beginning we used stars because it was
already, you know... wonderful. And then quickly we realised that even though the technique
was very powerful, it was out of our limits in it's capabilities. Just because you needed
such a bright star to analyse the waveforms. So in 1985 actually the first civillians, ah, astronomers actually
a researcher, came up with the concept of shooting your own laser to actually create an artificial
guide star in the upper atmosphere at about a hundred kilometres. And since then people
have been working on intergrating these lasers with adaptive optic systems to make them all
encompassing if you want. We can now find them everywhere, and get nice images.
So you can put a star anywhere in the sky and that's a huge advantage we have (exactly) nowadays.
Yep, so Céline, the lasers are often the technically hardest part of the thing to get right. At least they
seem to be broken most of the time and everytime I hear a telescope bulletin saying "delay" it's
usually because of the lasers rather than anything else.
What are the challenges? Why is it so hard to get a laser to make a star like this?
Well, to get started there isn't that many people interested in producing powerful lasers at that kind of wavelength.
If we're talking five eighty nine nanometre, that's the sodium line, it's bright orange
and we want typically ten watt per... laser guide stars. So one is a guide star watt
five as a guide star is fifty watt... this kind of laser power you don't get off the shelf.
Actually there's no... there was a company until maybe two years ago
that could deliver such a system.
So I guess most lasers are used for fibre optic communications and they work in the infrared, not at these wavelengths.
Well there's lasers working all over the spectrum
from the UV to the infrared really, but at this particular wavelength typically it's
been used for spectroscopy experiments in the lab and they use just a few milliwatts for that.
So, for astronomy we need, you know, watts. And tens of watts. (Yeah, so ten watts is a really big laser)
It's a pretty powerful laser that one, and so you have to develop technology to
get there, and there's been several generations of sodium lasers. So the first lasers were
liquid lasers, they used liquid dye as the laser medium, and they've done a good job
but you know, they were cumbersome to use and a bit unsafe in that they run with alcohol
and so there was a flammable issue with those. So eventually people went to solid state types
of lasers, and that's what... for instance, runs on the Gemini North and Gemini South
telescopes in Hawaii and Chile. And the next generation ones are going to be fibre lasers.
So, the technology is definitely improving. I would say at this point, the lasers run well.
We know how to operate them, they're not the issue. The problem is with the logistics
of propagating a powerful laser beam into the sky. So typically you have to deal with
aircraft going over the observatory and you don't want to hit those. So you need some
sort of, either spotters watching the night sky, or some automated system based on cameras
to make sure that you're going to terminate the beam propagation when an aircraft is flying over.
That means turn them off when any aircraft's coming over, yes?
Yeah, you just close the shutter (I see, ok)
and you know, shutter the beam. But the rest of the system is still operating normally.
Yep.
The other thing you have to take care of at least for observatories that
have the US as a partner, is to make sure that they're not going to fire the laser into
a satellite and damage it. So they have to work with US space command to get, you know
approval to propagate the laser at any given time.
And they don't just tell you "here are the orbits of satellites to avoid"
you have to tell them where you're going to look basically, to say yes or no.
Exactly you can't know where the satellite is, because it's usually NASA
they don't really want to reveal. But you send them the list of what you want to observe
through the night, and they send you a list back saying
"Ok from that time to that time, you can't propagate."