ESOcast 34: How To Stop a Star's Twinkle - The astronomy podcast exploring the cosmic frontier

Uploaded by nvdktube on 16.09.2012

The Allgäu public observatory
lies amidst the picturesque landscape of southern Germany.
As night falls,
a team of scientists and engineers
prepares to field test a very cool piece of technology:
a laser guide star unit,
which will soon be on its way to ESO’s Paranal Observatory.
This is the ESOcast!
Cutting-edge science and life behind the scenes at ESO,
the European Southern Observatory.
Exploring the ultimate frontier with our host Dr J, a.k.a. Dr Joe Liske.
Hello and welcome to the ESOcast.
Today we’re at the Allgäu public observatory in southern Germany
because this is where a team of scientists and engineers from ESO
is testing a brand new laser guide star unit.
‘What’s that?’ you ask.
Let me explain.
Now, we have all looked at the sky at night and seen the stars twinkling.
Now, the stars themselves, of course, don’t do any twinkling.
The twinkling is caused by turbulence in the Earth’s atmosphere.
As the starlight crosses the atmosphere
it encounters different pockets of air
with different temperature and pressure
which bend the light in different ways,
thus causing distortions.
In fact you can see this effect often in broad daylight,
whenever you look towards a distant object on the horizon
on a hot day.
Now the twinkling is all very pretty and even romantic,
but for us astronomers it’s actually a real problem
because it means that our images are blurred
and less sharp than they could be
if it wasn’t for the atmosphere.
So, what do we do about it?
Essentially we need a method to cancel out the distortions,
in effect, to “un-twinkle” the stars.
The way to do it is to bounce the starlight off a mirror
that is slightly deformed in exactly the right manner
to cancel out the distortions.
But how do you know how to deform your mirror?
As ESO’s Very Large Telescope observes the sky,
a specialised computer can pick a bright star
and constantly monitor how it twinkles
— deducing the atmospheric conditions above the telescope
many hundreds of times a second.
The computer then sends commands
to a series of devices attached to a mirror in the telescope,
bending and flexing it precisely in time with the atmospheric turbulence,
cancelling out the distortion in the images.
So, for this correction process to work
you need a really bright star
in the field of view of your telescope.
But bright stars are very few and far between,
and remember that the VLT was designed
to image only a very small part of the sky at any given time.
So for most observations
there just won’t be a bright star in the field of view of the VLT.
So what do we do now?
we make our own.
90 kilometres above our heads,
in the upper atmosphere,
is a relatively thin layer of sodium.
If you fire a powerful laser beam into the sky
you can make these sodium atoms glow,
thereby effectively creating an artificial star
for the computer to lock on to.
In 2006,
ESO installed the Southern Hemisphere’s first laser guide star on the VLT.
This system greatly improves the telescope’s power,
meaning the VLT can even make sharper images than Hubble
for certain types of observation.
But this existing system has limitations.
It can only create one artificial star at once
meaning it can only correct the telescope’s vision
for a small part of the sky at any one time.
It’s also very bulky
– the equipment has to be kept in a separate laboratory
and the laser beam fed along an optical fibre to the telescope.
Based on the experience obtained with its first system,
ESO engineers have been working to build a much improved,
new laser guide star unit.
So, Domenicos, this is it — this is the laser.
It’s incredibly small,
it fits on the back of this small telescope,
that’s amazing.
Yes. So this is what we’ve been working on for the past five years,
to make a 20-watt laser, very compact
and lightweight
so that it can be mounted directly on the back of the telescope.
So we had to develop fibre lasers first
and then developed these kinds of laser heads.
So, you’ve just said it, it’s a 20-watt laser.
That’s quite a bit of power isn’t it?
Yes. This is the power we’ll need, actually, for
the next generation of laser guide star systems.
And right now, for example, at Paranal
we have about 5-watt in the sky,
so this is quite a jump in power.
Is the laser beam that comes out of the end of this telescope dangerous?
What happens if I put my hand into it?
If you put your hand in, you’ll feel warmth.
But don’t have to look into the beam.
OK, so it won’t burn my hand.
But what about aeroplanes,
is it dangerous for them?
It’s not dangerous for the equipment or for the aeroplane,
it’s dangerous for the eyes of the passengers.
And, this laser is above the maximum permitted exposure so
we have to avoid planes crossing the beam.
In fact, here where we are now
we have obtained a no-fly zone above us,
so we don’t risk hitting a plane.
The new device is more reliable,
easier to maintain, and much smaller.
In fact, as we’ve just seen,
the whole unit fits into one small package
which is easy to mount on the launch telescope.
Because it’s so much smaller,
up to four of these lasers can be installed on a single telescope,
correcting the VLT’s image over a much wider field of view.
So what’s happening here in Germany
is that our team is testing the new prototype
to make sure that it works perfectly before it gets shipped to Paranal.
The facilities here at the Allgäu public observatory
are perfect for this
— and, what’s more,
they’re only a short drive from ESO Headquarters.
Laser guide stars like this will be crucial
for the forthcoming European Extremely Large Telescope,
which will use adaptive optics routinely.
The telescope will be many times the size of today’s biggest telescopes,
which should mean much sharper image quality.
But this great image quality will depend on how well the adaptive optics
and the laser guide stars work.
Pioneering new technologies like these
will make a big difference to the world’s most advanced observatories of the future,
especially the E-ELT.
This is Dr J signing off for the ESOcast.
Join me again next time for another cosmic adventure.
While we were filming this episode,
we got a stark reminder of why ESO’s telescopes
are located on mountaintops of Northern Chile,
and not here in the hills of Southern Germany.
Thankfully, storms like this are not something you ever see at Paranal.
ESOcast is produced by ESO, the European Southern Observatory.
ESO, the European Southern Observatory, is the pre-eminent intergovernmental science and technology organisation in astronomy,
designing, constructing and operating the world’s most advanced ground-based telescopes.
Transcription by ESO ; translation by —
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