The Earth’s atmosphere keeps us safe from the harsh environment of space, but it also
obscures our view into the cosmos.
No matter how powerful a telescope you build, the turbulence of the atmosphere limits your
resolution.
But astronomers and engineers have an amazing technology that allows a telescope to peer
into space as if the atmosphere isn’t even there, producing images from here on the ground
which are as sharp and clear as if the telescope was out in space.
It’s called adaptive optics, and we’re now at the point where the most powerful ground-based
telescopes have matched and even exceeded the capability of space telescopes.
Let’s start with an amazing photograph now that brings the capability of adaptive optics
into focus.
This is a photograph of Neptune captured by the European Southern Observatory’s Very
Large Telescope, using its new adaptive optics instrument called MUSE.
We’ll talk more about adaptive optics and MUSE in a second, but check this out.
Compare this with the best image ever taken by the Hubble Space Telescope.
Now, the surface features aren’t going to match up because they were taken at different
times, but wow.
And here’s a photo of Neptune taken by Voyager 2.
Okay, fine, this one is better, but keep in mind that Voyager 2 was only 7 million kilometers
away from the planet.
While Earth and Hubble-based images are more than 600 times farther away.
So it’s not exactly fair.
Let’s look at another image recently released from the European Southern Observatory, zooming
in and in on the globular star cluster NGC 6388.
This is the field of view captured in the Very Large Telescope’s Wide Field Mode,
and this is the improvement using the MUSE Narrow Field Mode.
Again, wow, it’s clearly full of stars.
Let’s talk about adaptive optics.
The turbulence of the Earth’s atmosphere is a big problem for astronomers, and this
is why space telescopes like Hubble were developed.
Of course, for all their clarity, space telescopes have their downsides; they’re expensive
and difficult to maintain and upgrade.
And if anything goes wrong, you need to send spacewalking astronauts to fix them.
When a powerful ground-based telescope is pointed at an object in space, like Neptune,
for example, the Earth’s atmosphere blurs that object.
Let’s go back to that photo of Neptune and show a comparison of with and without adaptive
optics.
What’s really happening here is that the image of Neptune is shifting around in the
telescope’s field of view.
If you do a really long exposure, all you get is more blur.
You can never resolve your image with pinpoint accuracy.
In fact, the most powerful telescopes on Earth have no better spatial resolution than a typical
backyard telescope, thanks to the atmosphere.
As far back as the 1950s, engineers proposed that if you could somehow deform the mirror
of a telescope in real time to account for the atmospheric turbulence, you should be
able to remove its distortions.
In the 1960s and 70s, the US military developed the first adaptive optics systems, such as
the Starfire Optical Range in New Mexico and the Advanced Electro-Optical Facility in Hawaii.
They found that they could deform a mirror in just one dimension, they could sharpen
the image seen by their telescopes.
Based these advances, more sensitive adaptive optics were attached to the huge twin Keck
observatories in Hawaii.
The European Southern Observatory has been using adaptive optics on their telescopes
for almost 20 years now.
So here’s exactly how they work.
Let’s say a telescope is looking at a faint galaxy in the sky.
Astronomers locate a relatively bright star within about 10 arcseconds of the target.
Just to give you an idea of how small that is, one arcsecond is a dime viewed about 2
kilometers away.
Light from both the guide star and the galaxy pass through the telescope’s optics and
a special camera called a wavefront sensor measures the guide star’s distortions, hundreds
or even thousands of times a second.
That’s why the guide star has to be bright; if it’s too faint, you can’t see it changing
quickly enough to provide any data.
This information is passed along to a computer that calculates the distortions and then sends
instructions to a deformable mirror on the telescope.
This mirror is tweaked and deformed in real time to reverse the effects of that atmospheric
turbulence.
Deformable mirrors are usually a piece of glass that’s a few centimeters thick, which
has a series of actuators behind that can be shortened or lengthened.
As the waves of light from the telescope strike the mirror, they’re corrected in real time
to remove the atmospheric distortions and passed along to the telescope’s camera system.
So what do you do if there isn’t a bright star near the object you want to photograph?
For these situations, astronomers create an artificial guide star.
They shoot a powerful laser up from the observatory that’s tuned to a very specific wavelength
of light that causes sodium atoms to glow in a region that’s about 100 km in altitude.
This glowing region creates an artificial star that’s bright enough to serve as a
guide star for the astronomer’s observations.
Now that you understand how adaptive optics works in general, I’d like to talk about
some of these specific instruments.
And I’ll get to that in a second, but first I’d like to thank:
Stephen Fleetwood Jörg Karwath
Dale Kirkwood Brian Westcott
Kirk Briggs Scotty Jones
And the rest of our 827 patrons for their generous support.
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That photo of Neptune I showed you earlier was taken by the European Southern Observatory’s
MUSE instrument, which stands for the Multi-Unit Spectroscopic Explorer.
It’s attached to one of the Very Large Telescope’s four 8.4 meter telescopes.
Instead of a single guide star, the telescope fires 4 separate lasers into space, giving
MUSE 4 artificial guide stars to compensate for atmospheric distortion.
The original Wide-Field Mode was unveiled in the summer of 2017, and allowed astronomers
to observe a region of the sky that was about an arcminute across.
If you want something that’s only about an arcminute, the Saturn Nebula is a great
fit.
Here’s a stunning image that ESO released in September of 2017 showing the Saturn Nebula,
also known as NGC 7009.
Located about 5,000 light-years from Earth, this was once a star like our Sun that reached
the end of its life, and expanded into a red giant, throwing its outer layers off into
space.
This is Wide-Field Mode.
The big announcement just a few weeks ago is that its Narrow-Field Mode is now operational.
Just like Wide-Field, it uses adaptive optics to remove the turbulence from the atmosphere,
but it can show a dramatically smaller piece of the sky.
Remember, this is incredibly difficult because you’re not likely to get a usable guide
star in a smaller field of view.
That’s why the lasers are so important.
This new mode allows the telescope to see a region just 7.5 arcseconds across.
Neptune, if you’re wondering, is just 2.4 arcseconds across at its closest, which is
why this was one of the targets chosen to celebrate the Narrow-Field Mode.
It’s not just visible light that takes advantage of adaptive optics.
The Very Large Telescope is also equipped with an instrument called GRAAL, or the Ground
Layer Adaptive Optics module, which allows the telescope to see objects in the infrared
spectrum.
This is when you want to see cooler objects like brown dwarfs, newly forming planetary
systems and extrasolar planets.
Again, from the ground.
And ESO is planning to release another instrument shortly called ERIS, or the Enhanced Resolution
Imager and Spectograph, which will take its infrared capabilities to the next level.
It’s expected to see first light in 2020.
Perhaps the most exciting part about this whole announcement is that this adaptive optics
suite of instruments on the Very Large Telescope is really just a proof of concept; a way to
test out this next level of adaptive optics in preparation for the next generation of
supertelescopes.
A larger, more capable version of this technology is going to be attached to the European Southern
Observatory’s upcoming Extremely Large Telescope, which will measure 39-meters across and gather
15 times more light than any operational telescope today.
The EELT will bounce the light off 5 separate mirrors, sampling the wavefront of light up
to 1000 times a second, and then using 5000 separate actuators to adjust for the turbulence
of the atmosphere.
In fact, several instruments attached to the telescope will have their own adaptive optics
systems, letting them tune the light.
If all goes well, it’ll see first light in 2024.
And as soon as it does, expect to see those lasers zapping into space.
A telescope 39 meters across, with a state of the art adaptive optics system.
I can’t wait.
Similar systems will be attached to the Giant Magellan Telescope and the 30-Meter Telescope.
I’ve attached a link to a paper that details precisely how all these next generation adaptive
optics systems will work, so if you really want the technical details, you should check
them out.
Conclusion I really think we’re at a turning point
now, where scientists are going to be making enormous advances thanks to both ground and
space-based observatories.
Instead of being trapped under the blanket of the Earth’s atmosphere, astronomers have
figured out how to compensate for it.
To ignore it.
What do you think?
Let me know your thoughts in the comments.
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