ADAPTIVE OPTICS

 

ADAPTIVE OPTICS -REVOLUTIONISING GROUND-BASED ASTRONOMY 

 

Ever wondered why stars twinkle at night, or, why Moon, when taken a video of, appears distorted? The reason is our atmosphere. When light enters our atmosphere, it is affected by winds in the
 atmosphere and by areas with different temperatures and densities. Different densities have different refractive indices, which are governed by Snell's law. So, the waves emitted by stars undergo continuous refraction in our atmosphere, which gives them the distorted look when seen from Earth.

 

This possesses a huge challenge for ground-based telescopes. These telescopes collect faint lights from far-away objects and atmospheric turbulence ruins the pictures.

To overcome this problem, we can do two things. First, is to take the telescope out of our atmosphere, i.e., send it to space. But this is expensive.

 

Fun Fact: James Webb, a space-based telescope, costs 10 times the estimated value of Monalisa or about buying 24 Boeing 747-8s.

 

The other way is to use adaptive optics. In simple words, adaptive optics is changing the shape of mirror to nullify the effects of atmosphere. It involves state-of-the-art deformable mirrors that change their shape using hundreds of actuators, which are governed in real-time by computers. This is so effective that the images turn out to be as good as when taken from space.

 

Credit: ESO/P. Weilbacher (AIP)

 

Fun Fact: VLT's 1.12m secondary mirror has about 1170 actuators!

 

Adaptive optics requires a fairly bright star to measure the distortion caused by the local atmosphere. Since, it is not always possible to find a bright-enough star, astronomers can create artificial stars instead by shining a powerful laser beam into the Earth's upper atmosphere. This cool-looking laser shot into the sky acts as a guide star to help measure the atmospheric effect and compensate it using deformable mirrors.

 

 Credit: ESO/M. Zamani

The incoming light coming from the guide star is passed through a wave front sensor. The most popular one is Shack-Hartmann WFS. It has several lenses called lenslets which creates images on sub apertures. These images are displaced from a perfect grid by distortions, and their displacements can be used to calculate the distortions so we can correct them.

 

Without Atmospheric Turbulence

With Atmospheric Turbulence

AO requires high-speed computer processing and unique deformable mirrors. It is still very much in development stage and big advancements are expected in the upcoming future.

 

Upcoming telescopes are making use of this amazing technology. ESO's upcoming ELT, scheduled to be completed by 2025, will have about 5000 actuators in its quaternary mirror. The Giant Magellan Telescope (GMT) at Las Campanas observatory in Chile, will have a large segmented adaptive secondary mirror with roughly the same number of actuators as the ELT.

credit: ESO