Adaptive Optics in Astronomy and Eyes

I was talking to a person a while back who happens to be an eye surgeon. I was telling them about my new book about observatories. We got to comparing notes about technology in our respective fields and they mentioned a procedure they use for measurements INSIDE the eye. It turns out to very closely resemble adaptive optics used for astronomy. I thought to myself, “How cool is that!”

And, it turns out, it’s pretty cool. The application of the tech is saving people’s eyesight just as the astronomers are clarifying our view of the cosmos.

Clearing the View with Adaptive Optics

So, adaptive optics has been a lifesaver for ground-based telescopes in particular. It is not a brand-new technology. Some version has been around since the second half of the 20th century when it started in military applications. Then, the technology migrated to observatories. Its main purpose is to clarify the views that telescopes get.

All telescopes on the ground have to “look” through our blanket of air. It muddies the view. Instead of seeing a clear, strong point of light when we look at a star, the atmosphere “messes with” the light, making the star look wavery. It’s what makes stars twinkle, actually. But, to get good astrophysical data, astronomers need a clear, steady view of their targets. This is where adaptive optics comes in handy. It takes the wavefront(s) of light coming into the mirror and deforms the telescope mirrors to account for distortions in the wavefronts caused by our atmosphere.

Lasers to the Rescue

If there’s not a nearby guide star, then astronomers create one. They shine a laser beam to a point in our atmosphere about 90 or so kilometers up. The laser light energizes gas atoms to create an artificial guide star. The telescope then focuses on that, the computers analyze the light that has traveled through the atmosphere from the laser guide star. That data tells the actuators to “actuate”, the mirrors deform, and astronomy gets done. (If you want more complex info on how this works, check out the European Southern Observatory page about the process, or the Gemini Observatory pages on the instruments they use. Gemini has done many, many observations using AO, including this one.

Of course, the telescope system has to know about the atmospheric distortions. One way to “know” is to look at the light of a bright star near the field of view of the target. It’s called a “guide star”. Computers analyze the distorted light from the guide star and use that information to send commands to the deformable mirrors. Those mirrors are then reshaped from behind (using little motors called “actuators”). That “cancels out” the incoming distortions and produces a sharp clear image or data set for the astronomers to study.

Measuring Inner Space

So, that’s a rough idea of how we use adaptive optics for space studies. How does that translate to measuring eyeballs? As with the telescope, astronomers measure the distortions in the propagation of light from a target in space. The distortions are also called aberrations.

Our eyes also take in light, and the structure of the eye itself can cause aberrations to the light that travels through the iris to the back of the eye and the retina. In a perfectly working eye, wavefronts of light smack into the retina perfectly. The person sees a sharp, clear image of whatever they’re looking at.

Fixing Imperfections

Unfortunately, virtually nobody has a perfect eye. We’ve all got little imperfections. Our eyes aren’t perfectly round, or we may have cataracts in the front (which causes the light to spread out as it passes through), or we can have other problems. The end result is that we don’t see perfectly and those imperfections cause aberrations.

So, how to measure the amount of aberration? Eye doctors have various technological instruments that apply this technique as they study the interiors of people’s eyes. The end result helps them determine how much correction a person needs. If you’ve ever had an image taken of the inside of your eye, it’s likely that wavefront aberrometry was used.

Lasers and Cataracts

Sometimes, eye doctors use lasers as part of the way they measure the wavefronts of light as they pass through the eye. This is particularly useful during such procedures as cataract surgery. During the operation, the surgeon takes out the cloudy lens affecting the person’s vision. Then, they use laser light to measure the wavefront aberrations through the eye. That helps them to determine the exact power of the lens to put into the eye. Then, once it’s in, they measure again.

If it’s good, then the operation is done. If it’s not, they try another lens. It’s a closely related procedure to what astronomers use when they study distant objects such as star clusters, nebulae, and galaxies. Interested in reading more about that procedure? Check out Eye Wiki.

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