This article presents the challenges in observing the sky with a ground telescope and present the need and use of adaptive optics.
Observations of the sky Astronomers and engineers : a partnership to the skies
When thinking about astronomers, people imagine an old guy with white hair and a beard wearing a lab coat looking through the lenses of a telescope and trying to find a phenomenon in the sky. But nowadays, an astronomer is someone working behind a computer, analysing data (numbers or images).
These data are obtained through digital cameras installed on instruments designed specifically for their research work. To build these instruments, astronomers work with engineers who will take into account what the astronomers want to observe. Then they will design the instrument and provide data to the astronomer.
The observation goals can vary greatly because different areas of astronomy exist : planetary science, star formation, galaxies... In this article, we will focus on a specific observation technique that provides astronomers with images : through the mirrors of a telescope.
Pushing the edge of sky discoveries
Telescopes have been around since the 17th century but advances in technology allow engineer to design bigger and more accurate telescopes that collect more and more light to observe objects that are not visible to the unaided eye or a smaller telescope.
As you may have understood, the farther away the object observed is, the dimmer the light is. And these are the objects that astronomers want to get to know better.
They need bigger telescope to collect more light and therefore get better images quality. A problem rose from these bigger telescope : the image quality didn’t get better. And the culprit is the atmosphere.
Why is the atmosphere not relevant for astronomers ? To illustrate the effect of the atmosphere on the images observed through a telescope, let’s set up a simple example : it’s summertime, the weather is hot and your family is gathered outside. You’re sipping a cold lemonade while daddy is taking care of the barbecue. Behind the barbecue, all the sauce bottles are lined up and you’re trying to read the label of those bottles through the heat of the barbecue. Everything is blurry and wavy due to the warmth generated by the fire.
The difference of temperature is the cause of the blur. The same phenomena of temperature differences leads to the deformed images in telescopes. But a solution has been developed to correct these images and render them as if the atmosphere was not there : the adaptive optics.
The adaptive optics
Sensing the deformation : wave-front sensor
First, we need to know how the light comes to the sensor and most importantly how deformed it arrives to the system. A device called the wave-front sensor is used for that purpose.
Correcting the deformation : Deformable mirror
Then after computation in the system, a deformable mirror changes its shape to reform the light as it should have been without the atmosphere. And finally, the image that the scientist expect is ready to be analysed.
A simple adaptive optics system
Here is a picture describing a simple adaptive optics system. The light comes from a star near the object that we want to study, it reaches the telescope and the adaptive optics system measures the deformation of the light and corrects it before it reaches the science camera that records the data for the astronomer.
Astronomers rely on engineers to observe the sky. Together, they build instruments such as telescopes and have to work as partners to overcome challenges that are presenting themselves in the form of nature. The adaptive optics are a perfect example of that partnership.