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Evolution of adaptive optics devices with improved wavefront sensor performance
Published on October 16, 2025
There are several methods for observing exoplanets, and about 4000 exoplanets have been discovered so far using these methods. Of these, only around 10 to 20 exoplanets have been confirmed by “direct imaging” with an observation device using a telescope. The main mission of our research is to improve the accuracy of direct imaging observation equipment and capture the light emitted by as many planets as possible.
Adaptive optics is a technique that immediately corrects a wavefront disturbed by atmospheric fluctuations and obtains the clearest star image without distortion within the performance limits of a telescope. It is also a key component of the observation device.
In order to achieve real-time and highly accurate wavefront correction, high-speed readout performance and high resolution are required. In addition, some times wavefront correction is performed in situations where the number of photons is very small, such as darker celestial bodies and laser artificial stars, which requires high sensitivity in the camera.
What is needed to improve performance, and what will be achieved once the ultimate performance is reached? We asked Dr. Kodai Yamamoto of the Astronomical Observatory, Graduate School of Science, Kyoto University, who is working on direct imaging of exoplanets using a giant telescope, about the current performance of adaptive optics and future prospects of using them.
- The desire to directly image the actual light emitted by planets using telescopic observation equipment
- Lay the foundation for direct imaging of planets with a large telescope
- Improving the performance of wave-front sensors is the key to evolution
- Search for life outside of Earth
- Aiming for a device that eliminates the effects of turbulence
- Cameras are also an important factor in improving device performance
- Researcher profile
The desire to directly image the actual light emitted by planets using telescopic observation equipment
－Can you tell us about the current status of planetary observation using adaptive optics?
We are currently building an observation device to be installed on the newly built “Seimei”, the largest telescope in Japan, in Okayama Prefecture. This device will be used for direct imaging of exoplanets.
The worlds first direct optical detection of an exoplanet occurred in around 2008. Since that time only 1nly 1nly 10-20 others have been confirmed this way, out of around 4000 exoplanet discoveries. So the fact that we have directly imaged only 10 to 20 planets means that the detected number of planets is very small.
The reason for the small number is that the performance of observation devices for direct imaging is still lacking. However, an additional challenge is that exoplanets are very dark objects located near fixed stars that emit their own light, so it is very difficult to detect their light.
We have developed an exoplanet imager “Second-generation Exoplanet Imager with Coronagraphic AO (SEICA)” for the previously mentioned telescope “Seimei” and are trying to find darker celestial bodies. However, we have not yet achieved the performance needed to detect undiscovered planets.
Some of the world’s largest telescopes, such as the Subaru Telescope and the VLT in Chile, have apertures of 8 m to 10 m, and high detection performance. Since the detection performance of a telescope increases with its size, the performance of “Seimei”, whose aperture is less than half of those larger ones, is inferior. Although the performance of observation equipment is getting better and better, it is still not good enough to detect very dark objects near fixed stars.
Lay the foundation for direct imaging of planets with a large telescope
－The adaptive optics device “SEICA” was developed to find exoplanets by direct imaging. I’m sure that further performance improvements will be required in the future. But what kind of results do you expect to achieve by improving its performance?
There are two goals for “SEICA”. One is to detect exoplanets, and the other is to serve as a test-bed for technologies that can be applied to large telescopes in the future.
As mentioned earlier, when trying to develop a new technology it is actually very difficult because the target is dark and very close to a fixed star. This is not the only reason why it is difficult to observe. Another major influence on the quality of an image is the atmosphere. When the atmosphere is disturbed, the image is distorted in the form of the so-called blinking of stars, and the outline of the image is blurred or shifted, making it difficult to obtain a clear image. Adaptive optics have been developed to counteract the effects of such atmospheric turbulence.
While our main goal is to elucidate the actual state of planets by direct imaging of planets, we also hope that our research will result in improving the performance of the adaptive optics system and laying the foundation for direct imaging of planets in the future with a large optical telescope equipped with adaptive optics.
Adaptive optics device “SEICA”
Improving the performance of wave-front sensors is the key to evolution
－I have heard that improving the performance of adaptive optics is essential for achieving the first goal, “Detection of exoplanets”. What is the most necessary thing for that?
Among adaptive optics devices is an extreme adaptive optics device called “Extreme AO”. The Extreme AO is also used in the Subaru Telescope, but the one we have developed uses an FPGA to control the wavefront more quickly and precisely. The reason why we are pursuing such high-precision wavefront control is that, again, how to cancel out atmospheric turbulence is directly related to the accuracy of imaging.
In fact, the shape of the atmosphere itself does not change so much. If you look at it at a frequency of about 1 kHz or 6.5 kHz, it keeps almost the same shape, but when the wind blows, it gets swept away. So the wavefront shape you are watching thinking “this is it” will drift away the next moment and you will be looking at another shape. Therefore, the time-delay between measurement and correction appears as a shape of error. Strictly speaking, it not only corrects while measuring the wavefront in real time, but also controls by combining a control-algorithm that predicts and corrects the wavefront ahead of the wind.
The conventional “tile-measurement” wavefront sensors check the shape of a wavefront by looking at the inclination of each point on the wavefront and integrating it back to the original shape, but because of the integral process, measurement errors at each point propagate to the overall shape measurement. However, with the “phase measurement” used in our adaptive optics equipment, you can directly measure “how high is this position on the wavefront.”
As the phase can be measured directly, errors are less likely to occur. Since adaptive optics for direct imaging of exoplanets requires high-speed and high-precision wavefront control, so does wavefront measurement. Improving the performance of wavefront sensors is essential for the evolution of adaptive optics.
Search for life outside of Earth
－I would like to ask you in detail more about the second question, “application to large telescopes”. Furthermore, can you tell us about your ideas as you pursue your research?
At the level of studying exoplanets, I’m sure that what researchers think is the ultimate is “searching for life in places other than the earth.” Of course, it is absolutely impossible to optically find life on the surface of a planet, so I think there is a desire to properly investigate things such as the appearance of plants, the presence of oxygen, and the composition of the atmosphere, which would bring us a little closer to discovering life.
In other words, direct imaging means that you can do spectroscopy. If you can do spectroscopy, you can get crucial clues as to what kinds of molecules are present. I believe that the ultimate goal of our generation is to use direct imaging to get clues as to whether life exists or not, and if so, what kind of life it is.
Aiming for a device that eliminates the effects of turbulence
When we say “ultimate” in terms of device development, the ultimate goal is to create an adaptive optics device that completely eliminates the effects of atmospheric turbulence. Compared to the adaptive optics devices that existed about 20 years ago, the ones that are currently being developed have much better performance, and they are getting closer to the level of making a beautiful wavefront, but are still not the ultimate adaptive optics in the true sense. It requires faster measurement, computing, and correction. Without these pursuits, the ultimate goal cannot be achieved.
Cameras are also an important factor in improving device performance
－The fact that a high-speed and high-precision wavefront sensor is essential for faster calculation, computing, and correction means that the 2D sensor that captures images; that is, the camera side, is also required to have the same high level of performance, right?
That’s right. After all, what we want most is high speed and low noise. As I said earlier, there is a delay in the time from measuring a wavefront to applying correction, and that delay causes a temporal error. So corrected wavefront images taken by the camera are almost speed limiting to it. The faster the acquisition time, the shorter the exposure time, and the number of photons that can be detected will decrease per frame. That’s why I said high speed and low noise, but there is a real trade-off between the two, so the faster you go, the higher the readout noise and photon noise will be, and in contrast, if you try to lower them, you will naturally see temporal errors in the image.
In order to resolve the trade-off between the two, predictive control has been introduced into the control algorithm. However, it is difficult to implement complex control because of the cost of the computation, both in terms of hardware and software. If you build a dedicated circuit from measurement to control, you can shorten the calculation time to the limit, but the dedicated circuit is expensive and not easy to modify. A general PC also has the advantage of being able to freely develop and change algorithm. Therefore, we have adopted an FPGA, which is an intermediate device between a PC and a dedicated circuit, as a control device.
Even if the algorithm eliminates the trade-off, it is still very important to select a camera to be used for the wavefront sensor. We try to carefully select and adopt the one that has both high speed and low noise.
Researcher profile
Kodai Yamamoto, Ph.D.
Researcher at the Astronomical Observatory of the Graduate School of Science, Kyoto University (inter-university collaboration).
Completed doctoral program at the Graduate School of Science, Osaka University in 2014.
Yamamoto specializes in infrared astronomy, and engages in direct imaging observations of extrasolar planets, and development of observation equipment.
*The content presented on this page is based on an interview conducted in January 2021.
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