'Smoothed' light will help search for Earth's twins

The first exoplanets (extra solar planets), which are the planets outside our solar system, had been discovered in the late 20th century, and now we have detected of more than two thousand of them. It is almost impossible to see the faint light of the planets themselves without special tools -- it is saturated "overshadowed" by the radiation of parent star. Therefore exoplanets are discovered by indirect methods: by registration of the weak periodic fluctuations in the luminosity of the star when a planet passes in front of its disk (the transit method), or by spectral translational vibrations of the star itself from the impact of the planet's gravity (the radial-velocity method). For the first time, in the late 2000s, astronomers were able to directly obtain images of exoplanets. So far we have about 65 of such images. To obtain them, the scientists use stellar coronagraphs first created in 1930s for observations of the solar corona outside eclipses known as solar coronagraphs. These devices have a focal mask -- an "artificial moon" inside them, which blocks some part of the field of view -- ultimately, it covers the solar disk, allowing you to see the dim solar corona.

To repeat this technique for the stars, we need a much higher level of accuracy and much higher resolution of the telescope, which accommodates a coronagraph. Apparent size of the orbit of Earth-type planets, nearest to us, is about 0.1 arcseconds. This is close to the resolution limit of modern space telescopes (for example, the resolution of the space telescope Hubble is about 0.05 seconds). To remove the effects of atmospheric distortions in ground-based telescopes, scientists use adaptive optics -- mirrors that can change shape while adjusting to the state of the atmosphere. In some cases, the mirror shape can be maintained with an accuracy of 1 nanometer, but such systems do not keep pace with the dynamics of atmospheric changes and are extremely expensive.

A team led by Alexander Tavrov, an associate professor at MIPT and the Head of the Planetary Astronomy Laboratory at the Space Research Institute of the Russian Academy of Sciences, has found a way to obtain the highest resolution, while using relatively simple and inexpensive systems of adaptive optics.

They used the idea of a EUI (Extremely Unbalanced Interferometer) proposed by one of the article's authors -- Juno Nishikawa, a Japanese scientist working at the National Astronomical Observatory of Japan. Conventional interferometry implies using the waves with approximately equal intensity for combining them into a single wavefront with the purpose of producing a clear and sharp image. The EUI light is divided into two beams (weak and strong), whose amplitudes have an approximate preset ratio of 1:10. A weak beam passes through the adaptive optics system, after which the two beams are brought together again and interfere with each other. As a result, the weak beam, so to say, "smoothes out" the light of the strong beam, which can significantly reduce both the distortion of the wavefront and the contribution of stellar speckle patterns (a random interference pattern).

"Through the use of a relatively simple optical set-up, we can obtain the image contrast at the quality necessary for the direct observation of Earth-type planets by means of coronagraphs. Of course, compared to foreign developments, our system requires a more complex control technique, but at the same time it is much less dependent on the temperature stability that greatly simplifies its operation in space," the team leader Alexander Tavrov says.

With the help of computer simulation, they have determined approximate characteristics of the system developed by them. According to calculations, the resulting scheme provides the image contrast of about 10-9. Furthermore, it was demonstrated that EUI shows achromatism, i.e. the reduction of aberrations with increasing wavelength.

In the future, scientists plan to create a laboratory prototype and perform a number of experiments on it. As Alexander Tavrov notes, "We want to see the distant worlds through a telescope, but it implies that the distant worlds might see us as well. An advanced technology -- by only some of 50 to 100 years -- could be enough to do it many times more precisely than we are able to do it now."