Smoothed Light Will Help Search for Earth’s Twins

Schematic diagram of the significantly unbalanced interferometer (Source: MIPT)

Schematic diagram of the significantly unbalanced interferometer (Source: MIPT)

Physi­cists of the Moscow Institute of Physics and Techno­logy MIPT and the Space Research Institute of the Russian Academy of Sciences developed optical technology for the “correction” of light coming from distant stars, which will signi­ficantly improve the “seeing” of teles­copes and therefore will enable us to directly observe exo­planets as Earth-twins.

The first exo­planets, 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 the 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, astro­nomers were able to directly obtain images of exoplanets in the late 2000s. 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.

“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. 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.” (Source: MIPT)

Reference: I. Shashkova et al.: Extremely unbalanced interferometer for precise wavefront control in stellar coronagraphs, J. Astron. Telesc. Instrum. Syst. 2(1), 011011 (2016), DOI: 10.1117/1.JATIS.2.1.011011

Link: Moscow Institute of Physics and Technology, Moscow, Russia • IKI-RAS Space Research Institute of RAS, Moscow, Russia

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