A group of researchers led by Takeo Minezaki at Institute of Astronomy (IoA), the University of Tokyo (UT), and Leonardo Vanzi at the Astro Engineering Center (AIUC), the Pontifical Catholic University of Chile (PUC), succeeded to achieve an angular resolution close to the diffraction limit by an adaptive optics system in visible wavelength mounted on the 1-m telescope of the European Southern Observatory (ESO) of La Silla in Chile.
We can see stars twinkling in the night sky. This is caused by atmospheric turbulence, which has bad influence on the propagation of light from stars. This atmospheric turbulence makes stars not only twinkle but also blurred. So it constrains the angular resolution of ground-based telescopes, which makes it difficult to study the detailed structure of distant celestial objects. Adaptive optics, or AO, is a technology to overcome this problem. AO systems measure and correct the image errors caused by the atmospheric turbulence in real time to obtain sharp images.
Most of existing AO systems in practical use are large in size and very expensive, and are usually mounted on large telescopes for observations in near-infrared wavelength. But in fact, comparably high angular resolution can be achieved by AO systems mounted on small telescopes for shorter wavelength, namely, visible light, according to the theory of diffraction of light. If such AO systems for small telescopes become widely available, many unique science programs can be performed. By this motivation, a group of researchers led by Takeo Minezaki at IoA, UT, developed an optical AO system for small telescopes. The most distinctive feature is that it is compact and lightweight, and easy to transport. It is also less expensive compared to other optical AO systems for small telescopes by adopting inexpensive products for the key components of the AO technology that become recently available.
The test observation of the AO system had been carried out in Japan, though the atmospheric turbulence is usually large, which makes it difficult to correct the blurred image accurately. Obviously, smaller atmospheric turbulence improves the AO performance. In pursuit of good atmospheric conditions, Minezaki started to discuss with Keiichi Ohnaka at the Catholic University of the North (UCN) three years ago on possible observations with small telescopes in Chile, and started to prepare for the observation of the AO system on the 1-m telescope of the La Silla observatory under the collaboration with Leonardo Vanzi at AIUC, PUC from last year. On the 1-m telescope, UCN agreed with ESO on its exclusive use, and the PUC team developed its new control and instrumentation system under their collaboration.
In March 2018, the AO system was transported from Japan to Chile and mounted on the 1-m telescope, then test observations were carried out. We find that stellar images became remarkably sharp by the AO system (Figure 1), and the achieved angular resolution was 0.18 arcsec in full width at half maximum, which is close to the theoretical limit of diffraction of light for the telescope aperture of 1 m and the observing wavelength of 0.65 µm (Figure 2). This result demonstrates a new capability of the AO technology for small telescopes. In addition, the experience and knowledge for the AO technology obtained here will be applied to the next generation instruments for the University of Tokyo Atacama Observatory (TAO) 6.5m telescope. An instrument is under development jointly with PUC and UT.
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Figure 1 : The obtained stellar images (colored darker when brighter). The left side shows the image corrected by the AO system, and the right side shows the uncorrected image. The arrow represents the angular size of 1 arcsec. |
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Figure 2 : The radial brightness profile of the stellar image corrected by the AO system. The full width at half maximum of the stellar image is 0.18 arcsec, which corresponds to the angular resolution. |
The web news on this research at the School of Engineering, PUC
The University of Tokyo Atacama Observatory (TAO) web page at IoA, UT
Takeo Minezaki (The University of Tokyo, Institute of Astronomy, Associate Professor)
Yukihiro Kono (The University of Tokyo, graduate student)
Leonardo Vanzi (Pontificia Universidad Católica de Chile, Departamento de IngenieriaElectrica, Centro de Astro-Ingeniería, Associate Professor)
Abner Zapata (Pontificia Universidad Católica de Chile)
Mauricio Flores (Pontificia Universidad Católica de Chile)
Sebastián Ramírez (Pontificia Universidad Católica de Chile)
Keiichi Ohnaka (Universidad Católica del Norte, Instituto de Astronomia, Associate Professor)
This work was partly supported by the Grant-in-Aid (No. 25287033) from the Japan Society for the Promotion of Science (JSPS), Re-Inventing Japan Project Science and Engineering Exchange program with Latin America (SEELA), Strategic Partnerships Project (SGU), and Graduate Research Abroad in Science Program of the University of Tokyo (GRASP). The facilities of the Advanced Technology Center of National Astronomical Observatory of Japan were used.
