Before dawn on March 3, 2022 (Hawai`i Standard Time), Subaru Telescope achieved the first launch of a new laser guide star system, upgraded and used for the Subaru Telescope’s adaptive optics system.
Overcoming Atmospheric Turbulence
Stars appear to twinkle in the night sky. This is because light from stars is disturbed by fluctuations in the refractive index of air (atmospheric turbulence). For observation through ground-based telescopes, atmospheric turbulence causes undesirable effects of blurring images of stars. To minimize the effects of turbulence as much as possible, the Subaru Telescope is located on the summit region of Maunakea, 4,200 m above sea level, for its thin, stable air. Still, it is affected by the turbulence which blurs the images of stars typically about 0.5 second of arc (1 second of arc = 1/3600 of a degree), 10 times larger than the telescope’s own imaging capability can yield.
A technology employed to overcome the effects of atmospheric turbulence is called adaptive optics. Adaptive optics works by capturing a wavefront of light from a bright star that is close to the target (a guide star) with a device called wavefront sensor to measure distortions introduced by the turbulence, and controlling a deformable mirror whose surface can be deformed to compensate for the distortions. We correct the effects of the atmospheric turbulence by observing the light from the target that is reflected by this mirror so that ground-based telescopes can take sharp images as if they were taken from space telescopes.
Yet, since it is sometime very hard to find stars that are both bright and close to targets, adaptive optics can be only used to observe a limited number of objects. This is where a laser guide star system provides solution by generating an artificial guide star by the laser beam (a laser guide star) (Figure 2). Within Earth’s mesosphere, a layer of sodium atoms lie at the altitudes of around 90 to 100 km. The 589-nm laser beam, projected into the sodium layer, can excite sodium atoms to emit light. The laser guide star produced by the luminous phenomenon in the sodium layer enables us to apply adaptive optics for observing parts of the sky where no bright stars are nearby (Note 1).
Latest laser enhances performance of adaptive optics
In 2011, the Subaru Telescope introduced a laser guide star system using a 4W-class all-solid-state laser, which led through many breakthroughs in science (Note 2). As its brightness declined with increasing age, we decided to discontinue the use in 2019, and set up a project to upgrade the system. The laser output determines the laser guide star’s brightness. Our development team upgraded the system by selecting a high-brighness laser dedicated to generation of a laser guide star by Toptica Projects in Germany. This laser achieves a high output power of 22W with technology called Raman fiber amplification, despite a smaller design than conventional lasers. This laser comes as standard for laser guide star systems, employed by all 8-m class telescopes in the world and selected by 30-m class telescopes under construction.
The greatest problem we ran into when upgrading the system was how to transmit such high-power laser. In the former system, the 4W laser beam generated within the laser room was transmitted through an optical fiber to the laser-launching telescope behind the telescope’s secondary mirror. However, the option to use optical fibers was ruled out due to the new laser’s high output of 22W. We instead developed a new transmission optics system with use of mirror reflection to be mounted on the telescope (Figure 3). This transmission optics system posed us another challenge: for a long distance of about 20 m where the laser beam is relayed multiple times via several different mirrors, the direction of the beam which is emitted upward in the sky is slightly altered by distortions caused by changes in the telescope’s posture and by the temperature change. To address this challenge, several sensors were installed on the transmission optics to detect the change in the beam path. Using the information from the sensors, the new transmission optics system has a mechanism of fine-tuning the inclination of the mirrors in the optical path to stablize the alterations of the beam within 1 arcsecond.
This development project is the product of the efforts orchestrated by the observatory staff who worked on the designing, production, installation, and testing. With a delay due to difficulties caused by the COVID pandemic, the team was tenaciously committed to the work, and finally achieved the first launch of the new laser guide star system before dawn on March 3, 2022. The system is offered for open use from the second half of 2022 when adjustment is completed for operation. Another project is underway to generate 4 laser guide stars in the sky by dividing the laser beam into 4 separate beams for a wavefront sensor of the laser tomography adaptive optics system, named ULTIMATE-START (Note 3) which is being developed by Tohoku University in Japan.
Yosuke Minowa, Subaru Telescope’s Associate Professor who led the upgrading work, comments, "Along with the upgrading of the laser system, the telescope needed to undergo large-scale renovation. We had to find a way for every aspect of the work, but we were able to make it by combining the efforts of the observatory staff. I am delighted that obsevation with the laser guide star adaptive optics system restarted with the strengthened capability, and also appreciate all the constant efforts devoted by the staff." Hajime Ogane, a graduate student of Tohoku University who came from Japan to participate in the observation with a carry-in prototype instrument to measure wavefronts through laser guide stars, remarks, "Seeing the completion of the new laser system up close gave me a great opportunity to learn. A bright laser beam lauching straight up to the sky from the telescope was so impressive. That makes me more excited about the laser tomography adaptive optics system with use of the wavefront sensor that we are currently developing."
The introduced high-power laser is also planned for application in the ULTIMATE-Subaru project led by Subaru Telescope (Note 4). The project aims to further widen the field of view that is observable with the adaptive optics system by expanding the location of 4 laser guide stars generated by a laser guide star system which will be upgraded. In addition to providing a new capability of the science instrument, the high-power laser offers a venue for verifying the technology for realization of a more complicated system, allowing us to take the first step toward a large-project for further upgrade. A brighter future of the Subaru Telescope lies ahead of the bright laser beam straight up into the sky.
This research was funded as part of the project ULTIMATE-START by Japan Society for the Promotion of Science’s Grant-in-Aid for Scientific Research (S) (No. 17H06129) titled Establishing processes of galaxy structure revealed by a Subaru tomographic adaptive optics.
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(Note 1) Science result of Subaru Telescope, dated November 20, 2006
(Note 2) Science result of Subaru Telescope, dated July 6, 2011
(Note 3) The project called ULTIMATE-START aims at development of a laser tomography adaptive optics system for tomographic measurement of atmospheric turbulence through generation of several laser guide stars around targets, which will enable high-resolution observation in a wide range of wavelengths from visible to near infrared bands.
(Note 4) ULTIMATE-Subaru is a project to develop next-generation instruments of the Subaru Telescope, geared toward near-infrared observation furnished with the wide field of view and high-resolution capabilities. The project aims to develop a ground-layer adaptive optics system to compensate for atomospheric turbulance in a wide field of view.