Subaru's Improved AO System Resumes Operation

January 24, 2011

Subaru Telescope's adaptive optics (AO) system resumed operation in November 2010 with a new deformable mirror, upgrades of various components, and improvement in its alignment process. The AO development team is also fine-tuning the laser guide star system. These efforts are part of Subaru's ongoing process of improving its suite of instruments to enhance the telescope's powers of observation.

An AO system is very important for a ground-based telescope such as Subaru. Although its location on the Mauna Kea summit area on the island of Hawaii is one of the best sites in the world for astronomical observations, atmospheric turbulence can still degrade the telescope's images.

The benefits of AO have been available to users of the Subaru Telescope since 2000, only a year after its first light. An AO system compensates for image distortions caused by turbulence in the Earth's atmosphere and sharpens the image. Although the wavefront of light from a star travels smoothly in space, atmospheric turbulence from the interaction between different temperatures and wind speeds disturbs the wavefront and blurs the image.

Subaru began with a 36-element AO system, which utilizes the light of a bright star near the target as a guide or reference star. After the guide star's light hits the deformable mirror on its path to the observing instrument, a beam splitter sends its infrared portion to the instrument while the optical portion moves to the wavefront sensor that measures its degree of distortion. The wavefront sensor's measurement of distortion prompts electrodes to control the shape of the 36 sections of the deformable mirror so that it compensates in real time for the blurring. Once activated, the mirror continues to compensate for the distorted light before it enters the instrument. In reality, the process is nearly instantaneous since the calculations preceding the adjustment of the mirror occur about a thousand times per second.

Subaru's AO development team worked for over five years to upgrade the 36-element system with a much-improved 188-element system, which has been used for observations since October 9, 2006. This system operates at the Nasmyth focus, not at the Cassegrain focus of the earlier system. The wavefront sensor has an array of small lenses that divide the guide star's light into 188 parts rather than 36, making the measurements of image distortion even more precise. More parts in the current system can compensate for subtle errors in the image and even improve resolution in optical wavelengths. A super-sensitive detector then senses the brightness of each of these parts at a high speed. Variation in the brightness is used to determine the distortion in the guide star's wavefront and to adjust the shape of the deformable mirror.

The 188-element system uses a Piezo-electric deformable mirror made of a special ceramic that expands and contracts in response to the voltage applied. This small, extremely important mirror is just 130 mm (5.07") in diameter and 2 mm (.078") thick, only slightly larger than its 36-element predecessor with a 110 mm (4.29") diameter. Unfortunately, a serious problem with the deformable mirror occurred during the fine-tuning of the AO system and interrupted the use of the AO system from early 2010. Nevertheless, immediate responses to the problem led to further improvements in the system and the resumption of its use in November 2010.

Overall, the 188-element AO system continues to significantly increase the resolution of images and gives astronomers a clearer view of the universe. Its high spatial resolution benefits spectroscopic observations and helps in the detection of detailed structures like stellar birth nebulae, or dim objects such as very distant galaxies.



Figure 1: The right panel shows a close-up of the crowded center of the M15 globular cluster, taken by IRCS + AO188 at Subaru Telescope. The field-of-view (FOV) is 1 arcminute by 1 arcminute. The spatial resolution is 0.1 arcseconds, which is much finer than the natural seeing size, thanks to the refurbished AO188. The left panel shows an overview of M15 in visible light, a photo taken by a 50 cm reflector at the headquarters of the National Astronomical Observatory of Japan.

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