Figure 1. Laser launching experiment at RIKEN. A sodium laser generated artificial guide star will be visible at the end point of the laser beam when the laser frequency is finely stabilized. (Larger Image)

The National Astronomical Observatory of Japan (NAOJ) and the Institute of Physical and Chemical Research (RIKEN) have successfully developed an all solid-state laser to produce an "artificial star" in the upper atmosphere to expand operation of the Subaru telescope's adaptive optics system. An adaptive optics system has been in operation at the Cassegrain focus of the Subaru telescope providing high-resolution images by compensating for atmospheric turbulence since December, 2000. With the new laser system and a new adaptive optics system under construction at Nasmyth focus, Subaru will acquire a new capability to obtain diffraction limited imaging anywhere in the sky even when a bright natural guide star is not available.

The Subaru telescope is built upon the summit ridge of Mauna Kea (4200 m), where the astronomical seeing is the best among existing telescope sites. A superb primary mirror and specially designed enclosure enables Subaru to obtain the sharpest imaging among eight meter telescopes. Nevertheless, the spatial resolution achieved by any large ground-based telescope, including Subaru, is far poorer than the theoretical diffraction limit because of atmospheric turbulence (Note 1).

The Cassegrain focus of Subaru is equipped with an adaptive optics system to compensate for image blurring due to turbulence in Earth's atmosphere. The adaptive optics (AO) system measures the perturbing effect of the turbulent atmosphere by measuring the wavefront of light from a bright guide star near the target object. The measurement is repeated about 1000 times per second and a deformable mirror with 36 driving electrodes is used to cancel the wavefront distortion about 100 times a second. With such a device, one can get an image much sharper than those obtained with usual imaging.

NAOJ is currently developing a new adaptive optics system with 188 control elements to be installed at the Nasmyth focus. However, the both the old and the new system requires a bright guide star near the target field to measure the atmospheric turbulence. Only a few lucky targets can enjoy these sophisticated technologies if the system must rely on existing "natural" stars in the sky (Note 2).

Figure 2. Laser Guide Star Adaptive Optics System (Larger Image)

To make the benefit of the adaptive optics system available for any target, a Laser Guide Star system is under development (Figure 2). The system generates an "artificial laser star" by illuminating the sodium layer (Note 3) at about 100 km altitude using a powerful sodium laser. With this new facility, Subaru Telescope will offer a diffraction limited imaging capability for any target in the sky for which no nearby bright natural guide star is available.

The key issue of the laser guide star system has been the development of a powerful sodium laser yielding the 589 nm sodium D line (Note 4). NAOJ and MegaOpto Co., Ltd., have been developing an all solid state, sodium laser under the guidance of the Solid State Device Unit (Tomoyuki Wada, Unit Leader) of RIKEN (Note 5) and succeeded in launching the orange laser beam into the sky using the fiber relaying system at the RIKEN campus in June.

The laser guide star system will be delivered to Subaru Telescope later this year for installation on the telescope. The first light of the laser guide star system is expected toward the end of the year 2006. When completed, the new Nasmyth adaptive optics system with 188 control elements will improve the spatial resolution at 2 micrometers to its theoretical limit of 0.07 arcseconds, about four times better than without the adaptive optics system. In near infrared wavelengths, the Subaru telescope with the adaptive optics system will offer about three times better spatial resolution than that of the Hubble Space Telescope (Note 6).

Figure 3. Subaru's Laser Guide Star Adaptive Optics system: An adaptive optics system instantaneously measures the wavefront distortion due to the atmospheric turbulence and compensates for the effect by driving a "deformable mirror" in real time. The laser guide star generation system consists of the laser, relaying optical fiber, and the launching telescope mounted on the back side of the secondary mirror of the Subaru telescope (Larger Image)

Figure 4. The sum-frequency Nd:YAG laser developed at RIKEN to generate a powerful 589 nm laser beam: A Nd:YAG laser is known to give rise to two laser lights at 1319 nm and 1064 nm. It is an interesting coincidence that by mixing these two laser beams with a non-linear crystal one can generate a laser beam at 589n nm. This is because the sum of the frequencies of the two incident lasers happens to coincide with the frequency of the 589 nm laser (1/1319 +1/1064 = 1/589). RIKEN and MegaOpto succeeded in generating a 4 W laser at the sodium D line (589 nm). To make this idea a useful reality, several new developments were necessary : 1) stabilizing the resonance cavity under the severe environment at the summit of Mauna Kea, 2) amplifying the output laser power while keeping the beam quality, 3) frequency transformation based on non-linear optics, and 4) locking the resulting frequency to the sodium D-line with an error less than 2x10-7. (Larger image).

Figure 5. All solid-state 589nm laser (Larger Image)

Figure 6. Comparison of the image quality: Image without LGSAO (left) and with LGSAO (right) (Larger Image).

• Note 1: The theoretical limit of the spatial resolution for a given telescope with aperture D[m] to image a star at a wavelength λ [μm] is defined by the diffraction limit 1.2 λ/D radian. For imaging observation at 2.2 μm with the 8.2 m Subaru Telescope, the diffraction limit is as small as 0.07 arcsec. Whereas the actual image size of a star blurred by the atmospheric turbulence, called the seeing size, typically about 0.4 arcsec, 5 times larger than the diffraction limit in the near infrared.

• Note 2: To operate an adaptive optics system, a guide star brighter than 15th magnitude is necessary to measure the atmospheric turbulence. However, the chance that any target object happens to have a nearby bright star available is a mere 2%. Therefore the current Cassegrain AO
  system is fully useful only for those lucky targets.

• Note 3: There is a layer called the "sodium layer" at about 100 km above the ground, where the density of sodium atoms is significantly large. A similar sodium layer is also found around the planet Mars. Although it is not strictly understood, the presence of such a layer is ascribed to the vaporization of meteors in the upper atmosphere. For some photochemical reasons sodium atoms accumulate at a specific height. A sodium laser guide star takes advantage of this phenomenon.

• Note 4: In order to excite the sodium atoms in the upper atmosphere to emit enough light as an artificial star, a powerful laser beam emitting the 589 nm sodium D line photons is required to illuminate the sodium layer. Orange colored sodium lamps used as illuminating lamps on highways emit this special light. The sodium laser system reaches the upper atmosphere and the sodium atoms in a column about 50 cm in diameter, 5 km in length and a height of about 100 km. The sodium atoms are excited by this laser beam and give forth the orange colored photons. Apparent magnitude of this artificial star will be 11th to 12th magnitude for a 4 W laser.

• Note 5: This is an achievement of the NAOJ group (M. Iye, Y. Hayano et al.) and the RIKEN (K. Kaya, director) Solid-State Optical Science Research Unit (S. Wada, N. Saito et al.).

• Note 6: The 8.2 m Subaru telescope, 3.4 times larger than the 2.4 m Hubble Space Telescope (HST), gives a diffraction limited image size 3.4 times smaller than the HST.