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Upgrading the laser-guide-star system at the Keck II telescope

New technology lasers will allow astronomical adaptive optics systems to provide significantly better correction for the effects of atmospheric turbulence.
28 May 2013, SPIE Newsroom. DOI: 10.1117/2.1201305.004881

Adaptive optics (AO) are used on ground-based astronomical telescopes to measure and compensate for the blurring effects of turbulence in the earth's atmosphere. This blurring limits the angular resolution of uncorrected ground-based telescopes to about 0.5 arcseconds at the best sites, as compared to the diffraction limit of 0.02 arcseconds at 1μm wavelength for a telescope with a 10m-diameter primary mirror. A light source above the turbulence (e.g., a star) is needed to measure the wavefront distortion. Because stars of sufficient brightness are only available over 1 or 2% of the sky, astronomers create their own ‘star’ using lasers. The systems currently in scientific operation use 589nm lasers to excite the sodium atoms in the mesosphere, at about 90km altitude, to create a laser guide star (LGS).

W. M. Keck Observatory (WMKO) operates two 10m-diameter optical/infrared telescopes, each equipped with LGS AO systems: see Figure 1. (The laser light visible in this photograph is due to scattering by atoms and molecules in the first 10 to 20km.) The Keck II telescope LGS AO system1, 2 began operation in 2004, followed by the Keck I system3 in 2012. Data from the Keck II LGS AO system has been used in 152 refereed astronomical science papers through 2012, representing 69% of all the science papers based on data from LGS AO systems worldwide.4 The range of science targets has been large, including Kuiper belt objects, brown dwarfs, black holes, gravitational lenses, supernovae, and high-redshift galaxies.

Figure 1. The Keck I and Keck II laser-guide-star (LGS) adaptive-optics (AO) systems being used to observe the center of our galaxy. (Photo by Ethan Tweedie Photography.)

Although the science productivity has been excellent, there is considerable room for improvement. The dominant sources of performance degradation in the WMKO systems are the abilities to measure the wavefront distortion and the image motion (i.e., the tip and tilt of the wavefront). At WMKO, we are aiming to reduce the wavefront measurement error by improving the LGS in two steps: reducing the LGS spot size (by projecting the laser from behind the Keck II telescope's secondary mirror) and improving the brightness of the LGS.

Since the image motion of the laser on its upward journey to the mesosphere is not known, the LGS cannot be used to measure image motion. A natural star is therefore needed. However, all the light collected by the 10m primary mirror can be used, so this star can be roughly 300 times fainter than if it were used to measure the wavefront error. We are working toward implementing a near-infrared sensor (to replace the existing visible sensor) in order to improve the tip-tilt measurement at WMKO.

The existing Keck II LGS AO system uses a dye master oscillator tuned to 589nm and two stages of dye amplifiers. Pumping is performed by a total of six neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers. (The more advanced Keck I system uses a solid-state, mode-locked, continuous-wave laser.5) The 13W of dye laser output is pulsed at 26kHz. This laser will be replaced in 2014 with a 20W continuous-wave Raman fiber-amplifier laser being fabricated by a consortium of TOPTICA Photonics AG and MPB Communications Inc.6 This sodium-wavelength laser technology was first demonstrated by the European Southern Observatory7 (ESO) and the commercialization of this technology has been supported by a collaboration of ESO and several US observatories led by WMKO. The new WMKO laser is currently on order with the laser vendor, which has already assembled and is testing a similar laser for ESO.

The primary advantage of the new laser is the predicted factor-of-ten increase in coupling efficiency to the sodium atoms for a continuous versus a pulsed laser. The old and new lasers pump the D2a line of the sodium doublet. The fiber laser will also have 10% of its power tuned to the D2b line, which will allow atoms that would otherwise be ‘lost’ to be repumped into a state where they can be used again. Other benefits of the new laser system include a dramatic reduction in size and input power (3kW versus 50kW).

One exciting science application of the WMKO LGS AO systems has been tracking the orbits of stars around the supermassive black hole at the center of our galaxy.8 The new laser system is predicted to improve the Strehl ratio of these observations by a factor of 1.5 to 2.5 (see Figure 2), leading to a corresponding improvement in the precision of the orbital measurements.

Figure 2. Simulations of the current performance of the Keck II LGS AO system on the Galactic Center (left) and the predicted performance with the new laser system (right) over a 10-arcseconds-square field at 2.2μm wavelength. The Strehl ratio is improved by a factor of two. (Simulations performed by researchers at the Thirty Meter Telescope project, WMKO, and the University of California, Los Angeles.)

In summary, we are working on a major upgrade to the Keck II telescope's LGS AO system. We recently passed a system design review for the overall implementation of the new laser with the Keck II telescope and AO system, and we are now moving into the preliminary design phase for the telescope and laser system modifications. This laser is the first of three lasers, producing a total of seven LGSs, that we hope to implement on the Keck II telescope as part of WMKO's next-generation adaptive-optics facility.9 This facility will offer excellent AO correction in the near-infrared and good correction at optical wavelengths, and will allow astronomers to observe targets everywhere in the sky.

The procurement and implementation of the new Keck laser has been made possible by grants from the Gordon and Betty Moore Foundation, the W. M. Keck Foundation, The Bob & Renee Parsons Foundation, and Friends of Keck Observatory.

Peter Wizinowich
W. M. Keck Observatory
Kamuela, HI

Peter Wizinowich is optical systems manager at W. M. Keck Observatory, where he has played lead roles in the implementation of telescope optics, adaptive optics, and interferometer. He previously worked at Steward Observatory in Tucson, AZ, and the Canada-France-Hawaii Telescope on Mauna Kea, HI. His PhD is from the University of Arizona Optical Sciences Center. He has been a conference chair or program committee member for each of the Adaptive Optics conferences in the SPIE Astronomical Telescopes and Instrumentation symposia since 1998.

1. P. Wizinowich, The W. M. Keck Observatory laser guide star adaptive optics system: overview, Publ. Astron. Soc. Pac. 118, p. 297-309, 2006.
2. M. van Dam, The W. M. Keck Observatory laser guide star adaptive optics system: performance characterization, Publ. Astron. Soc. Pac. 118, p. 310-318, 2006.
3. J. Chin, Keck I laser guide star adaptive optics system, Proc. SPIE 8447, p. 84474F, 2012. doi:10.1117/12.925807
4. P. Wizinowich, Progress in laser guide star adaptive optics and lessons learned, Proc. SPIE 8447, p. 84470D, 2012. doi:10.1117/12.925093
5. N. Sawruk, System overview of 30W and 55W sodium guide star laser systems, Proc. SPIE 7736, p. 77361Y, 2010. doi:10.1117/12.858218
6. A. Friedenauer, RFA-based 589-nm guide star lasers for ESO VLT: a paradigm shift in performance, operational simplicity, reliability, and maintenance, Proc. SPIE 8447, p. 84470F, 2012. doi:10.1117/12.923869
7. Y. Fang, L. R. Taylor, D. Bonaccini Calia, 25W Raman-fiber-amplifier-based 589nm laser for laser guide star, Opt. Express 17, p. 19021-19026, 2009.
8. L. Meyer, The shortest-known–period star orbiting our galaxy's supermassive black hole, Science 338, p. 84-87, 2012.
9. P. Wizinowich, W. M. Keck Observatory's next generation adaptive optics facility, Proc. SPIE 7736, p. 77360K, 2010. doi:10.1117/12.857628