Thomas Pfrommer

Progress toward developing the TMT adaptive optical systems and their components

Atmospheric turbulence compensation via adaptive optics (AO) will be essential for achieving most objectives of the TMT science case. The performance requirements for the initial implementation of the observatorys facility AO system include diffraction-limited performance in the near IR with 50 per cent sky coverage at the galactic pole. This capability will be achieved via an order 60x60 multi-conjugate AO system (NFIRAOS) with two deformable mirrors optically conjugate to ranges of 0 and 12 km, six high-order wavefront sensors observing laser guide stars in the mesospheric sodium layer, and up to three low-order, IR, natural guide star wavefront sensors located within each client instrument. The associated laser guide star facility (LGSF) will consist of 3 50W class, solid state, sum frequency lasers, conventional beam transport optics, and a launch telescope located behind the TMT secondary mirror. In this paper, we report on the progress made in designing, modeling, and validating these systems and their components over the last two years. This includes work on the overall layout and detailed opto-mechanical designs of NFIRAOS and the LGSF; reliable wavefront sensing methods for use with elongated and time-varying sodium laser guide stars; developing and validating a robust tip/tilt control architecture and its components; computationally efficient algorithms for very high order wavefront control; detailed AO system modeling and performance optimization incorporating all of these effects; and a range of supporting lab/field tests and component prototyping activities at TMT partners. Further details may be found in the additional papers on each of the above topics.

First Assessment of Mountains on Northwestern Ellesmere Island, Nunavut, as Potential Astronomical Observing Sites

Ellesmere Island, at the most northerly tip of Canada, possesses the highest mountain peaks within 10 degrees of the pole. The highest is 2616 m, with many summits over 1000 m, high enough to place them above a stable low-elevation thermal inversion that persists through winter darkness. Our group has studied four mountains along the northwestern coast which have the additional benefit of smooth onshore airflow from the ice-locked Arctic Ocean. We deployed small robotic site testing stations at three sites, the highest of which is over 1600 m and within 8 degrees of the pole. Basic weather and sky clarity data for over three years beginning in 2006 are presented here, and compared with available nearby sea-level data and one manned mid-elevation site. Our results point to coastal mountain sites experiencing good weather: low median wind speed, high clear-sky fraction and the expectation of excellent seeing. Some practical aspects of access to these remote locations and operation and maintenance of equipment there are also discussed.

The ESO transportable LGS Unit for measurements of the LGS photon return and other experiments

Sodium laser guide stars (LGS) are used, or planned to be used, as single or multiple artificial beacons for Adaptive Optics in many present or future large and extremely large telescopes projects. In our opinion, several aspects of the LGS have not been studied systematically and thoroughly enough in the past to ensure optimal system designs. ESO has designed and built, with support from industry, an experimental transportable laser guide star unit, composed of a compact laser based on the ESO narrow-band Raman Fiber Amplifier patented technology, attached to a 30cm launch telescope. Besides field tests of the new laser technology, the purpose of the transportable unit is to conduct field experiments related to LGS and LGS-AO, useful for the optimization of future LGS-AO systems. Among the proposed ones are the validation of ESO LGS return flux simulations as a function of CW and pulsed laser properties, the feasibility of line-of-sight sodium profile measurements via partial CW laser modulation and tests of AO operation with elongated LGS in the EELT geometry configuration. After a description of the WLGSU and its main capabilities, results on the WLGSU commissioning and LGS return flux measurements are presented.

First light adaptive optics systems and components for the Thirty Meter Telescope

Adaptive optics (AO) is essential for many elements of the science case for the Thirty Meter Telescope (TMT). The initial requirements for the observatorys facility AO system include diffraction-limited performance in the near IR, with 50 per cent sky coverage at the galactic pole. Point spread function uniformity and stability over a 30 arc sec field-ofview are also required for precision photometry and astrometry. These capabilities will be achieved via an order 60×60 multi-conjugate AO system (NFIRAOS) with two deformable mirrors, six laser guide star wavefront sensors, and three low-order, IR, natural guide star wavefront sensors within each client instrument. The associated laser guide star facility (LGSF) will employ 150W of laser power at a wavelength of 589 nm to generate the six laser guide stars. We provide an update on the progress in designing, modeling, and validating these systems and their components over the last two years. This includes work on the layouts and detailed designs of NFIRAOS and the LGSF; fabrication and test of a full-scale prototype tip/tilt stage (TTS); Conceptual Designs Studies for the real time controller (RTC) hardware and algorithms; fabrication and test of the detectors for the laser- and natural-guide star wavefront sensors; AO system modeling and performance optimization; lab tests of wavefront sensing algorithms for use with elongated laser guide stars; and high resolution LIDAR measurements of the mesospheric sodium layer. Further details may be found in specific papers on each of these topics.

The Large Zenith Telescope: A 6 m Liquid‐Mirror Telescope

The Large Zenith Telescope is a 6 m optical telescope employing a rotating primary mirror coated with a film of liquid mercury. Located at an altitude of 400 m in the Coast Mountains of southwestern British Columbia, this telescope began regular operation in 2005 October. Equipped with a four-element Richardson prime-focus corrector and thinned 2048 × 2048 pixel drift-scanning CCD imaging camera, it is used for astronomical survey observations and also serves as an engineering test facility for further development of liquid-mirror technology. Built at a cost of less than

Simulations of pulsed sodium laser guide stars: an overview

1 million dollars, it achieves an image quality and sensitivity comparable to that of a conventional telescope of equal aperture and is limited primarily by the astronomical quality of the site.

ESO adaptive optics facility progress and first laboratory test results

Almost all sodium laser guide star (LGS) systems in the world are based on pulsed lasers. We review the relevant sodium physics and compare different laser pulse formats. Selected formats are discussed on the basis of numerical simulation results. One of the key findings is that the brightness of most existing LGS facilities could be boosted at, as we argue, reasonable expense. Recommendations are presented to enhance the LGS return flux and to design future LGS lasers, including those suitable for spot tracking in the mesosphere to mitigate the spot elongation problem.

Laser guide star return flux simulations based on observed sodium density profiles

The Adaptive Optics Facility project is completing the integration of its systems at ESO Headquarters in Garching. The main test bench ASSIST and the 2nd Generation M2-Unit (hosting the Deformable Secondary Mirror) have been granted acceptance late 2012. The DSM has undergone a series of tests on ASSIST in 2013 which have validated its optical performance and launched the System Test Phase of the AOF. This has been followed by the performance evaluation of the GRAAL natural guide star mode on-axis and will continue in 2014 with its Ground Layer AO mode. The GALACSI module (for MUSE) Wide-Field-Mode (GLAO) and the more challenging Narrow-Field-Mode (LTAO) will then be tested. The AOF has also taken delivery of the second scientific thin shell mirror and the first 22 Watt Sodium laser Unit. We will report on the system tests status, the performances evaluated on the ASSIST bench and advancement of the 4Laser Guide Star Facility. We will also present the near future plans for commissioning on the telescope and some considerations on tools to ensure an efficient operation of the Facility in Paranal.

High-resolution mesospheric sodium observations for extremely large telescopes

We extend previous sodium LGS models by integrating the return flux across the mesosphere, taking into account variable mesospheric gas density, temperature, and local sodium density. This method allows us to produce accurate predictions of the actual return flux on the ground, relevant for determining the performance of adaptive-opticsassisted instruments. We find that the flux distribution across the sky depends strongly on geographic location and laser parameters. Almost independent of location, future sodium LGS will be about three times brighter at zenith than at the observing horizon.

Boundary layer seeing measurements in the Canadian High Arctic

Variations in density structure and altitude of mesospheric sodium impact the performance of adaptive optics systems employing sodium laser guide stars. The associated wave-front errors grow as the square of the telescope aperture and will be very significant for the next generation of large-aperture ground-based optical/infrared telescopes. To support the adaptive optics program for the Thirty Meter Telescope and European Extremely Large Telescope, we are conducting a program of sodium monitoring using a high-resolution sodium lidar system on the 6-meter Large Zenith Telescope (LZT). Located at 49°N latitude, the LZT lidar system provides density profiles with spatial and temporal resolution sampling of 4.8 m and 20 ms. In this paper we report highlights of results obtained over two years of observations.