I. Guidolin

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 of the ESO laser guide star facility

Two teams of scientists and engineers at Max Planck Institut fuer Extraterrestrische Physik and at the European Southern Observatory have joined forces to design, build and install the Laser Guide Star Facility for the VLT. The Laser Guide Star Facility has now been completed and installed on the VLT Yepun telescope at Cerro Paranal. In this paper we report on the first light and first results from the Commissioning of the LGSF.

ESO adaptive optics facility progress and first laboratory test results

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.

PM fiber lasers at 589nm: a 20W transportable laser system for LGS return flux studies

In this paper we present the rationale and design of a compact, transportable, modular Laser Guide Star Unit, comprising a 589nm laser mounted on a 300mm launch telescope, to be used in future experiments probing the mesospheric sodium properties and to validate existing LGS return flux simulations. The 20W CW 589nm Laser is based on the ESO developed concept of 589nm lasers based on Raman Fiber Amplifiers, refined and assembled together with industry. It has the same laser architecture as the laser which will be used for the VLT Adaptive Optics Facility. We have added to the 20W CW laser system the capabilities of changing output polarization, D2b emission levels, power level, linewidth and to operate as pulsed laser with amplitude modulation. We focus in this paper on the laser description and test results.

Laser-guide-star-related activities at ESO

We report in this paper on the current activities of the Laser Guide Star group at ESO. We are building the Laser Guide Star Facility on the VLT UT4, we are pushing new technologies in view of multiple LGS and future ELT LGS-AO systems. We are studying new LGS propagation/sensing schemes, to get rid of the LGS cone effect. We are also fostering in member states countries the development of fast sensors to deal with pulsed lasers atmospheric layer sensing.

On‐sky demonstration of matched filters for wavefront measurements using ELT‐scale elongated laser guide stars

The performance of adaptive optics systems is partially dependent on the algorithms used within the real‐time control system to compute wavefront slope measurements. We demonstrate the use of a matched filter algorithm for the processing of elongated laser guide star (LGS) Shack‐Hartmann images, using the CANARY adaptive optics instrument on the 4.2 m William Herschel Telescope and the European Southern Observatory Wendelstein LGS Unit placed 40 m away. This algorithm has been selected for use with the forthcoming Thirty Meter Telescope, but until now had not been demonstrated on‐sky. From the results of a first observing run, we show that the use of matched filtering improves our adaptive optics system performance, with increases in on‐sky H‐band Strehl measured up to about a factor of 1.1 with respect to a conventional centre of gravity approach. We describe the algorithm used, and the methods that we implemented to enable on‐sky demonstration.

ESO Adaptive Optics Facility Progress Report

The ESO Adaptive Optics Facility (AOF) consists in an evolution of one of the ESO VLT unit telescopes to a laser driven adaptive telescope with a deformable mirror in its optical train. The project has completed the procurement phase and several large structures have been delivered to Garching (Germany) and are being integrated (the AO modules GRAAL and GALACSI and the ASSIST test bench). The 4LGSF Laser (TOPTICA) has undergone final design review and a pre-production unit has been built and successfully tested. The Deformable Secondary Mirror is fully integrated and system tests have started with the first science grade thin shell mirror delivered by SAGEM. The integrated modules will be tested in stand-alone mode in 2012 and upon delivery of the DSM in late 2012, the system test phase will start. A commissioning strategy has been developed and will be updated before delivery to Paranal. A substantial effort has been spent in 2011-2012 to prepare the unit telescope to receive the AOF by preparing the mechanical interfaces and upgrading the cooling and electrical network. This preparation will also simplify the final installation of the facility on the telescope. A lot of attention is given to the system calibration, how to record and correct any misalignment and control the whole facility. A plan is being developed to efficiently operate the AOF after commissioning. This includes monitoring a relevant set of atmospheric parameters for scheduling and a Laser Traffic control system to assist the operator during the night and help/support the observing block preparation.

MELT: an optomechanical emulation testbench for ELT wavefront control and phasing strategy

We present an optomechanical test bench setup (MELT) for testing and validating key functionalities to be used on the Extremely Large Telescope (ELT) during the periods of system verification, wavefront control commissioning, through the handover to science, up to regular diagnostic, monitoring, and validation tasks during operations. The main objectives of MELT are to deploy and validate the telescope control system, to deploy and validate wavefront control algorithms for commissioning and operations, as well as to produce and validate key requirements for the phasing and diagnostic station (PDS) of the ELT. The purpose of MELT is to deploy optomechanical key components such as a segmented primary mirror, a secondary mirror on a hexapod, an adaptive fourth mirror, and a fast tip/tilt mirror together with their control interfaces that emulate the real telescope optomechanical conditions. The telescope control system, deployed on MELT can test control schemes with the active mounts emulating the real ELT optomechanical control interfaces. The presented optomechanical setup uses the Active Segmented Mirror (ASM) with its piezo-driven 61 segments and a diameter of 15 cm. It was designed, built, and used on sky during the Active Phasing Experiment (APE). Several beam paths after the telescope optical train on MELT are conditioned and guided to wavefront sensors and cameras, sensitive to wavelength bands in the visible and infrared to emulate wavefront commissioning and phasing tasks. This optical path resembles part of the phasing and diagnostics station (PDS) of the ELT, which is used to acquire the first star photons through the ELT and to learn the usage and control of all the ELT optomechanics. The PDS will be developed, designed, and built in-house at ESO. MELT helps its design by providing a detailed test setup for defining and deploying system engineering tasks, such as detailed functional analysis, definition of tasks to be carried out, and technical requirements, as well as operational commissioning aspects. The bench test facility MELT will in the end help us to be as much as possible prepared when the telescope sends the first star light through the optical train to be able to tackle the unforeseeable problems and not be caught up with the foreseeable ones.

Comparison between observation and simulation of sodium LGS return flux with a 20W CW laser on Tenerife

We report on the comparison between observations and simulations of a completed 12-month field observation campaign at Observatorio del Teide, Tenerife, using ESOs transportable 20 watt CW Wendelstein laser guide star system. This mission has provided sodium photon return flux measurements of unprecedented detail regarding variation of laser power, polarization and sodium D2b repumping. The Raman fiber laser and projector technology are very similar to that employed in the 4LGSF/AOF laser facility, recently installed and commissioned at the VLT in Paranal. The simulations are based on the open source LGSBloch density matrix simulation package and we find good overall agreement with experimental data.

Extremely Large Telescope Prefocal Station A system concept

The Prefocal Station (PFS) is the last opto-mechanical unit before the telescope focal plane in the Extremely Large Telescope (ELT) optical train. The PFS distributes the telescope optical beam to the Nasmyth and Coudé instrument focal stations and it contains all of the sky metrology (imaging and wavefront sensing) that will be used by the active optics of the telescope and to support operations such as phasing the primary mirror (phasing and diagnostic station). It also hosts local metrology that will be used for coarse alignment and maintenance. We present the main results of a concept design study for the Nasmyth A prefocal station.