The ELT prefocal stations provide wavefront sensing to support the active control of the telescope during observations; they also include mirrors to distribute the telescope optical beam to the scientific instrument or diagnostic tool that finally uses the light collected by the ELT. Built in to the prefocal stations is a hosted metrology positioning system where metrology measuring instruments including a laser tracker and alignment telescope will be installed. This metrology will be used during coarse alignment of the ELT, to maintain the internal alignment of the prefocal stations, and to locate them with respect to their surroundings. The detailed design and application of these instruments is described here, together with a first set of test results.
The Extremely Large Telescope (ELT) hosts two Prefocal Stations, one on each of its Nasmyth Platforms A and B. Both units will alternately distribute the light collected by the telescope’s giant optical system into the science instruments and other test equipment, deliver the focal surface images and pupil images of three natural guide stars for acquisition, guiding and wavefront sensing purposes and provide optical sensing to support phasing of the ELT primary mirrors, diagnostics, and maintenance of the optics as part of the Phasing and Diagnostics Station (supplied by ESO). As the subsystems were being assembled, the integration and verification activities proceeded gradually, following the typical V-process model approach. The full PFS Main System test without the M6 optics, which are being tested on their mirror mounts separately at the polisher premises, is now coming to its end and the overall system is ready for final testing.
The ELT Phasing and Diagnostic Station (PDS), is a multi-purpose optomechanical system providing metrology tools to phase the segmented primary mirror of the ELT and hosting the sensors required to verify AO-assisted diffraction limited image quality at the ELT. The purpose of the PDS in the context of the ELT lifecycle is twofold. On one side, during the AIV phase of the ELT, the PDS will be the fundamental tool enabling commissioning of the telescope. On the other hand, during operation, the PDS will provide the essential metrology means to monitor performance and detect and isolate potential failures within the observatory. The project, which is one of the most important internal development endeavors at ESO, passed PDR in mid-2021 and underwent an optical final design review in late 2021, where challenges associated to the schedule were identified and more time was given to optimize the design. The project underwent a restructuring in early 2022 before starting its final design phase which has been successfully completed in 2023. In the same period all procurements concerning critical long lead items have been launched. The present contribution first introduces the project in the context of the ELT construction programme, outlining the project structure and the project management tools employed for planning and progress monitoring. Subsequently, the main system engineering processes used within the project will be described. Finally, we report on the main technical results obtained during the final design phase and the plans for the assembly, integration and test of the system.
The tremendous costs and complexity of instruments for the next generation of telescopes (ELT, GMT, TMT) has led to increasing research into photonic devices as cheaper and compact alternatives to bulk optic components. The widespread availability of adaptive optics and diffraction-limited image quality of telescopes in the infrared enables the use of single-mode photonic devices. A possible single-mode counterpart to conventional, bulk optic spectrographs are photonic spectrographs, based e.g. on arrayed waveguide gratings (AWG). AWGs are widely used in the telecommunication industries for wavelength multiplexing and demultiplexing. As a result, commercial AWGs are typically operated around the 1550nm communication window and their potential application as an astrophotonic spectrograph has been studied mainly in the astronomical H band (1500-1700 nm). In order to extend the wavelength range, we developed an AWG for the astronomical J band (center wavelength 1250 nm). The device was ordered for fabrication and was characterized using a test bench built in-house. The AWG chip is found to exhibit spectral resolutions close to the design resolution (R∼12500) over a large wavelength range from at least 1150nm to 1500 nm, with an intrinsic peak transmission close to 80% at the center wavelength. Our results demonstrate the potential to use AWG’s in astronomical spectrographs in future instruments.
The Natural Guide Star Adapter (NGSA), a circular structure that is part of the Extremely Large Telescope (ELT) Pre-Focal Station (PFS) [6], defines with its rotation axis one of the reference coordinate systems of the entire telescope and has a key role in the performance of all the instruments. The PFS NGSA hosts three sensor arms (SA), which are free to move in a roughly annulus area to support to support closing control loops on natural guide stars when the light beam is controlled by the telescope[2]. The actuation of these SA’s causes a deformation of the mechanical structure and consequently uncertainty on the position and orientation of the reference coordinate system. Starting from the Finite Element Analysis (FEA) of the PFS, we develop a model that reconstructs the behavior of the structure for all the possible combinations of SA positions, and we conceive strategies for a robust definition of the reference coordinate system, as described in this paper.
The Prefocal Stations (PFS-A and PFS-B) of the Extremely Large Telescope (ELT) are the last component in the telescope’s light path, right before the light is delivered to the science instruments at the telescope focus. Following the Final Design Review passed in 2020 the efforts were focused on the preparation of the manufacturing documentation and on launching the manufacturing of the Prefocal Stations. The fabrication of the equipment was divided into seven main manufacturing lots based on the nature of the parts with the main aim of minimizing the project risk and optimize the schedule. The ESO's approval of each manufacturing documentation package was obtained by means of individual Manufacturing Readiness Reviews (MRR). The manufacturing of the parts and procurement of the long lead time COTS was launched in 2021 and is coming to an end. The Assembly, Integration and Verification (AIV) activities begin with the end of the manufacturing. The AIV strategy follows the well-established “V-model” methodology. The initial phases are focused on subsystem level and progressively evolves until the work is concluded with the fully assembled, integrated and verified Prefocal Stations in the ELT (Chile). Throughout this process, the fully integrated Prefocal Stations will be completely tested in the factory acceptance test (FAT) campaign in Europe. This paper summarizes the manufacturing status and describes the way Prefocal Stations AIV is planned to be carried out.
Structural, thermal, and optical performance (STOP) analysis is gradually becoming a crucial tool in the development of optical systems in general and astronomical telescopes and instruments in particular. We describe the approach of the European Southern Observatory to STOP analysis at the examples of the phasing and diagnostic station and of the pre-focal station of the European Extremely Large Telescope under construction. Further, we discuss the fundamentals of the estimation of thermal effects on optical performance and how to carry it out with the in-house developed software tool Sensitizer.
Located at the Nasmyth Platforms A and B, the Prefocal Stations of the Extremely Large Telescope (ELT) are the last opto-mechanical components before the light from the giant telescope comes to a focus. The mission of the Prefocal Stations are threefold. Firstly, these high-precision opto-mechanical and optical sensing devices propagate the light collected on the telescope into science instruments and other test equipment. Very high optical quality, stability, and low vibration are key characteristics of the deployable M6N and M6C mirrors, that provide the optical propagation function. Secondly, by means of three Sensor Arms, they pick and adapt the light from up to three guide stars for its use in the Acquisition, Guiding and Wavefront Sensing to support the telescope active and adaptive optics. The active optics stabilize the images delivered to the science instruments, despite the constantly changing effects of wind and other disturbances on the telescope, and periodically realign the telescope to keep the adaptive optics working in their operating range. The adaptive optics compensate for the wavefront distortion caused by the atmospheric turbulence by acting on the deformable mirror (M4). Thirdly, the Prefocal Stations provide optical sensing to support phasing of the ELT primary mirrors, diagnostics, and maintenance of the optics. These tasks are performed by the Phasing and Diagnostic Station, which is located on the Coudé path. The functions provided by the Prefocal Stations are critical for the commissioning and operation of the ELT telescope. Here we report on the final design of the Prefocal Stations, with an emphasis on the Prefocal Station Main System.
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.
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.
A suite of seven instruments and associated AO systems have been planned as the "E-ELT Instrumentation Roadmap". Following the E-ELT project approval in December 2014, rapid progress has been made in organising and signing the agreements for construction with European universities and institutes. Three instruments (HARMONI, MICADO and METIS) and one MCAO module (MAORY) have now been approved for construction. In addition, Phase-A studies have begun for the next two instruments - a multi-object spectrograph and high-resolution spectrograph. Technology development is also ongoing in preparation for the final instrument in the roadmap, the planetary camera and spectrograph. We present a summary of the status and capabilities of this first set of instruments for the E-ELT.
We report on the comparison between observations and simulations of a completed 12-month field observation campaign at Observatorio del Teide, Tenerife, using ESO's 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.
The Nasmyth platforms of the E-ELT will contain one Prefocal Station (PFS) each. The main PFS functional requirements are to provide a focal plane to the three Nasmyth focal stations and the Coudé focus, optical sensing supporting telescope low order optimisation and seeing limited image quality, and optical sensing supporting characterising and phasing of M1 and other telescope subsystems. The PFS user requirements are used to derive the PFS technical requirements specification that will form the basis for design, development and production of the system. This specification process includes high-level architectural decisions and technical performance budget allocations. The mechanical design concepts reported here have been developed in order to validate key system specifications and associated technical budgets.
Recent numerical simulations and experiments on sodium Laser Guide Star (LGS) have shown that a continuous wave (CW) laser with circular polarization and re-pumping should maximize the fluorescent photon return flux to the wavefront sensor for adaptive optics applications. The orientation and strength of the geomagnetic field in the sodium layer also play an important role affecting the LGS return ux. Field measurements of the LGS return flux show agreement with the CW LGS model, however, fluctuations in the sodium column abundance and geomagnetic field intensity, as well as atmospheric turbulence, induce experimental uncertainties. We describe a laboratory experiment to measure the photon return flux from a sodium vapor cell illuminated with a 589 nm CW laser beam, designed to approximately emulate a LGS under controlled conditions. Return flux measurements are carried out controlling polarization, power density, re-pumping, laser linewidth, and magnetic field intensity and orientation. Comparison with the numerical CW simulation package Atomic Density Matrix are presented and discussed.
ESO has a very active on-going AO WFS detector development program to not only meet the needs of the current crop of instruments for the VLT, but also has been actively involved in gathering requirements, planning, and developing detectors and controllers/cameras for the instruments in design and being proposed for the E-ELT.
This paper provides an overall summary of the AO WFS Detector requirements of the E-ELT instruments currently in design and telescope focal units. This is followed by a description of the many interesting detector, controller, and camera developments underway at ESO to meet these needs; a) the rationale behind and plan to upgrade the 240x240 pixels, 2000fps, “zero noise”, L3Vision CCD220 sensor based AONGC camera; b) status of the LGSD/NGSD High QE, 3e- RoN, fast 700fps, 1760x1680 pixels, Visible CMOS Imager and camera development; c) status of and development plans for the Selex SAPHIRA NIR eAPD and controller.
Most of the instruments and detector/camera developments are described in more detail in other papers at this conference.
The Four Laser Guide Star Facility (4LGSF) is part of the ESO Adaptive Optics Facility, in which one of the VLT telescopes, UT4, is transformed in an adaptive telescope-equipped with a deformable secondary mirror, two adaptive optics systems at the Nasmyth focii and four sodium laser guide star modular units. In this paper we present the design, the assembly and validation test performed so far in Europe on the first laser guide star unit.
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.
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.
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.
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.
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