Lorenzo Zago

Challenges in optics for Extremely Large Telescope instrumentation

Zeeko Ltd, 4 Vulcan Court, Hermitage Industrial Estate, Coalville, Leichestershire, England, LE67 3FW, UKReceived 3 March 200, accepted 27 March 2006Published online laterKey words Optics – ELTs – spectrographs – gratings – new materialsWe describe and summarize the optical challenges for future instrumentation for Extremely Large Telescopes (ELTs).Knowing the complex instrumental requirements is crucial for the successful design of 30-60m aperture telescopes. Afterall, the success of ELTs will heavily rely on its instrumentation and this, in turn, will depend on the ability to produce largeand ultra-precise optical components like light-weight mirrors, aspheric lenses, segmented filters, and large gratin gs. Newmaterials and manufacturing processes are currently under study, both at research institutes and in industry. In the presentpaper, we report on its progress with particular emphasize on volume-phase-holographic gratings, photochromic materials,sintered silicon-carbide mirrors, ion-beam figuring, ultra-precision surfaces, and free-form optics. All are promising tech-nologies opening new degrees of freedom to optical designers. New optronic-mechanical systems will enable efficient useof the very large focal planes. We also provide exploratory descriptions of “old” and “new” optical technologies togeth erwith suggestions to instrument designers to overcome some of the challenges placed by ELT instrumentation.

The ESPRI project: astrometric exoplanet search with PRIMA

PRIMA, the instrument for Phase-Referenced Imaging and Micro-arcsecond Astrometry at the VLTI, is currently being developed at ESO. PRIMA will implement the dual-feed capability, at first for two UTs or ATs, to enable simultaneous interferometric observations of two objects that are separated by up to 1 arcmin. PRIMA is designed to perform narrow-angle astrometry in K-band with two ATs as well as phase-referenced aperture synthesis imaging with instruments like Amber and Midi. In order to speed up the full implementation of the 10 microarcsec astrometric capability of the VLTI and to carry out a large astrometric planet search program, a consortium lead by the Observatoire de Genève, Max Planck Institute for Astronomy, and Landessternwarte Heidelberg, has built Differential Delay Lines for PRIMA and is developing the astrometric observation preparation and data reduction software. When the facility becomes fully operational in 2009, we will use PRIMA to carry out a systematic astrometric Exoplanet Search program, called ESPRI. In this paper, we describe the narrow-angle astrometry measurement principle, give an overview of the ongoing hardand software developments, and outline our anticipated astrometric exoplanet search program.

Small parallel manipulator for the active alignment and focusing of the secondary mirror of the VLTI ATS

The paper describes the mechanism made for the 140-mm secondary mirror of the VLTI Auxiliary Telescopes. This mechanism consists of an hexapod parallel manipulator, capable of orienting its mobile base along all six degrees of freedom by means of six independent linear actuators.

The ESPRI project: astrometric exoplanet search with PRIMA I. Instrument description and performance of first light observations

Context. The ESPRI project relies on the astrometric capabilities offered by the PRIMA facility of the Very Large Telescope Interferometer for discovering and studying planetary systems. Our survey consists of obtaining high-precision astrometry for a large sample of stars over several years to detect their barycentric motions due to orbiting planets. We present the operations principle, the instruments implementation, and the results of a first series of test observations. Aims. We give a comprehensive overview of the instrument infrastructure and present the observation strategy for dual-field relative astrometry in the infrared K-band. We describe the differential delay lines, a key component of the PRIMA facility that was delivered by the ESPRI consortium, and discuss their performance within the facility. This paper serves as reference for future ESPRI publications and for the users of the PRIMA facility. Methods. Observations of bright visual binaries were used to test the observation procedures and to establish the instruments astrometric precision and accuracy. The data reduction strategy for the astrometry and the necessary corrections to the raw data are presented. Adaptive optics observations with NACO were used as an independent verification of PRIMA astrometric observations. Results. The PRIMA facility was used to carry out tests of astrometric observations. The astrometric performance in terms of precision is limited by the atmospheric turbulence at a level close to the theoretical expectations and a precision of 30 mu as was achieved. In contrast, the astrometric accuracy is insufficient for the goals of the ESPRI project and is currently limited by systematic errors that originate in the part of the interferometer beamtrain that is not monitored by the internal metrology system. Conclusions. Our observations led to defining corrective actions required to make the facility ready for carrying out the ESPRI search for extrasolar planets.

Extremely high-resolution tip-tilt-piston mirror mechanism for the VLT-NAOS field selector

NAOS (Nasmyth Adaptive Optics System) is the adaptive optics system presently developed for the ESO VLT. The field selectors are to feed the NAOS wavefront sensor with the light coming from an appropriate reference source which can be up to 1 arcmin (on the sky) distant from the center of the field of view. A large input tip-tilt-piston mirror selects the required part of the field of view. A second active mirror redirects the selected field to the wavefront sensor. The displacement of both mirrors are synchronized. The NAOS Field Selector consists of two extremely accurate tip-tilt-piston mirror mechanisms controlled in closed loop. Each mechanism provides a mechanical angle amplitude of +/- 6 degrees with a resolution and mechanical stability of 0.42 arcsec rms over 20 minutes. This implies a dynamic range of 100,000 which requires an extremely accurate, very high resolution closed loop control. Both mirrors are made in SiC for low mass and inertia. The design configuration of the mechanism in based on three electromagnetic actuators 120 degrees apart with the mobile magnets mounted on flexure guides. The mirror is supported by a combination of flex pivots and a membrane for flexibility in tilt and high radial stiffness. All kinematic joints consist of flexure elements so that the mechanism is essentially frictionless. The control system is implemented on a VME bus operated with the VXWorks OS with high electrical resolution (>= 18-bit) AD and DA interface boards. The controller has been carefully designed to achieve the best overall performances, i.e., a very good noise rejection, and a relatively low settling time.

The ESPRI project: Differential Delay Lines for PRIMA

ESPRI is a project which aims at searching for and characterizing extra-solar planets by dual-beam astrometry with PRIMA@VLTI. Differential Delay Lines (DDL) are fundamental for achieving the micro-arcseconds accuracy required by the scientific objective. Our Consortium, consisting of the Geneva Observatory, the Max-Planck Institut for Astronomy Heidelberg, and the Landessternwarte Heidelberg, in collaboration with ESO, has built and tested these DDLs successfully and will install them in summer 2008 at the VLTI. These DDLs consist of high quality cats eyes displaced on a parallel beam-mechanics and by means of a two-stage actuation with a precision of 5 nm over a stroke length of 70 mm. Over the full range, a bandwidth of about 400 Hz is achieved. The DDLs are operated in vacuum. We shall present, in this paper, their design and their exceptional performances.

VISTA secondary mirror drive performance and test results

This paper summarizes the main aspects of the design and qualification test results of the secondary mirror mechanism for the VISTA Telescope. A design overview is presented, with detailed description of the main aspects of the system including the electromechanical part and the control system. Also a description of the test facilities and test methodologies is provided prior to the presentation and discussion of the performance test results.

The EMIR detector translation unit: a cryogenic high-precision 3-DoF parallel mechanism

The paper describes a 3-DoF translational (XYZ) mechanism for the main detector of the EMIR multi-object spectrograph, developed for the GTC telescope. This mechanism is designed for the cryogenic environment (77 K) and consists of a parallel manipulator with flexure joints, actuated by three identical and symmetrically located linear actuators.

Inflatable Domes For Astronomical Telescopes

The current design of large telescopes calls on innovative technology to achieve high performance while keeping cost at an acceptable level.

The performances of the GTC secondary mirror drive unit

This paper summarizes the main aspects of the design and qualification test results of the secondary mirror mechanism for the 10.4-m Gran Telescopio Canarias (GTC). The design of the M2DS consists of a two stage mechanism, a hexapod for alignment using six linear actuators and a compensated tilt/chop stage with three voice coils taking its base on the hexapod mobile plate. The system has been tested after servos adjustment and calibration and the latest results are presented, which illustrate the quality and accuracy of this mechanism in both alignment and chop performances. Finally, the results and experiences are summarized in order to provide useful information for new developments of such systems.