Davide Fierro

Active optics primary mirror support system for the 2.6 m VST telescope.

The Very Large Telescope Survey Telescope (VST) is equipped with an active optics system in order to correct low-order aberrations. The 2.6 m primary mirror is supported both axially and laterally and is surrounded by several safety devices for earthquake protection. We describe the mirror support system and discuss the results of the qualification test campaign.

VST project: technical overview

The VST (Very Large Telescope Survey Telescope) is an 2.6 m class Alt-Az telescope which will be installed in the European Southern Observatory (ESO) Paranal site, Chile. It has been designed by the Technology Working Group of the Astronomical Observatory of Capodimonte, Italy. The VST is an 1 degree(s) X 1 degree(s) wide-field imaging facility planned to supply databases for the ESO VLT science and carry out stand-alone observations in the UV to I spectral range starting in the year 2001. All the solutions adopted in the VST design comply to the ESO VLT standards. This paper reports a technical overview of the telescope design.

Active optics control of VST telescope secondary mirror

In telescopes based on active optics, defocus and coma are usually compensated for by secondary mirror movements. They are performed at the Very Large Telescope Survey Telescope (VST) with a hexapod--a parallel robot with six degrees of freedom positioning capability. We describe the application of the two-mirror telescope theory to the VST case and the solutions adopted for the hexapod control. We present the results of performance and reliability tests performed both in the laboratory and at the telescope.

The VST active primary mirror support system

The 2.6-m primary mirror of the VST telescope is equipped with an active optics system in order to correct low-order aberrations, constantly monitoring the optical quality of the image and controlling the relative position and the shape of the optical elements. Periodically an image analyser calculates the deviation of the image from the best quality. VST is equipped with both a Shack-Hartmann in the probe system and a curvature sensor embedded in the OmegaCAM instrument. The telescope control software decomposes the deviation into single optical contributions and calculates the force correction that each active element has to perform to achieve the optimal quality. The set of correction forces, one for each axial actuator, is computed by the telescope central computer and transmitted to the local control unit of the primary mirror system for execution. The most important element of the VST active optics is the primary mirror, with its active support system located within the primary mirror cell structure. The primary mirror support system is composed by an axial and a lateral independent systems and includes an earthquake safety system. The system is described and the results of the qualification test campaign are discussed.

Performance of the VST secondary mirror support system

The VST telescope is equipped with an active optics system based on a wavefront sensor, a set of axial actuators to change the primary mirror shape and a secondary mirror positioner stage. The secondary mirror positioning capability allows the correction of defocus and coma, caused by incorrect relative positions of the two mirrors arising from the deformation of the telescope tube and of the optical train under the effect of gravity and thermal espansion. Periodically the image analyser calculates the deviation of the image from the best quality and the telescope control software decomposes the deviation into the single optical contributions. The new position and orientation of the secondary mirror is computed by the telescope control software and transmitted to the secondary mirror support system for execution. The secondary mirror positioner is a hexapod, i.e. a parallel robot with a mobile platform moved by six linear actuators acting simultaneously. This paper describes the secondary mirror support system and the qualification test campaign performed both in laboratory and at the telescope.

The ADC for the VST Telescope: theory and preliminary test of the electromechanical system

The VST telescope is equipped with an Atmospheric Dispersion Corrector to counterbalance the spectral dispersion introduced by the atmosphere. The well known effect of atmospheric refraction is the bending of incoming light due to variable atmospheric density along the light path. This effect depends on the tangent of the zenith angle and also varies with altitude, humidity and wavelength. Since the magnitude of refraction depends on the wavelength, the resulting effect is not only a deviation of the light beam from its original direction but also a spectral dispersion of the beam. This effect can be corrected by introducing a dispersing element in the instrument. In the VST case the device that compensates for this effect is based on a set of four prisms in two cemented doublet pairs. The system provides an adjustable counter dispersion by counter-rotating the two pairs of prisms. The counter-rotating angle depends on the atmospheric dispersion, which is computed with an atmospheric model using both environmental data (temperature, pressure, humidity) and the telescope position. Two different approaches have been compared for the computations to cross-check the results. The electromechanical system has been assembled, tested and debugged prior to the shipping to Chile. This paper describes the atmospheric models used in the VST case and the most recent phases of work.

Active optics system of the VLT Survey Telescope.

This paper describes the active optics system of the VLT Survey Telescope, the 2.6-m survey telescope designed for visible wavelengths of the European Southern Observatory at Cerro Paranal, in the Atacama desert. The telescope is characterized by a wide field of view (1.42 deg diameter), leading to tighter active optics than in conventional telescopes, in particular for the alignment requirements. We discuss the effects of typical error sources on the image quality and present the specific solutions adopted for wavefront sensing and correction of the aberrations, which are based on the shaping of a monolithic primary mirror and the positioning of the secondary in five degrees of freedom.

The VST auxiliary units: a status report before their commissioning in Paranal

The VST telescope is going to be commissioned in Paranal, together with its main sub-systems, such as the Image Analysis and Auto-Guiding system. A preliminary work of fine tuning of each sub-system has been performed in Italy before their shipping to Paranal, where they are waiting for the telescope AIV to be completed in a way to start the final commissioning of the overall system. Each unit has been extensively characterized and tested, with particular care to the Active Optics Shack-Hartmann sensor and to the Auto-Guiding arm. We describe here the phases concerning the integration and test of all the VST Auxiliary Units performed in Italy before their shipping to Paranal.

VST optomechanical technical specifications versus error budget

This paper concerns optomechanics tolerances specifications for VST telescope. It shows the strategy of tolerances definition for optomechanical systems. These prescriptions are the baseline for development and tests of VST telescope optomechanic components. The telescope is provided with an active optics control system, so some tolerances may be relaxed, respect to passive systems designs since they can be actively compensated. Gravitational and thermal deformations have been also considered. The design error budget strategy is described. Manufacturing, mounting and alignment tolerances have been evaluated within the whole telescope image quality error budget, in terms of rms spot radius. Since the telescope is seeing limited, effects of atmospheric seeing have also been considered in the error budget in terms of CIR. Do to its large field of view (1 degree square), the VST optical design (optomechanics tolerances included) is the first source of error if compared to a classical telescope design that has a small field of view. The overall optical quality depends also on telescope configuration (ADC and one-lens corrector or two-lens corrector configuration) and on observational zenith angle (0÷50°).

VST project: mechanical design optimization

The VLT Survey Telescope (VST) is a cooperative program between the European Southern Observatory (ESO) and the INAF Capodimonte Astronomical Observatory (OAC), Naples, for the study, design, and realization of a 2.6-m wide-field optical imaging telescope to be operated at the Paranal Observatory, Chile. The VST has been specifically designed to carry out stand-alone observations in the UV to I spectral range and to supply target databases for the ESO Very Large Telescope (VLT). The telescope design, manufacturing and integration are responsibility of TWG and have been carried out on the base of a model of optimized design not only for mechanics but for all telescope subsystems. The paper is an overview on the telescope mechanical design and optimization.