L. Marty

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.

Removing static aberrations from the active optics system of a wide-field telescope

The wavefront sensor in active and adaptive telescopes is usually not in the optical path toward the scientific detector. It may generate additional wavefront aberrations, which have to be separated from the errors due to the telescope optics. The aberrations that are not rotationally symmetric can be disentangled from the telescope aberrations by a series of measurements taken in the center of the field, with the wavefront sensor at different orientation angles with respect to the focal plane. This method has been applied at the VLT Survey Telescope on the ESO Paranal observatory.

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.

The primary mirror system control software for the VST

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 primary mirror system software has been designed with a system engineering approach. The software has to change the mirror shape during observations, but also shall allow the user to perform a number of other activities. It has to support: periodic maintenance operations like the alignment, the mirror removal and installation for recoating; the functional tests; the engineering operations; the recalibration of several parameters. This paper describes how the primary mirror system software has been developed to support both the observations and engineering activities.

VST: from commissioning to science

The VLT Survey Telescope (VST) has started the scientific operations on the ESO Paranal observatory after a successful commissioning period. It is currently the largest telescope in the world specially designed for surveying the sky in visible light. The VST is dedicated to survey programmes, supporting the VLT with wide-angle imaging by detecting and pre-characterising sources, which the VLT Unit Telescopes can then observe further.

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 active optics control software for the VST telescope

The VST active optics software must basically provide the analysis of the image coming from the wavefront sensor (a 10×10 subpupils Shack Hartmann device) and the calculation of primary mirror forces and secondary mirror displacements to correct the intrinsic aberrations of the optical system and the ones originated for thermal or gravity reasons. The software architecture, the simulation code to validate the results and the status of work are here described.

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.

VST telescope primary mirror active optics actuators firmware implementation

The VST (VLT Survey Telescope) is a 2.6 m class Alt-Az telescope in installation phase at Cerro Paranal in Northern Chile, at the European Southern Observatory (ESO) site. The VST is a wide-field imaging telescope dedicated to supply databases for the ESO Very Large Telescope (VLT) science and to carry out stand-alone observations in the Ultraviolet to Infrared spectral range. The VST is provided with an active optics control system to actively compensate the optical aberrations; it is based on 84 actuators controlling the shape of the primary mirror and a hexapode for secondary mirror positioning. The present paper focuses on the implementation of the microcontroller programming firmware for the Primary Mirror Actuators Electronic Control Board. The most relevant problems encountered during the implementation of this real time multitasking distributed control application are described; optimization problems due to low performing hardware platform, not provided with operating system, are also reported. Several described topics are applicable to other distributed control systems, requiring closed loop control system and communication capability with a higher level computer.