Stefano Stanghellini

Design and construction of the VLT primary mirror cell: support of the large, thin primary mirror

The primary mirror cell of the very large telescope supports the primary mirror, the tertiary tower and mirror, and the Cassegrain instrumentation. Stringent requirements have been set to achieve the desired image quality, flexibility of use, and the necessary mirror safety. This paper describes the most important requirements set on the system and some of the design solutions which were chosen.

VLT secondary mirror unit performance and test results

This paper explains the overall mechanical functions of the Very Large Telescope Secondary Mirror Unit (VLT-M2 Unit), and the method of their verification by tests inside two different test adapters. The cinematic mechanisms of the M2 Unit are the Focusing Stage, Centering Stage and the Chopping Assembly which enables to adjust the position of the secondary mirror along 5 degrees of freedom. All these mechanisms are characterized by very high stiffness and accuracy, long lifetime and high reliability. The presentation of the finally achieved performance test results of the M2 Unit, realized by their high precision mechanisms, are the major points of this presentation.

Design and preliminary tests of the VLT secondary mirror unit

The mechanics of the secondary unit are located behind the secondary mirror as seen from the telescope focus. The secondary is undersized and defines the pupil of the telescope. There is a focusing and centering drive for slow adjustments of the secondary mirror. In addition, the secondary mirror has a fast beam steering mechanism for chopping and rapid guiding to remove atmospheric wavefront tilt during observations. The specified square wave chopping frequency is 5 Hz with a duty cycle larger than 80%. To achieve a high bandwidth, the secondary mirror is manufactured of light-weighted beryllium coated with nickel. The beam steering mechanism has a counter-vibrating mass to compensate for dynamic forces and moments. The chopping mechanism has been successfully tested. The code of the digital control used during the tests was generated using Matlab real time toolbox. The servos were implemented on a digital signal processor card equipped with a TMS 320C40. To compensate for resonances inside the bandwidth of the servos, a special filter is applied in the velocity loop. The design of the secondary unit is now completed and fabrication and assembly have begun.

Use of beryllium for the VLT secondary mirror

REOSC has been selected for the design, manufacturing and integration of the four ESO very large telescope (VLT) secondary mirrors. The VLT secondary mirrors are 1.12 m lightweight convex hyperbolic mirrors made of beryllium. Despite the VLT active optics correction capabilities, the use of a metal for the mirror structure implies specific manufacturing processes and associated design rules in order to ensure its dimensional stability during the telescope required life time. This paper describes how the fabrication process of the VLT secondary mirror has been optimized in order to maximize the dimensional stability of its structure. The beryllium properties are analyzed in parallel with the mirror requirements, the choices for the manufacturing, at all levels, are presented. A short work progress is presented, with the achieved mirror properties.

From 250 to 90 tonnes: systems engineering in the VISTA conceptual design development

Systems Engineering has been used throughout the development of the Visible and Infrared Survey Telescope for Astronomy (VISTA). VISTA was originally conceived as being a classic 4m telescope with wide-field imaging capability. The UK Astronomy Technology Centre (UK ATC) radically changed this thinking by treating the whole design as one system, integrating the camera optics into the telescope design. To maximise the performance, an f/1 primary mirror was adopted resulting in a very compact telescope and enclosure. Amongst other benefits, this reduced the overall mass of the telescope from 250 to 90 tonnes. During this optimisation process, the concept of a direct imaging K-short camera was developed. This development, in conjunction with an increase in IR field of view, produced a system with uniform image quality and throughput across a 350 mm diameter focal plane, 1.65 degree field. While this has presented some major engineering challenges, the approach has produced a system which is both scientifically rewarding and achievable. The optimisation, design trade-offs and Technical Specification developed in the conceptual design phase were achieved through a systems analysis approach. This paper describes some of the key systems engineering decisions and the tools employed to achieve them. Current systems engineering activities are described and future plans outlined.

Design and tests of the VLT M1 mirror passive and active supporting system

Keywords : Very Large Telescope, Active force control, Hydraulic supporting system design, M I mirror position loop, M 1 supporting dynamic behaviour

Experience with the operation of the European ALMA antennas

The 25 European antennas of ALMA were delivered by ESO to the ALMA project in Chile between April 2011 and September 2013. Their combined time of operation is already significant and allows us to draw conclusions regarding their ability to fulfil the original specification, in terms of both scientific performance and operational availability. In this paper, we will summarize the experience gained during the past five years of operation. We will characterize the performance of the antennas in routine operation and compare with the data obtained during acceptance testing. We will also describe the few technical issues experienced while operating at 5000m and the way in which these were treated during these first years of operation. We will evaluate the effective reliability obtained in service based on field data and draw some conclusions as to the way in which reliability and maintainability aspects were covered during the process which led to the final design of the antenna. We will discuss the smart use of software to handle redundancy in a flexible way and to exclude failed components without affecting overall antenna operability. The use of low-level diagnostics enabled by remote access allows us to shorten the trouble-shooting cycle and to optimise physical interventions on the antennas. Finally, the paper will cover Antenna maintenance manuals edited using an industrial interactive standard. It will be explained why this advanced and innovative concept has not achieved the success that was expected, and why the traditional form is preferred at the ALMA Observatory.

Integrated approach to the design of a 4-m survey telescope (VISTA) and its instrumentation

The design of VISTA (Visible and Infrared Survey Telescope for Astronomy) requires close interaction between the science requirements, the optical and active mechanical design of the telescope and its instrumentation with the wavefront sensing. The optical design is based on an integrated approach of the telescope with tow separate cameras, one working in the IR waveband and the other working in the Visible waveband. The large field of view (2 degrees in the visible and 1.65 degrees in the IR), the seeing-limited resolution required (FWHM of 0.4 arcsec for the visible and 0.5 arcsec for the IR), the technological advance in active telescopes and large IR arrays and the f/1 quasi Ritchey-Chretien telescope design, makes this telescope a very powerful tool in performing high resolution and large astronomical surveys. A system analysis, modeling the various sources of errors such as optical aberrations, surface errors, control errors, environmental effects and detector effects is presented in this paper.