The design, manufacture and construction of the four 8 m VLT telescopes has been a major endeavor of the European Southern Observatory and European astronomy over the past twenty years. The final stages of this project, the assembly, integration and verification of the succession of four 8 m Unit Telescopes in the last three years, is the culmination of this great project. The successful achievement of this activity has been the result of a carefully planned campaign, using the best of telescope building methods developed on other projects over many years.

The HET is unique among 9-meter class telescopes in featuring an Arecibo-like design with a focal surface tracker. The focal surface tracker causes image quality and pointing/tracking performance to interact in a complex way that has no precedent in astronomical telescope system design and that has presented unusual demands upon commissioning. The fixed-elevation, segmented primary-mirror array offers some simplifications over traditional telescope design in principle, but has presented challenges in practice. The sky access characteristics of the HET also place unique demands on observational planning and discipline. The HET is distinguished by uniquely low construction and operating costs which affected commissioning. In this contribution, we describe those aspects of our commissioning experience that may impact how similar telescopes are designed, especially those with larger aperture, and review the challenges and lessons learned from commissioning a 9-meter class telescope with a small technical team.

The GTC (Gran Telescopio Canarias) Project, a 10 meter segmented telescope to be installed at the ORM in La Palma, Spain, is moving forward at full steam. Main science drivers for the GTC are image quality, operational efficiency and reliability. First light is planned for end-2002.

The Large Binocular Telescope (LBT) Project is a collaboration between institutions in Arizona, Germany, Italy, and Ohio. The telescope will have two 8.4 meter diameter primary mirrors phased on a common mounting with a 22.8 meter baseline. The second of two borosilicate honeycomb primary mirrors for LBT is being case at the Steward Observatory Mirror Lab this year. The baseline optical configuration of LBT includes adaptive infrared secondaries of a Gregorian design. The F/15 secondaries are undersized to provide a low thermal background focal plane which is unvignetted over a 4 arcminute diameter field-of- view. The interferometric focus combining the light from the two 8.4 meter primaries will reimage the two folded Gregorian focal planes to three central locations. The telescope elevation structure accommodates swing arm spiders which allow rapid interchange of the various secondary and tertiary mirrors as well as prime focus cameras. Maximum stiffness and minimal thermal disturbance were important drivers for the design of the telescope in order to provide the best possible images for interferometric observations. The telescope structure accommodates installation of a vacuum bell jar for aluminizing the primary mirrors in-situ on the telescope. The telescope structure is being fabricated in Italy by Ansaldo Energia S.p.A. in Milan. After pre-erection in the factory, the telescope will be shipped to Arizona in early 2001. The enclosure is being built on Mt. Graham under the auspices of Hart Construction Management Services of Safford, Arizona. The enclosure will be completed by late 2001 and ready for telescope installation.

The Magellan Project is a collaborative effort by the Carnegie Institution of Washington, the University of Arizona, Harvard University, Massachusetts Institute of Technology and the University of Michigan, to build two 6.5- meter telescopes at the Las Campanas Observatory in Chile. First light for Magellan 1 is scheduled to occur in early 2000. The telescope mount and dome are complete and installed on the mountain. The mount and its associated control system have passed preliminary pointing and tracking tests using a small finder telescope. Active positioning of the secondary cage to maintain focus and collimation is also operational.

Development of the SOAR telescope is currently underway. Project plans include many tactics for smooth assembly, integration, and validation of this new facility to be located on Cerro Pachon at the Cerro Tololo Inter-American Observatory in Chile. A small project team has been established to manage and engineer the development of the major subsystems that are combined in this high image quality 4.2-meter diameter telescope. The status and plans for the development of the 28m telescope are discussed. A modest-sized facility building is under construction by CTIO, the organization appointed to operate the facility for the SOAR partners. Each telescope subsystem is contracted on a firm fixed price basis and will include complete performance testing at the contractor's facility before acceptance and shipment to the site. To ensure seamless integration, representatives of each contractor will come to the site for assembly and testing in place. They join personnel from the Project Office, the new operations staff, and the CTIO maintenance organization to form integrated product teams for subsystem integration, SOAR eases integration by using and mandating common, commercial control software. The contractors, the SOAR team, and the instrument buildings are making extensive use of LabVIEW/BridgeVIEW running under Linux (with real-time extensions as necessary) on compactPCI chassis. The telescope will include sufficient instrumentation, including a possible adaptive optics system, to allow system testing and optimization. An exceptionally large instrument payload ensures that instruments can remain in place upon the telescope as they are delivered and brought on line.

The Visual and Infra-red Survey Telescope for Astronomy, or VISTA, is a UK funded four meter class wide-field infra-red and optical survey telescope to be situated in Chile. The telescope, which is regarded as a two-channel camera, was funded on the basis of having a one-degree, infra-red field at the Cassegrain focus and a two degree, optical field at prime focus. The re-use, development or sharing of existing telescope system designs will play a major role in the project and thus pose particular design challenges and trades. This paper briefly outlines the science specification and the functional requirements of the telescope/camera together with the initial technical concepts and options. The unique or interesting features of this type of system are also discussed. The newly appointed project office, its project organization and plan are briefly described. An integrated systems engineering approach to the project, which is being developed, is also outlined.

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.

We discuss many enhancements and refinements to the WIYN Telescope that have demonstrably improved the imaging performance over the past few years. Currently, WIYN yields a median delivered image quality of 0.8 arc-seconds for 10 second exposures in the R-Band, and images better than 0.6 arc-seconds 20% of the time Peak yielding excellent atmospheric seeing conditions. The telescope and enclosure were designed to exploit the good seeing conditions with features such as superb optical components, a primary mirror that is actively controlled for low-order aberrations, a thermally controlled primary mirror, active and passive enclosure ventilation, real-time focus and collimation control, and precise tracking and guiding control. The WIYN organization is committed to continuously enhance the scientific performance of the observatory and through a long-term commitment of increased technical support we have made significant progress on many technical aspects for improved imaging performance. These improvements, which are described in this paper, include optimization of the wavefront curvature measurement process, better control of secondary tilt and piston (focus), feedback mechanisms for structural thermo-mechanical effects, careful timing of the active thermal control for the primary mirror, damping of structural vibrations, and the implementation of a closed-loop focus sensor. WIYN is also developing an adaptive optics tip-tilt system which is described elsewhere in this conference.

The WIYN 3.5 meter telescope uses active thermal control of the primary mirror and both active and passive ventilation of the observatory enclosure. These features have proven effective for delivering consistently excellent images, and make the WIYN facility an ideal test bed for quantitative measurements of the effects of temperature and ventilation on mirror and dome seeing. We describe the results of seeing experiments conducted over the first four years of operations at the WIYN Observatory.

The McDonald Observatory 2.7-meter telescope was built in the 1960s and has been scientifically productive for 30 years. A program of upgrades was undertaken to improve the image quality and modernize the control system in order to enhance the telescope's capability for science observations and also to address maintenance issues of growing concern. Retirement and/or replacement of numerous obsolete components was also a key goal because of a growing risk to operation due to the age of the telescope. This paper reports on several phases of this ongoing program, specifically those undertaken in conjunction with renaming the 2.7-meter the Harlan J. Smith Telescope on its 25-year anniversary.

The use of a team approach by contractors, engineers and management to build the Large Binocular Telescope (LBT) has been successful in maintaining quality construction at a reasonable price. No matter how efficient the team, the building of a 16-story building, with a totally unique design, and on just 1.2 acres of land at an elevation of 3191 meters does present formidable problems. This paper will present the current status of the LBT construction on Mt. Graham and how the team approach has continued to be successful in providing quality solutions on a timely basis while keeping the costs of construction to a minimum. The paper will discuss many issues that project managers must plan for when undertaking new and unique designs and what steps managers can take to avoid costly delays.

The success of the VLT telescopes is largely linked to the excellent performance and reliability of the primary mirror and secondary mirror systems. By March 2000, three sets of these mirrors will have been successfully integrated, aligned, tuned and tested. As with all advanced and complex opto-mechanical systems, there has been the usual teething problems and trouble shooting. In addition the VLT primary mirrors, being a particularly thin and fragile meniscus, require careful manipulation during transport, installation in the M1 Cell and during the periodic removal for coating. During the VLT design, significant engineering effort was devoted to this problem. This has resulted in the design of dedicated subsystems and in the preparation of a series of procedures for mirror handling which are safe and practical.

To improve the image quality performance of the Hobby-Eberly Telescope's (HET) segmented primary mirror and to assist in the requirements definition for an optical alignment sensing and control system, multiple engineering tests have been designed and executed. The most significant of these tests have been the alignment maintenance baseline and solid mount tests. Together, these engineering tests defined the complex thermal and non-thermal response modes of the steel HET primary mirror truss and quantified the performance of the segment support system. We discuss the configuration and performance of the HET primary mirror, and discuss our engineering test motivation, goals, design, implementation and results. We also discuss the implications of our primary mirror performance test results for conceptually similar next generation telescope designs, such as the Extremely Large Telescope.

We report on the design of the two tertiary mirrors of the Large Binocular Telescope. The tertiary mirrors are flat octagonal shaped 540 X 640 mm Hextex honeycombs made of Schott borosilicate. Each mirror cell is mounted on three linear actuators for the active control of the mirror pointing and for the adjustment of the telescope optical path length. Each tertiary mirror unit embeds a rotator stage to point at four different instrument stations on the telescope. Particular effort is developed to the optimization of the honeycomb mirror support system to minimize the optical surface RMS deformation at the different mirror attitudes.

The large direct drive motors and encoders form together with the control system a high performance telescope exhibiting very high tracking accuracy. This paper describes the integration and fine-tuning of the VLT Drive Systems. It discusses the different problems encountered during the integration. The servo model that was used to simulate the problems and to find new solutions is described as well as test results and advanced analysis methods.

The purpose of the Gran Telescopio de CANARIAS (GTC) project is the design, construction, erection and startup of a telescope with a segmented primary mirror equivalent to a circular aperture of 10 m in diameter. The GTC project was created to satisfy the needs of Spanish astronomers for more telescope time and larger telescopes. The GTC project does more than simply increment the list of `8-10 meter class' telescopes. Its requirement for higher specifications differentiates it from other telescopes of its generation. The GTC will combine a large collecting surface with excellent image quality and suitably optimized observation in both the visible and the infrared. The Gran Telescopio de CANARIAS Project Office Telescope Group, based in the Canary Islands, together with a joint venture of Spanish companies, are developing the telescope mechanics systems, taking into account the scientific requirements established. This paper summarizes the scientific requirements that have technically led the design process of the telescope mechanics and the features that have been incorporated to meet said requirements.

The upgraded 3.8 m UK Infrared Telescope employs active control of the primary mirror figure and secondary mirror alignment to constrain intrinsic wavefront errors, currently to approximately 180 nm, while a fast guider controls a tip- tilt secondary to remove telescope vibrations and tracking errors. It routinely produces images with FWHM below 0.'5 at 2.2 micrometers wavelength (the K-band). The best fully-sampled image yet recorded has FWHM equals 0.'171 and is believed still to be the best ever achieved by a ground-based telescope without the use of higher-order adaptive optics.

The Large Binocular Telescope is currently in the pre- assembly phase at the Ansaldo Energia workshop in Milan. Since late 1998 the manufacturing of the Azimuth and Elevation structures has been taken place in North Italy along with the main auxiliary equipments, and since September 1999, the Azimuth Ring have been assembled and aligned on the new concrete foundation poured months before in the same area. The pre-assembly activity in Italy will take some months more from now as the final acceptance tests are scheduled now for December 2000; then the whole telescope steel structure will be disassembled and shipped to Mt. Graham where the final assembling phase will start in spring 2001. In this paper, the authors, part of some industrial companies and public institutes main character in this scientific and technical challenges, briefly describe the manufacturing and the machining processes of the main telescope components, the reached results and the procedures adopted during the pre-assembling as overall test bench for the final erection in Arizona.

The telescope mount is an important component for the success of the Southern Observatory for Astronomical Research (SOAR) scientific mission. The SOAR telescope structure must have the best combination of extremely high structural stiffness, low torque bearings, sophisticated encoder pick-offs and smooth drive trains so that the servo system can achieve closed loop control in the sub-arc second regime. While challenging, these parameters are achievable. Once assembled, this mount will enable the telescope to have superior image viewing quality and large payload capacities. This paper will address the telescope mount structure, drive system performance, structural analysis and thermal design.

The Large Binocular Telescope (LBT) under construction on Mount Graham, Arizona is a unique instrument which supports two 8.4-meter primary mirrors on the same mount. This unique optical support structure configuration presented new challenges in the design and construction of the telescope enclosure building. The LBT is a project managed by Steward Observatory at the University of Arizona and Arcetri Observatory in Italy. This paper discusses the design and analysis of the steel structure that encloses the telescope, including solutions to the problems presented by the design criteria.

The GTC Dome is entering into manufacture. Once the design has been frozen, it is time to expose the main features of the Dome and to exhibit the key aspects that will introduce an improvement on the GTC performance with respect to existing telescopes. The implementation of the natural ventilation for the telescope chamber, the use of an improved classical up-and-over shutter or the use of dome shell ventilation are described along the paper, as well as other features that have been laid-out to be absolutely ready to be `switch-on' after day one.

The SOAR telescope dome is a 20 meter diameter 5/8 spherical structure built on a rotating steel frame with an over the top nesting shutter and covered with a fiberglass panel system. The insulated fiberglass panel system can be self- supporting and is typically used for radomes on ground based tracking systems. The enclosed observing area is ventilated using a down draft ventilation system. The rotating steel frame is comprised of a ring beam and dual arch girders to provide support to the panel system sections and guide the shutter. The dual door shutter incorporates a unique differential drive system that reduces the complexity of the control system. The dome, shutter and windscreen `track' the telescope for maximum wind protection. The dome rotates on sixteen fixed compliant bogie assemblies. The dome is designed for assembly in sections off the facility and lifted into place for minimal impact on assembly of other telescope systems. The expected cost of the complete dome; including structure, drives, and controls is under 1.7 million. The details covered in this paper are the initial trade-offs and rationale required by SOAR to define the dome, the detailed design performed by M3 Engineering and Technology, and the choices made during the design.

LAMOST is a reflecting Schmidt telescope that lies on the ground. It is consists of three main parts: Schmidt plate- M_A, spherical primary mirror M_B and focal plane unit. Its optical axis is on the meridian plane with a 25- degree inclination to the horizontal that is different to traditional telescope. Therefore LAMOST enclosure will be consists of two parts accordant: the dome enclosure of M_A and the tunnel enclosure from focus to M_B. This paper describes the major design consideration of the LAMOST enclosures, that featured with a better seeing performance, a good wind buffet function and an acceptable optical baffle, its structure are more simple with good operability and maintainability.

Telescope Technologies Limited have optimized the structural design of their range of 2.0 m telescopes for operation in the `open-air' during optical and near infra-red observations, in order to obtain the benefits of avoiding dome `seeing' and vibration effects from a rotating dome.

A unit device for supporting a telescope primary mirror in its cell is described. It replaces the traditional roller- ball or oil-bellows support unit. The device utilizes the levitating field from opposing magnets to support the primary's weight above the cell's surface. This frees the bearings of the device so that the primary may expand or contract smoothly, unimpaired with `sticky', loaded bearings. The mechanics of the device restrain the opposing magnets from drifting inappropriately and work to isolate the primary from undesirable bending moments. Supplying the near-cell magnet, which may advance toward the near-primary magnet, with the standard counterweight and fulcrum commonly seen behind the cell assures the primary/device, weight/force balance remains for any orientation. Design, forces, and ongoing research for levitated support is discussed. A prototype is under construction.

During the design phase of the ESO Very Large Telescope considerably emphasis was placed on providing the means of controlling and monitoring thermal and wind disturbances. Today, about one year after the start of operation of the first Unit Telescope, much has been learned about the behavior of the telescope, and also about the optimum control strategies to reduce such disturbances. This paper outlines the current strategy for the control of environmental disturbances and discuses some of the lessons that have been learned.

The adaptive optics system of the Multiple Mirror Telescope is going to realize a high speed (1 kHz bandwidth) and high order (336 actuators) wavefront correction. However, to achieve the required 0.08 arcsec pointing stability the focal point of the Shack-Hartman wavefront sensor must be kept aligned to the Cassegrain focus better than 10 micrometers in spite of the non-common path tip/tilt error due to mechanical and thermal deformation of the telescope structure. The wave-front sensor must also be rotated with high precision to keep it aligned with the deformable secondary mirror in spite of the parallactic angle correction of the telescope. Our approach is to use a feed- forward loop to eliminate the adverse effect of deformation. A fast, deterministic field bus is applied to interconnect the actuators, sensors and computers. The bandwidth (500 kbs) and latency (less than 1 ms) of the DeviceNet serial bus is adequate to support our distributed control system. The field bus architecture simplifies and standardizes the control software as well as improves the reliability of the electronics by reducing the wiring.

This paper describes the control system analysis and design for the three principal axes of the 2.5 mts SDSS Telescope. The telescope requirements are good tracking performance with errors lower than 165 marcsec rms in the speed range between 0 to 45 arcsec/sec for all the axes. The pointing error is about 2 arcsec rms per axis for a maximum absolute value of 5 arcsec. The telescope has the additional requirement of slewing, between tracking areas, with maximum speed of 3 degree/sec.

With its `first fringe' milestone planned within a year the VLT Interferometer (VLTI) has entered a phase where it becomes possible to have a preliminary validation of its system performance by combining results of tests obtained at sub-system level. This is also the time for a careful planning of integration and commissioning activities. After an overview of the system approach used in the course of the VLTI project to predict and control the system performance, we present the results of the last crucial tests done on the thermal behavior of the underground, air-filled Delay Line Tunnel. These results have enabled to validate the early assumptions made on the effects of internal air turbulence (so called `internal seeing'). Finally, the last section presents the basic approach to the integration and commissioning of the VLTI to start this year at Paranal.

The first VLT unit telescope, Antu, saw first light in May 1998 and started science operation in April 1999, roughly at the same time as first light of unit telescope 2, Kueyen, was achieved. The time between first light and science operation is used to verify, quantify, qualify and optimize the functionality and performance of the telescopes and their instruments.

Control systems and structural analysis technologies have advanced to a level where top-down design optimization can be realized for large optical telescopes. Although design criteria have continually progressed, only recently has prior experience been enhanced through the use of high-speed processor technologies and advanced, simulation modeling design tools. Thus, optimization of the complete electromechanical structure makes possible a cost effective, yet high performance, telescope design. The iterative process of selecting controls and mount structure, modeling, astronomer requirements and refinements are explored to demonstrate an optimized solution.

The Gran Telescopio Canarias (GTC) is a Spanish initiative, open to international partnership, to construct a 10-m segmented mirror telescope at the Roque de Los Muchachos Observatory, on the Island of La Palma. The GTC first light will be at the end of 2002, while it will be fully operational at the end of 2003. The GTC Global Model (GTCGM), using MATLAB/SIMULINK, aims at verifying the design of the telescope complies with the specifications set by the GTC Project Office. The GTCGM can also provide evaluations of the individual contributions of each simulated error, for the Error Budget verification.

Precise large astronomical telescopes require proper control and minimization of static and dynamic structural distortions caused by different loads, e.g. from gravity, wind, temperature and equipment noise. This equally holds for the main structure as well as for the component level. Examples for the first are the alignment of secondary and primary mirror, and for the latter the shape error minimization of the reflecting surfaces. In this paper, essential concepts for passive and active distortion control are presented and discussed. Some of these concepts are demonstrated for the case of a big representative truss structure and the influence of modal reduction is pointed out. The tools developed to simulate and optimize active damping systems on truss structures are flexible and extendable and could be applied to big telescope structures like for example the OWL structure.

The TNG is regularly working with external observers. After a proper tuning of the motion control system the tracking performance are well in spaces. This paper describes the handling of the external compounds by the drive control system and the diagnostic capabilities of the system.

The baseline concept for the OWL mechanical structure is further developed and studied on the basis of the `six mirrors optical concept'. The primary mirror supporting structure is elaborated in deeper detail, the impact on the design of using lightweight mirrors is analyzed and also a trade-off among the so-called iso-static and hyper-static configurations is discussed. The performance of the telescope under seismic and wind survival load cases is analyzed.

The paper discusses the requirements of the enclosure and infrastructure for OWL. Although predicted to have no serious technological risks, these items will constitute a significant investment within the OWL project. An enclosure for such a large telescope does not have to provide the same functions as the actual enclosures built as of today. Protection from wind disturbance is not provided as efficiently as by enclosures with dimensions in the order of 30 m. The conditioning of such large volumes is economically not viable. A none co-rotating enclosure is shortly discussed as a solution and the reasons, which could make it effective are analyzed. The pier of the telescope is sketched and its effect on the telescope dynamics is discussed.

This paper outlines the design of an auto guiding system and points out the potential pitfalls that often are not observed. Rather than describing the mathematics in detail, the fundamental principles are discussed at a more basic level, leaving the interested reader to explore the details in the mathematical textbooks himself. The diagrams in this paper all made through simulations rather than mathematical calculations.

We present an active, low cost hydrostatic shoe bearing system for the Mexican Infrared Telescope which solves the suspension and motion of a 100 ton, 7.8 m telescope. Different geometries are analyzed to optimize the shoe's pressure print. These designs offer a self-adjusting action between the shoe's sliding path and the girth track. Different parameters such as pressure, temperature and proximity are measured and implemented into a control system in order to stabilize the bearing from the fluid's thermal viscosity effects. A simple method for fluid injection is discussed.

This article presents the design of a capacitive sensor that can detect piston errors as well as lateral displacements between two adjacent MI segments. Due to the measurements' statistical analysis the resolution acquired will be nanometric. This precision is required to achieve the image quality expected for the whole primary segmented-mirror.

The applicability of the curvature method for co-phasing of segmented mirrors is investigated by means of simulations for the case of strongly defocused images. The simulations are performed for both the monochromatic and the white light as well. A simple wavefront reconstruction from curvature signal was made. The reconstruction quality of the piston modes and the aberrations up to the fourth order is analyzed. The dependence of the Central Intensity Ratio for a segmented mirror as a function of the rms segment's aberrations is presented. The effect of turbulence-induced distortions on the quality of mirror co-phasing is analyzed. It is shown that the local pistons and the local tip-tilts can be measured directly from the curvature signal without any phase recovering procedure. The results obtained show that, even in the presence of the atmospheric turbulence, the curvature method is sensitive enough to detect the errors of segmented mirrors.

Seeing is generally caused by non-homogeneous variations in the refractive index of the column of air in the observed light path. The refractive index is affected by wavelength, air pressure, temperature and relative humidity. In this paper the Cauchy-Lorenz formula for the refractive index of air is evaluated to find the relative dependence of seeing on each of these parameters. A standard atmospheric model is used to evaluate the effect of altitude on each term, and from this an expression for the variation of seeing as a function of altitude is derived.

The California Extremely Large Telescope is a study currently underway by the University of California and the California Institute of Technology, to assess the feasibility of building a 30-m ground based telescope that will push the frontiers to observational astronomy. The telescope will be fully steerable, with a large field of view, and be able to work in both a seeing-limited arena and as a diffraction-limited telescope, with adaptive optics.

The European Southern Observatory is developing a concept of ground-based, 100-m class optical telescope, with segmented primary and secondary mirrors, integrated active optics and multi-conjugate adaptive optics capabilities. Preliminary analysis have confirmed feasibility of the major telescope components within a cost on the order of 1,000 million Euros and within a competitive time frame. The modular design allows progressive transition between integration and science operation, and the telescope would be able to deliver full resolution and unequalled collecting power 11 to 12 years after project funding. The concept owes much of its design characteristics to features of existing telescopes, namely the Hobby-Eberly for optical fabrication, the Keck for optical segmentation, and the VLT for active optics control. The only critical area in terms of needed development seems to be multi-conjugate adaptive optics, but its principles have recently been confirmed experimentally and rapid progress in the underlying technologies is taking place and benefits from consumer applications. Further studies are progressing, confirming initial estimates, and a baseline design is taking shape. The primary objective of those studies is to demonstrate feasibility within proven technologies, but provisions are made for likely technological progress allowing either cost reduction or performance improvement, or both.

The design of any modern astronomical telescope requires close interaction between the science requirements, the optical and mechanical design of the telescope and its instrumentation. In addition new, large aperture, telescopes will need to have adaptive optics as an integral part of the concept. This paper discusses optical concepts for the telescope and instruments, highlighting technology challenges.

The Hobby-Eberly Telescope (HET) has been examined as a prototype for an Extremely Large Telescope (ELT) with a 33- meter diameter primary mirror. In this paper we examine the feasibility of scaling the HET/ELT up to 100-meters in diameter. In this 100-meter telescope design (called ELTX) the advantages of the tilted Arecibo concept seem to emerge even more strongly. For example the whole primary mirror is below grade and extremely well shielded from wind shake and the Stewart platform which carries the spherical aberration corrector and the instruments is capable of being scaled up to this massive size without any serious problems. Such a design is on track for probable science missions in the next half century.

The next generation large telescope is expected to be on the order of a 20 to 50 meter diameter aperture. The facilities required for these large telescopes must be structurally efficient in design to be cost effective. A geodesic type, rotating aluminum dome is one possibility. A geodesic structure has the advantages of high specific strength and stiffness, easy of deployment to the site, rapid on-site assembly and installation in parallel with other telescope assembly tasks, and long life. This paper presents a feasibility study, cost estimates, and concept design for a 91 meters (300 feet) diameter geodesic aluminum dome suitable for the next generation large telescopes.

Dynamic Radio and Optical Astronomy instruments are becoming larger and more sophisticated, so they demand better, more precise control to enable the narrow field optics to perform their functions. Coaxing performance into the sub arcsecond region of pointing and tracking requires much more than just reducing friction and getting stronger gearboxes. It requires the designers to develop a fundamental feel of the structure's personality and its dynamic properties, the system nonlinearities, and macro- and microscopic aspects of the drive systems, bearings and other motion components of the instrument.

the development of the large-scale telescope building is pressingly putting the question of the 5-Th generation supertelescope development with 25 m diameter primary mirror, having penetrating ability about 29 magnitude. The volume of information obtained on the supertelescope, will allow to expand the idea about the Universe origin and evolution greatly.

Recent incremental upgrades to the Phased Array Mirror Extendible Large Aperture (PAMELA) telescope testbed have enabled the demonstration of phasing (with a monochromatic source) of clusters of primary mirror segments down to the diffraction limit. PAMELA upgrades include an improved Shack-Hartmann wavefront sensor, passive viscoelastic damping treatments for the voice-coil actuators, and mechanical improvement of mirror surface figures. This report summarizes the recent PAMELA upgrades and presents a status of this unique testbed for wavefront sensing and control. The Marshall Space Flight Center acquired the PAMELA telescope in 1993 after Kaman Aerospace was unable to complete integration and testing under the limited SDIO and DARPA funding. The PAMELA is a 36-segment, half-meter aperture, adaptive telescope which utilizes a Shack-Hartmann wavefront sensor, inductive coil edge sensors, voice coil actuators, imaging CCD cameras and interferometry for figure alignment, wavefront sensing and control. MSFC originally obtained the PAMELA to supplement its research in the interactions of control systems with flexible structures. In August 1994, complete tip, tilt and piston control was successfully demonstrated using the Shack-Hartmann wavefront sensor and the inductive edge sensors.

We explore the issues in the control and alignment of the primary mirror of the proposed 30 meter California Extremely Large Telescope and other very large telescopes with segmented primaries (consisting of 1000 or more segments). We show that as the number of segments increases, the noise in the telescope active control system (ACS) increases, roughly as (root)n. This likely means that, for a thousand segment telescope like CELT, Keck-style capacitive sensors will not be able to adequately monitor the lowest spatial frequency degrees of freedom of the primary mirror, and will therefore have to be supplemented by a Shack-Hartmann-type wavefront sensor. However, in the case of segment phasing, which is governed by a `control matrix' similar to that of the ACS, the corresponding noise is virtually independent of n. It follows that reasonably straightforward extensions of current techniques should be adequate to phase the extremely large telescopes of the future.

We are currently constructing a 0.5-meter and a 1-meter wide-field telescope that will operate in tandem to detect and track Near Earth Objects and space debris. The telescopes have a single flat-plane Cassegrain focus, are optimized for a specific range of wavelengths of light, a small focal ratio with a wide field of view, and hold a single multi-chip CCD camera as their only observational instrument. Because these telescopes are designed for a specific type of observation, the costs of the telescopes are greatly reduced compared to similar sized multipurpose telescopes systems. To achieve high-quality NEO/Space Debris observations, the manufacturers of the telescope systems, CCD cameras, facilities, and software work together to integrate their cutting-edge technologies into a single robust system. In this paper we discuss the strategy, design, and implementation of our manufacturing team approach to building cost-effective advanced technology telescope systems.

The Swedish ELT is intended to be a 50 m telescope with multiconjugate adaptive optics integrated directly as a crucial part of the optical design. In this paper we discuss the effects of the distributed atmospheric turbulence with regard to the choice of optimal geometry of the telescope. Originally the basic system was foreseen to be a Gregorian with an adaptive secondary correcting adequately for nearby turbulences in both the infrared and visual regions, but if the performance degradation expected from changing the basic system to a Cassegrain keeping the adaptive secondary could be accepted, the constructional costs would be significantly reduced. In order to clarify this question, a simple analytical model describing the performance employing a single deformable mirror for adaptive correction has been developed and used for analysis. The quantitative results shown here relates to a wavelength of 2.2 micrometers and are based on the seven layer atmospheric model for the Cerro Pachon site, which is believed to be a good representative of most good astronomical sites. As a consequence of the analysis no performance degradation is expected from changing the core telescope to a Cassegrain (Ritchey- Chretien). The paper presents the layout and optical performance of the new design.

We are exploring the feasibility of a very large telescope with a wide field of view for multi-object spectroscopic surveys. This paper presents a brief overview of the scientific need for such facility, a possible optical design for such a telescope, and a description of how such a telescope might function for both wide-field, seeing-limited spectroscopy and narrow-field, high-Strehl imaging and spectroscopy. The science is primarily driven by the fact that imaging surveys are now capable of cataloguing vast numbers of targets that would take a formidable amount of time on currently existing telescopes for spectroscopic followup. The telescope design, a 4-mirror extension of the Paul concept, is a 30-meter telescope that delivers a full 1 degree(s) field at f/4 with excellent image performance across the full field. The primary is an f/1, 30-meter. The secondary has a diameter of 5.3-meters and contains a pure conic surface that delivers an uncorrected focus between the primary and secondary. The tertiary mirror is located at the vertex of the primary and has a diameter of about 10.6- meters. The quaternary mirror, located at the position of the initial focus, is about 4.4-meters and images the final focus back at the vertex of the tertiary mirror. The initial design had both the tertiary and quaternary mirrors with high-order, even aspheric surfaces. Further study has led to a simplification of the design in which the tertiary is now a pure conic like the primary and secondary mirrors, and the quaternary mirror is something like a Schmidt corrector with only modest fourth and sixth order terms.

The next generation of large telescopes will be in the 25 - 100 m range. Feasibility studies for telescopes of this size are in progress at various locations and it is currently discussed which aperture size that will be optimal. For this purpose it is highly important to clarify whether there is an upper limit to the size of telescope enclosures that can be built with reasonable resources. First, a design of a telescope enclosure for the proposed Swedish 50 m telescope is presented. Secondly, based on this design, the influence of the aperture size on the enclosure design and the construction budget is discussed, and it is concluded that it is entirely feasible to construct an enclosure for a telescope as large as 100 m. Detailed budgets are given.

This presentation describes the verification and validation processes of an Extremely Large Telescope Project and outlines the key role System Engineering plays in these processes throughout all project phases. If these processes are implemented correctly into the project execution and are started at the proper time, namely at the very beginning of the project, and if all capabilities of experienced system engineers are used, the project costs and the life-cycle costs of the telescope system can be reduced between 25 and 50%. The intention of this article is, by explaining the importance of Systems Engineering in the AIV and validation processes, to motivate and encourage project managers of astronomical telescopes and scientific instruments to involve the entire spectrum of Systems Engineering capabilities performed by trained and experienced SYSTEM engineers for the benefit of the project.

The Mexican Infrared Telescope is one of the most important projects in the Institute for Astronomy of the National University of Mexico. As part of the design we pretend to simulate different components of the telescope by the Finite Element Method (FEM). One of the most important parts of the structure is the primary mirror support. This structure is under stress, causing deformations in the primary mirror; these deformations shouldn't be over 40 nanometers, which is the maximum permissible tolerance. One of the most interesting subjects to develop in this project is to make the segmented primary mirror to work like if it were a monolithic one. Each segment has six degrees of freedom, whose control needs actuators and sensors with stiff mechanical structures. Our purpose is to achieve these levels of design using FEM aided by computer and we pretend to study several models of the structure array using the Conceptual Design Method, in an effort to optimize the design.

In the year 2000, EOS Technologies, Inc. of Tucson, Arizona will complete six two-meter class telescopes for astronomy. Applications for these telescopes range from monitoring of active-galactic nuclei to the search for extra-solar planets. Four of the telescopes will form part of the Keck International Project. These telescopes meet the highest tracking and axis interaction specifications ever attempted in a two-meter class telescope. Each of these telescopes is capable of fully remote-control and semi-autonomous operation.

The VLT project considered from its beginning that maintenance was going to be an important issue to support the operation of this large observatory. A special maintenance philosophy and strategy was developed considering as its primary base preventive and predictive maintenance (PM and PdM). This paper describes the basic concepts under the VLT maintenance strategy and how to implement it in practice.

As the latest generation of large (8 - 10 meter) telescopes are taking their first observations of our expanding universe, a new age of 2.0 m class high performance telescopes is emerging to support them. Telescopes of this aperture can be used as part of an interferometric array to combine their science beams with that of a larger telescope, as with the W. M. Keck and VLT Outrigger Telescopes projects. These telescopes are also used for survey and target acquisition work.

In this paper four 30 m to 100 m telescope configurations are put forward. (1) A transit mounting telescope with spherical primary mirror which can be rotated continuously only around the level axis, and a corrector on the prime focus is used to track the objects during they passing through the meridian cycle about 1.5 hours. (2) The mounting is the same as the first one, but to get higher image quality and smaller corrector, we applied the idea, which Su et al put forward in 1986, to actively change the shape of the area in use on the primary mirror during tracking in this telescope. (3) The third one is a fully fixed mounting telescope, but it is composed of several telescopes like the first or the second configuration in parallel along south and north direction. (4) An alt-az mounting telescope with a f/0.6 aspherical primary mirror. There are three important ideas in this paper: (a) transit mounting; (b) primary mirror with continuous variable shape during observation; (c) very fast f ratio telescope with fixed shape aspherical primary mirror. All these configurations can be used in optical, infrared, sub-millimeter and radio telescopes and also can be used on the moon.

A thermally conductive, dimensionally stable optical bench has been fabricated from advanced composite materials for use at temperatures below 180 K (-93 degree(s)C). The optical bench comprises the main structure of the interferometer for the TES (Tropospheric Emission Spectrometer) instrument which will be part of the EOS CHEM satellite scheduled for launch in late 2002.

A unique set of instrument enclosures was implemented as part of an interferometric array now in place atop Mt. Wilson, California. These enclosures were designed in response to project criteria set forth in the planning phases of a new project by the Center for High Angular Resolution Astronomy at Georgia State University. The array is intended for high resolution imaging at optical and infrared wavelengths and is comprised of six 1-m diameter telescopes arranged in a Y-shaped configuration with a maximum baseline of approximately 350 m.

The GTC dome is a rotary, pre-raised, hemispherical structure with an external radius of 17528 mm. The dome design objectives pursued were as follows: (1) To reduce total weight to a minimum in order to diminish thermal inertia and the power required by dome and observation shutter drive mechanisms. (2) To obtain a rigid structure with high eigenfrequencies to reduce noise reaching the telescope. (3) To ensure good internal ventilation; to this end there are 16 ventilation doors and an air chamber inside the dome wall, and heat insulation on the inside surface of the wall.