The WIYN Observatory is a joint project of the University of Wisconsin, Indiana University, Yale University and the National Optical Astronomy Observatories to build a 3.5 meter ground-based telescope on Kitt Peak, Arizona. The observatory is currently under construction and nearing completion. This paper presents the current status of the project.

The Project Telescopio Nazionale GALILEO (TNG) will provide a 3.5-m telescope for the Italian astronomical community. Its main features closely parallel those of the ESO New Technology Telescope (NNT). We describe here its characteristics and its most important differences with respect to the NTT. The figuring of the three mirrors was successfully completed, with results exceeding the specifications. The telescope structure has been assembled in the workshop, and alignment tests are under way. The control system (motors, encoders, VME and workstation environment) is also in an advanced stage of development. The TNG will be part of the Observatorio del Roque de los Muchachos, in the Canary Islands. Its location is on the West side of the mountain, some 400-m from the NOT; excavation works are already well advanced. Civil works should be finished in the course of the current year. It is expected to complete the rotating building, and the erection of the telescope before the end of 1995, to start regular operations in 1996.

The status of the Magellan Project is discussed in detail with overall planning, cost, and manpower. The progress of the manufacturing of various telescope components, the work on the site, and the preparation of contracts are described. In particular the enclosure, the buildings and the telescope mount are shown in detail. Present plans concerning the telescope control system and the future operation are mentioned.

We describe the concept, basic design and capability of an eight meter class telescope currently being constructed by a international consortium of universities led by The Pennsylvania State University and The University of Texas at Austin. This unique telescope concept represents a deliberate trade between the science mission and technical cost drives. The basic science driver for the Spectroscopic Survey Telescope has traditionally been the need to obtain a large number of spectroscopic exposures in a short time. An efficient design that meets this need is a tilted Arecibo type telescope with a large segmented primary mirror. The SST has a number of other unique features that allow it to meet its science mission with unusual cost effectiveness.

The Gemini Project is an international partnership of Canada, the U.K., the U.S., Chile, Argentina, and Brazil, to build two 8-meter telescopes, one on Mauna Kea, Hawaii, and one on Cerro Pachon, Chile. The telescopes are to achieve an unprecedented combination of light-gathering power and image quality over the infrared, optical, and ultraviolet spectral regions observable from the ground. The key scientific requirement for the telescopes is that images at 2.2 microns delivered to the focal plane are not degraded more than 0.1 arcseconds over an hour's integration. In addition, these telescopes, in particular the Mauna Kea telescope, should be capable of reaching infrared emissivities of between 2 - 4% in operation. These requirements present special challenges for large telescope builders. To address these challenges, the Gemini project regards the entire observatory as a system. All aspects which may limit performance are tracked through the use of a systems error budget that includes the enclosure, telescope structure, both mirrors, control system and the instrumentation. This paper will highlight the meniscus mirror support system, the control philosophy to reduced wind buffeting and strategies to reduce thermal effects such as mirror and dome seeing.

The concepts and current status of Japanese 8.2 m optical and IR telescope project, SUBARU, are summarized. SUBARU aims to achieve imaging with very high resolution of 0.2 - 0.1 arc sec, and high observing capability through the wide wavelength region of optical and near-IR. The construction of SUBARU has started in 1991. So far the 8.3 m mirror blank was successfully formed, the construction of the lower fixed part of the enclosure is almost finished, and the main parts of the telescope are being fabricated. The first light of SUBARU is scheduled to be 1998 spring.

We describe the optical layout of the Large Binocular Telescope. Recent changes in the baseline optical configuration include moving the wide field foci above the primaries to allow a one degree field at F/4. The infrared F/15 secondaries are now a Gregorian design to allow maximum flexibility for adaptive optics. 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 two folded Gregorian focal planes to a central location. The telescope elevation structure accommodates swing arms which allow rapid interchange of the various secondary and tertiary mirrors. Using swing arms has allowed us to remove most of the superstructure supporting the spiders in an earlier version of the design. Maximum stiffness and minimal thermal disturbance continue to be important drivers for the detailed design of the telescope. By concentrating the structural mass between the two elevation C-rings, we are able to achieve a 2 Hz increase in the lowest eigenfrequency without increasing the mass of the elevation structure. The telescope structure accommodates installation of a vacuum bell jar for aluminizing the primary mirrors in-situ on the telescope. The detailed design of the telescope structure will be completed during 1994.

The performance goals of the telescope are reviewed and compared with the achieved values. The optical performance is close to the original goals, but our initial observations support evidence from other observatories that Mauna Kea seeing is even better than was assumed in setting the goals; so it is important not to lower our aims. The primary mirror active control system performance is summarized as well as the pointing the tracking performance. The telescope is substantially operational and since January 1994 we have been devoting the majority of nights to astronomical science.

Astronomical observations are now taking place on the Keck I telescope on a regular basis. We summarize here the status of the Keck I and II optics, and the current wavefront and image quality of the Keck I telescope as measured by in-telescope optical tests. Shack-Hartmann measurements of the individual primary mirror segments yield 80% encircled energy diameters that vary from 0.31 to 0.60 arc seconds. Full width at half maximum measurements of direct segment images obtained on a night of excellent seeing varied from 0.32 to 0.51 arcsec, and the combined image was 0.42 arcsec.

The active mirror control system of the W.M. Keck telescope maintains the optical figure of the segmented primary mirror under the changing influences of gravity and temperature. The ultimate performance of the system depends on the size of the calibration errors and on its stability. The design error budget calls for the calibrated mirror control system to contribute an image blur less than 0.1 arc seconds (80% enclosed energy) over the full range of operating conditions.

The achieved pointing and tracking performance of the telescope is presented and compared with the Keck goals. The implications of the current performance on observing are discussed, and planned remedies for deficiencies in pointing and tracking are proposed.

The azimuth and elevation sources of the W.M. Keck Telescope have been designed to meet stringent tracking, offsetting, and slewing requirements. The requirements and the achieved performance are presented. The feedback architecture of the position and rate loops is described. The analysis includes an identification of the telescope structure via frequency sweep test signals. The identified model is compared with the single resonance model used in the preliminary design. As expected there are numerous additional resonances, and the effects of these on performance and stability are discussed. The autoguider loop is also discussed. Shortcomings are noted and ideas for improved performance are examined.

We report on the casting of two 6.5 meter diameter borosilicate honeycomb mirrors at the Steward Observatory Mirror Laboratory. These F/1.25 primary mirror blanks are for the MMT Conversion Project in Arizona and the Magellan Project in Chile. The first 6.5 meter mirror blank was cast in April 1992. After a three month annealing cycle it was removed from the furnace using a fixture glued to the upper surface of the blank. It has since been stripped of its mold material in preparation for polishing. The mold material is broken up with a high pressure water spray. The second identical 6.5 meter was cast in February 1994. Each honeycomb 6.5 meter is cast from over 10 tons of E6 borosilicate glass manufactured by Ohara. This glass is melted onto a mold constructed of aluminosilicate fiber to produce a honeycomb structure with roughly 20% of solid density. The hexagonal voids in the honeycomb are produced by ceramic fiber boxes bolted to the bottom of the mold with SiC bolts. The furnace rotates at 7.4 rpm during the casting process to produce a F/1.25 parabola on the front surface. This rough parabola minimizes the amount of glass which must be removed during the grinding process. The front faceplate of the mirror will be 28 mm thick after generating. After the second 6.5 meter blank has been cast, minor modifications will be made to the furnace to allow casting of the first 8.4 meter mirror for the Large Binocular Telescope.

The Japan National Large Telescope (SUBARU) under construction adopted Corning ULE glass for its primary mirror, which is made from 44 hexagonal blank segments (hexes) sealed together. The optimized hexes placement pattern was determined using the simulated annealing method so as to minimize the thermal deformation due to the inhomogeneity of the coefficient of thermal expansion of the hexes. The sealing process has been successfully completed and now the flat-to-flat blank is under the grinding phase to be followed by the sag-down phase for meniscus forming. The present paper summarizes the hexes placement pattern optimization method and various analysis behind its actual application, including current status of the fabrication.

The spin casting technique has proved successful in the production of several raw blanks of the 8-m category. After the solution of start-up problems several castings could be successfully cooled down to room temperature. The quality inspections showed that the high requirements set for the inner quality of ZERODUR are completely fulfilled.

A novel computer-controlled method has been developed for grinding and polishing thin and a flexible mirrors and other optical components. The method makes use of the surface pressure induced by a partial vacuum between the tool and the workpiece. Based on the measured errors of the shape of the workpiece, the surface pressure is regulated dynamically by varying the partial vacuum under computer control while the tool is moving on the workpiece. To increase the figuring efficiency, several non-rotating tools are used simultaneously in a linear array across the optics. The method has been developed and tested using a 0.5 m thin blank and a small computer-controlled polishing machine. A description is presented of the developed equipment and of the experience gained in the use of the method.

Convex secondary mirrors are notoriously difficult to fabricate because of the tremendous cost and difficulty of making accurate measurements of the optical surfaces. The new 6.5- and 8-m-class telescopes require secondary mirrors that are larger, more aspheric, and more accurately figured than those for existing telescopes. The challenge of measuring these giant optics has spurred the development of a new measurement technique using holographic test plates. This test uses a full-aperture test plate with a computer-generated hologram fabricated onto the spherical reference surface. When supported a few millimeters from the secondary and properly illuminated with laser light, an interference pattern is formed that shows the secondary surface errors. The hologram consists of annular rings of metal drawn onto the curved test plate surface using a custom-built writing machine. The accuracy of the surface measurement using this technique is expected to be 35 nm P-V and 6 nm rms for a 1.65-m secondary mirror for the MMT. Considerably higher accuracy is expected for less aspheric surfaces.

The pursuit of economical fabrication of large (8 m) fast (< f/2), astronomical optics has led to the development of efficient fabrication and testing methods at the Mirror Lab. These methods rely on a mix of advanced technology blended with some traditional practices. Two fabrication strategies have been developed, one for primary mirrors and one for secondary mirrors. Both of these plans rely heavily on the use of the stressed lap both as a grinder as well as for polishing. For secondary fabrication novel methods of testing the convex, severely aspheric mirrors are used.

A large part of the active optics system and control environment for the Galileo telescope has been developed and tested. Presently the primary mirror support cell has been characterized for the mechanical and optical aspects. The primary mirror has also been characterized and tested with the active support system in work. Part of the mechanics for the secondary and tertiary active mirror supports has been constructed and we plan to start the characterization work in the second half of this year. An overview of the main results obtained during factory acceptance tests and a discussion about the general informatics implementation is here provided.

The primary mirror selected for the Gemini 8-m Telescopes is a thin meniscus made of Corning ULE^TM glass. The conceptual design of the Gemini support system has evolved in response to the properties of the meniscus mirror and the functional requirements of the Gemini Telescopes. This paper describes the design requirements, the design features, and predicted performance of this system.

This article first deals with general aspects of optimizing mirror supports. A wide variety of support topologies have been optimized by Nelson et al for unobscured entrance pupils. Optical forces and locations of point supports have been calculated here for annular pupils. Efficient topologies introducing a small amount of defocusing are also proposed for unobscured and annular pupils. Support efficiencies are given for each topology. Wavefront errors are estimated in the case of a defective cell, in order to specify tolerances on forces and geometries. The OVLA active optics is then discussed. The very thin, meniscus-shaped primary will be actively supported by 29 actuators and 3 fixed points. Actuator locations and forces have been calculated to minimize the mirror deflection under its own weight but also to allow a good control of astigmatism. We finally present a study of a concave adaptive secondary for the OVLA telescopes. As an initial result, we propose a defocus adaptive corrector with a variable thickness distribution. Conditions of use are defined, and performances are evaluated.

We describe the active support system and optimization of support forces for a 3.5-m honeycomb sandwich mirror. The optimization was based on interferometric figure measurements made in a vibration-isolated test tower. We obtained actuator influence functions by measurement and by finite-element analysis. The two sets of influence functions are similar in shape, but the computed figure changes are 25% smaller in magnitude than the measured figure changes. We achieved a figure accuracy of 25 nm rms surface error with the computed influence functions and only slightly worse with the measured influence functions, but were unable to reproduce the 21-nm rms surface error obtained on the passive polishing support. This implies that subtle differences between the polishing support and operational support caused small, uncorrectable figure changes. The optimization was performed by singular-value decomposition of the influence functions into normal modes. The best results were obtained using 20 - 30 out of a possible 37 modes.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) will be a 2.5-m clear-aperture telescope mounted in an open cavity in a modified Boeing 747 SP aircraft. SOFIA represents the next generation of the NASA Ames Research Center's infrared astronomy program. The existing airborne infrared telescope, the Kuiper Airborne Observatory (KAO), is a 0.91-m-aperture telescope flown on a Lockheed C-141 aircraft. The SOFIA telescope will have approximately eight times the sensitivity and three times the resolution of the KAO, and will be able to detect all of the far-infrared point sources detected by the Infrared Astronomical Satellite in 1983. A number of studies have been performed on the design of a large-aperture telescope capable of operating in the environment of an aircraft flying at 41,000 ft at Mach 0.85 while looking at astronomical sources through an open port. SOFIA poses a number of serious technical challenges for both the telescope designer and the system designer. This paper addresses one of these challenges, namely, the design of the telescope's primary mirror. Using new Russian technology will permit the fabrication of a lightweight, 2.7-m-diameter, f/1.3, primary mirror made of silicon carbide. The mirror and its graphite-aluminum mount will weight 650 kg, will not require any kind of active figure control or gravity sag compensation, will have a thermal time constant less than any other material, and will meet or exceed all of the requirements for the SOFIA mission.

A large, flexible mirror like the VLT Primary needs several sophisticated support systems, one for each operation during its lifetime. Typical mirror specifications, design loads and design requirements are given. For astronomical use ZEISS proposes a hydraulical passive support system for all 6 DOF of the mirror. For active correction an actuator with high dynamic bandwidth is connected to the axial passive support. Design features are discussed. Particularly the dynamic behavior of the support system is studied in an ACSL simulation. Active damping is foreseen as backup feature.

The Stratospheric Observatory For Infrared Astronomy is a planned 2.5 meter telescope that will be installed in a modified Boeing 747 aircraft and operated at altitudes above 40,000 feet. A concept for the telescope optical assembly which includes the primary mirror, secondary mirror, tertiary mirrors, and the metering structure will be discussed. The optical assembly is modular to facilitate assembly/disassembly operations. The optical assembly will have an image quality of less than 1.5 arcseconds, operate at elevation orientations from 15 degree(s) to 70 degree(s), and reach thermal equilibrium at -40 degree(s)C in less than 2 hours. A passive lightweighted monolithic primary mirror design based on proven technology has been shown to be a key element in meeting these requirements.

The lower stratosphere in the polar regions offers conditions for observation in the near-infrared comparable to those obtained from space. We describe a concept for a 6-meter, diluted aperture, near-infrared telescope carried by a tethered aerostat flying at 12 km altitude, to serve as a testbed for future space astronomical observatories while producing frontier science.

A study of a 25 m class telescope is presented. The scientific case is described together with imaging and spectroscopy aspects. Spectroscopy is found possible also at high resolution with the telescope proposed. Light efficiency and mirror coating are discussed. The optical design and corresponding performance requirements are presented. With a spherical segmented f/0.9 primary mirror and an exit focal ratio of f/2.86, an on-axis four mirror system with segmented primary and quaternary mirrors is found optimal. It gives an image quality of 0.27 arcsec FWHM over a field of 90 arcsec. Mechanical design is based on a tripod configuration similar to those of radio telescopes. The alignment system proposed is discussed. Total alignment is divided into three main sub-tasks. First, low frequency alignment is established using a slow wavefront sensor. Second, a high frequency alignment is maintained with an internal laser measurement system. Third, a high frequency correction for wavefront tilt errors is made with a correlation tracker. The enclosure is co-rotating with two sections sliding apart for observations. It has adjustable wind screens and double skin panels with internal air circulation. Control facilities are installed in a thermal jacket and the observing floor is cooled.

The Astrometric Imaging Telescope will detect extra-solar planetary systems with imaging and astrometry. The optical system contains a high-efficiency coronagraph and scatter-compensated mirrors to detect Jupiter-size planets around nearby stars. The optical system also is distortion free, tolerant to misalignments, and tolerant to optical surface contamination. This allows for the astrometric precision to detect Uranus-mass planets. A focal plane guider and fine guidance sensor are other elements of the optical design.

In China, astronomers and instrument experts are discussing and researching to build a new telescope. The formal decision will be done in the near future. Here two telescope configurations we proposed for China are presented.

The Galileo National Telescope (TNG) is an Italian 3.5 m class optical telescope to be located in La Palma (Spain), whose main elements have been derived from those of the ESO New Technology Telescope. Construction of TNG is well in progress, and the first light should be obtained in the second half of 1995. This paper gives a general overview on the drive system, that, together with a detailed diagnostic, should ensure high performances in pointing and tracking and a high level of reliability.

The structure of the Spectroscopic Survey Telescope is described, as developed in the author's office for the SST Project, McDonald Observatory, University of Texas at Austin, in the period November 1992 through March 1993. Performance requirements are listed, and the approach to design is discussed. The resulting structure and its performance characteristics are described, and illustrated in a number of drawings. The structure of the SST described herein is very stiff (lowest natural frequency of 7.05 Hz) while at the same time very light (less than 45 tons). The paper closes with a discussion of some recommendations for the design of similar future telescopes.

The Gemini project is an international collaboration to design, fabricate, and assemble two 8 M telescopes, one on Mauna Kea in Hawaii, the other on Cerro Pachon in Chile. The telescopes will be national facilities designed to meet the Gemini Science Requirements (GSR), a document developed by the Gemini Science Committee (GSC) and the national project scientists. The Gemini telescope group, based on Tucson, has developed a telescope structure to meet the GSR. This paper describes the science requirements that have technically driven the design, and the features that have been incorporated to meet these requirements. This is followed by a brief description of the telescope design. Finally, analyses that have been performed and development programs that have been undertaken are described briefly. Only the designs that have been performed by the Gemini Telescope Structure, Building and Enclosure Group are presented here; control, optical systems, acquisition and guiding, active and adaptive optics, Cassegrain rotator and instrumentation issues are designed and managed by others and will not be discussed here, except for a brief description of the telescope configurations to aid subsequent discussions.

The ESO VLT was first presented in April 1984 and its construction decided in December 1987. The VLT program consists of an array of four telescopes of 8 m diameter capable of working independently or in a combined mode. This paper gives the present status of the construction of the 8 meters Unit Telescope.

A 2.5 m-thick cinder layer immediately below the pier of Subaru (Japan National Large Telescope, JNLT) now under construction on the summit of Mauna Kea in Island of Hawaii was improved by recompacting volcanic cinders obtained by excavation at the site mixed with cement. The purpose was to increase the Young's modulus at very small strains of the supporting ground to 500 MPa so as to make the lowest natural frequency of the pier-ground system in its rocking motion to be more than 4.3 Hz. This requirement is essential for the telescope control system to correct as quickly as possible for external and internal disturbance of the telescope tracking. The results of the field and laboratory geotechnical tests including the measurements of elastic wave velocities and cyclic triaxial tests, which were performed for the design of the ground improvement work and the prediction and evaluation of the Young's modulus of the constructed layer, are described.

The Magellan Project 6.5 Meter Telescope is an alt-azimuth Tripod-Disk design currently under construction. The telescope, which is a joint project of the Carnegie Observatories and the University of Arizona, will be located at the Las Campanas Observatory, Chile. An overview of the structural design, including its evolutionary history, is presented. In addition, two critical mechanical systems which have been prototyped and tested are described. The direct friction drives are used on the telescope main axes and instrument rotators. The vane-end actuator system is used to maintain collimation and focus on any of the four secondary mirror assemblies for which the system is designed.

The rotational part of the enclosure of SUBARU telescope (Japan National Large Telescope, or JNLT) is under the fabrication in Canada, and on-site erection is scheduled to take place in May, 1994, with the expected completion in the third quarter of 1995. The enclosure is featured with various design consideration for a better seeing performance, a better wind buffet performance, and good operability and maintainability. This paper describes the major design consideration behind the final shape and function of the enclosure and the current status of its fabrication.

We describe the enclosure of the Large Binocular Telescope, now in a detailed design phase. The enclosure is co-rotating with the telescope, includes a service floor carrying all telescope utilities, and is characterized by two large portal shutters, which open laterally leaving the air flow at the front and at the back of the telescope essentially unobstructed. Large louvers on the side walls and wind shields with variable permeability at the front and back openings are used to control the air flow in different wind conditions. The inner and outer surfaces of the enclosure are designed to obtain short thermal time constant and close equilibrium with ambient air. Forced air circulation in the outer skin of the enclosure is used for better heat exchange and, with electrical heaters, for melting snow. The interaction of the whole building with the natural air flow in conditions representative of those encountered at the specific site on Mt. Graham was the subject of extensive water channel measurements on a model of the building and of the surrounding environment. The flow patterns obtained in the simulations show no mixing of lower air layers with those at the level of the primary mirrors or above. The handling scheme for large equipment, including the primary mirror cells and the bell-jar for aluminizing of the mirrors on board the telescope, is based on a large bridge crane that can transfer instruments and maintenance equipment to the telescope and to the service floor from the storage and maintenance area at ground level.

The Gemini project is an international collaboration between the USA, United Kingdom, Canada, Chile, Argentina, and Brazil, to design, fabricate and assemble two 8 M telescopes, one on Mauna Kea in Hawaii, the other on Cerro Pachon in Chile. The telescopes will be national facilities designed to meet the Gemini Science Requirements, a document developed by the Gemini Science Committee. This paper describes the design considerations that influence the scientific performance of the enclosure and support facility, and the features that have been incorporated to meet the demanding science requirements, particularly the 0.026 arc sec allowance for `enclosure seeing'. A description of the Gemini enclosure, support facilities and site plans for Mauna Kea is given here together with a brief description of the analysis and testing that has been performed to establish the performance of the facility.

This paper presents results of air flow simulations around and through the enclosures with two different computational fluid dynamics programs. Calculated flow patterns and pressure distributions are compared with wind tunnel test results. The presented analyses are a starting point in the development of the operational model for the VLT enclosures. More extended and detailed analytical models including coupled thermal and flow field simulations as well as additional wind tunnel tests and full scale measurements are necessary to develop the operational model for the ventilation system of the enclosures.

Heating of the air in astronomical telescopes is known to have a deteriorating effect on the image quality. Main heat sources are the primary mirror and localized electrical components in or near the light beam. To evaluate the effects of these localized heat sources we measured the image quality deterioration at the focus of the ESO-La Silla 2.2 meter telescopes with variable amounts of heating of a dummy electronics box and of a bar simulating one of the secondary mirror spiders. The effects on the FWHM of the image profile turned out to be remarkably small. Most of the effect of the heating showed up in the removal of energy of the core into the far wings of the image profile. For a heat input of 560 Watts (resulting in an excess temperature of 60 K) the amount of energy removed amounted to 13%. This behavior can be explained by a model in which the heating destroys the wave-front over only a part of the aperture while leaving it unaffected over most of the aperture. With such a model we predict the amount of energy removed by a 500 Watt heat source in an 8 meter telescope to be only 1%.

This paper summarizes the design and initial operation of the Starfire Optical Range 3.5-meter telescope. This facility is the centerpiece of the U.S. Air Force's strategic optical research program for high resolution imaging and laser beam propagation. Areas of research include high resolution imaging of low earth orbit satellites, laser power beaming to satellites, and deep space laser communications. The telescope and mount form the world's largest optical telescope capable of tracking low earth orbit satellites. A major emphasis in the research programs at the SOR is the development of adaptive optics, especially laser beacon adaptive optics, for large aperture telescopes.

In the 1970s the pioneering thin-mirror 3.8 m United Kingdom Infrared Telescope (UKIRT) of the UK Science and Engineering Research Council (SERC) was conceived as a low-cost `light bucket', with an 80% encircled-energy diameter <EQ 3'. However the delivered primary mirror had an 80 encircled- energy diameter of approximately 1' and the telescope has regularly delivered sub-arc-second images. To exploit this quality and to keep UKIRT competitive in a 21st century of 8-meter telescopes, in 1991 the SERC initiated an ambitious Upgrades Program, with the goal of routinely providing near- diffraction limited images at 2.2 microns. The major elements of the program are an adaptive tip-tilt secondary system, an active five-axis secondary collimation system, an upgraded primary mirror support system providing active control of the main optical aberrations, and modifications to the telescope and its enclosure to reduce or eliminate dome and mirror seeing, so as to take advantage of the excellent natural seeing on Mauna Kea. This paper outlines the overall project goals, the proposed strategies for upgrading the telescope and the progress to date.

A fast tip-tilt secondary is being implemented on the University of Hawaii 2.2-m telescope, to provide image quality to match the site characteristics of Mauna Kea, and complement the existing wide-field RC secondary.

This is a progress report on the development of the tip-tilt secondary mirror for the United Kingdom Infrared Telescope on Mauna Kea, Hawaii. The concept-- with emphasis on the electromechanical and optomechanical design--was published in an earlier paper. The reader is kindly requested to refer to the background information given there. Here, we present the electronics, system control and data handling considerations along with updated design drawings of the mirror and the combined piezoelectric/hexapod mirror mounts.

We review the status of the liquid mirror project. Interferometric tests of a f/1.2 2.5-m diameter liquid mirror carried out with a scatterplate interferometer show Strehl ratios of order 0.6, close to the value of 0.8 usually taken to signify that diffraction limit has been reached. The mirror is certainly better than implied by the data because the interferograms were taken with 1/500 second exposures and the wavefronts therefore include the effects of seeing in the testing tower. Correctable small variations of the rotational velocity account for another substantial fraction of the deviations from a parabola. We have videotaped hours of interferogram and PSF observations that show that those we analyze are representative.

The primary corrector for SUBARU telescope with a new type of atmospheric dispersion corrector allows better images in wider field of view compared to the corrector of the published design.

The Magellan Project 6.5-meter telescope is a collaboration of the Carnegie Institution of Washington and the University of Arizona. The telescope will be located on Cerro Manqui, at the Las Campanas Observatory, Chile. A design goal for the Magellan Project is to develop a telescope and enclosure system with a 1 degree(s) centigrade per hour time constant. The primary mirror is a University of Arizona borosilicate spincast honeycomb design. The enclosure is an octagonal shaped spaceframe structure with an insulated panel covering. The telescope fabrication is by L & F Industries. This paper will concentrate on thermal control of the telescope environment and the telescope thermal control interface to the enclosure.

The Magellan Project 6.5-meter telescope is a collaboration of the Carnegie Institution of Washington and the University of Arizona. The telescope will be located on Cerro Manqui, at the Las Campanas Observatory, Chile. At the beginning of the Magellan Project several schemes were investigated for realuminizing the primary mirror. We have chosen to leave the primary mirror in its cell with the mirror support system intact. Two major advantages of leaving the mirror in its cell are that it does not have to be lifted or handled and the support system does not have to be removed or reinstalled for aluminization.

The Magellan Project 6.5-meter telescope includes focal stations at both Cassegrain and Nasmyth locations. Instruments which use the f/15 infrared secondary will be mounted at the Cassegrain position. The Nasmyth foci will be fed by an f/11 Gregorian secondary. Rapid changes between the Cassegrain and Nasmyth configurations will be accomplished using a flip-tertiary diagonal mirror. The tertiary mirror turret includes a two-element field corrector and atmospheric dispersion compensator for use with the Gregorian secondary. The turret and the tertiary rotate in order to direct the light to one of 5 Gregorian focal stations, as well as to select or to bypass the field corrector/ADC for any focal station. The optical design of the corrector produces a 30 arc-min diameter field. The Gregorian focal ratio has been chosen to accurately match the field curvature of a particular collimator design, which is the first part of an imaging spectrograph with a 30 arc-min field of view. A variety of cameras will be used with this spectrograph. The wide-field camera should cover the entire field of view with a typical rms image diameter of 0.5 arc-sec. The high-resolution camera should cover the central 15 arc-min with a typical rms image diameter of 0.1 arc-sec.

The Spectroscopic Survey Telescope is being constructed by a consortium of universities at McDonald Observatory in the Davis Mountains of Texas. Principal partners are the University of Texas at Austin and the Pennsylvania State University. Also participating are Stanford University and the University of Munich and University of Gottingen in Germany. We describe the specific design attributes which enable the SST to be constructed for a fraction of the cost of astronomical telescopes of comparable size. Such unique features as identical spherical mirror segments, selective figuring for constant mirror mount deformation, air bearing azimuth rotation system, and pre-fabricated architectural type domes are employed. Emphasis is on simplification of design, reduction of part count and mass, and utilization of lessons learned from other recent large telescope projects.

Collimation techniques for the Hubble Space Telescope before and after the 1993 December servicing mission are discussed. As a first step we convert actuator positions of the HST secondary mirror into tilts and decenters, by means of a conversion matrix. Theoretical estimates of wavefront errors introduced by known amounts of tilts and decenters are related to each other by means of a conversion matrix. Several sets of stellar images taken with the Faint Object Camera over a two year period are analyzed and aberration coefficients derived. A residual coma of approximately 1/19th wave at 632.8 nm is found in the images. We report here the secondary mirror move made, after the refurbishment mission in 1993 December, to remove the coma and the analysis of the images taken after the move to verify the collimation.

The f/5 focus of the converted Multiple Mirror Telescope (MMT) has been designed for optical fiber spectroscopy over a 1 degree(s) diameter field as well as superlative imaging over a 0.5 degree(s) diameter field. The secondary mirror required for this purpose is nearly as large as one of the existing MMT primary mirrors (1.7 m). To attain the specified performance of the telescope support structure, this secondary must be made quite light (approximately 300 kg), without excessively compromising its stiffness. Our design calls for removing excess weight by machining hexagonal cells into a near zero-expansion materials such as ULE or Zerodur. The completed blank will be approximately 20 cm thick at the center, tapering to approximately 13 cm at the edge. The support of a secondary of this size must be considered as part of the blank design. We describe two possible axial support techniques: a vacuum support or a multiple actuator support similar to that adopted for the primary mirror of the converted MMT. Tangent flexures at the edge of the blank appear to be the most attractive radial support option.

A control system, consisting of an image movement sensor and movement compensator, is used to stabilize optical image in the focal plane of a spaceborne Earth's surveillance system. After an optoelectronic sensor for such a system had been elaborated by the authors, it was shown than its precision is affected with cloud movement in the field of view of the optical system. Moreover, even moderate cloudness appears to result in untolerable measuring errors. In this paper two techniques of diminishing measuring errors due to cloud movement are presented and their efficiency is estimated.

A Folded Astronomical Space Telescope is a 2.4-m Hubble Space Telescope class of telescope that can be packaged in a 1.5-m diameter cylinder through use of a single ring of eight deployable segments. Because it has less mass and uses a much smaller booster to inject it into orbit, the cost is greatly reduced. The enabling rationale, general configuration, and optical technologies for such a telescope are presented.

In this system, the experiment mirror has 500 mm aperture and 6 mm thickness. There are 58 actuators and three fixed points in it. A Shack-Hartmann test apparatus is used for the measurement of wavefront aberration. In this apparatus an ingenious equivalent of lenslet array is used. All image points formed by it appear very clear theoretical diffraction pattern. And a CCD from a TV camera is used. Like European Southern Observatory, we use quasi-Zernike polynomial to fit the wavefront aberration for correcting. But in our work correction is to the whole wavefront aberration (except lateral focus and longitudinal focus). In our work, another important character is that the damp least square method is used for determining the forces. The correction results are the root mean square of wavefront aberration about 0.02 - 0.04 micrometers . A circle including measuring and correcting the wavefront aberration takes about 3.3 minutes. A more precise algorithm proposed by us is used for calculating the wavefront aberration for checking.

We present data which illuminates the method and also discuss details of the operation of the Phasing Camera.

The computing environment and major components of the Keck telescope control system are discussed. This is followed by a discussion of calibration procedures and a review of current status and problems.

The Maksutov scheme for the compensation of an aspheric concave mirror by means of a single spherical mirror is described. The residual longitudinal spherical aberration and the corresponding Zernike polynomial coefficient are computed. The optical error budget for the scheme, as obtained from the analytical computations and by means of the CODE V program for design and engineering of optical systems, is presented. The application of such a scheme to the verification of the figuring of the hyperbolic primary mirror (D equals 1.70 m) of the T-170 Telescope to be launched on board of the Russia/Ukraine/Italy/Germany SPECTRUM UV satellite is discussed.

The Large Optical Generator (LOG) was originally installed as a precision generator at the University of Arizona. It has since been relocated to the Steward Observatory Mirror Laboratory, where, in addition to its tasks as generator, it can be reconfigured as a polishing machine. As such, utilizing the Mirror Lab's stressed-lap techniques, LOG has recently finished a series of three 3.5 meter mirrors to high accuracy. It is currently configured as a generator for work on the 6.5 meter MMT upgrade. LOG's operating parameters and level of performance both as generator and polisher will be discussed, along with some of the unique safety features that have been built into its operation.

The instruments used to interferometrically measure the optical surfaces of the 6.5-m f/1.25 primary mirrors for the MMT conversion and Magellan Telescopes must compensate over 800 micrometers surface departure from the best fitting sphere. The errors in the optical test must not contribute more than 0.04 arc seconds FWHM to the final image and the conic constant must be held to 0.01%. This paper presents the design, analysis, fabrication, and certification of the instruments used to measure these giant mirrors to such high accuracy.

The paper describes the communications infrastructure connecting the Isaac Newton Group of Telescopes at the Roque de Los Muchachos Observatory, La Palma to the Royal Greenwich Observatory, Cambridge. High-speed data access to La Palma has recently been improved by the installation of a 64 kilobit/s leased line which complements the academic network connections ordinarily available through SPAN and Internet. The cost of this connection is justified by improved day-to-day communication between users of the telescopes, administrative staff and their counterparts in the UK. The link is described in detail with reference to the cost-saving and technical benefits obtained. The link is planned to be available for astronomical use outside office hours, and related issues such as access to data for assessment, user confidence, analysis and archival, the control of telescopes and instruments, and the return of status to the user will be covered.

The macroscopic hidden logic in the evolution of astronomical telescopes, from the days of Galileo, points towards a cylindrical horse shoe design, resembling the fork. This design has the advantages of both equatorial as well as altazimuth mounts; but it inherits pointing problems inherent in the current logic of evolution. Whereas the orientation-analysis of stellar objects reveals that in spite of: (1) the lateral shift of the telescope due to spinning and two orbital motions of the earth and (2) the `big-bang expansion' of the universe, we can refer multi stellar objects simultaneously for error free offset tracking. The emerging fundamental logic demands no references to earth which is seen as a rattling platform and hence it circumvents the pointing problems caused by gravity, wind, ground noises, atmospheric refraction, wobbling of earth, etc. The resulting new concept telescope has the following features: (1) It is floated on multi layer air cushions; (2) two bright stars are used for auto guiding; (3) the central shadowed portion of the primary mirror is utilized as the reflector of guide field; (4) telescope shape is spherical; (5) tube is of classical Serrurier strut design and (6) four to eight folded Cassegrain stations are located around the primary mirror.

This paper will describe and discuss the methods which are being developed to support the large borosilicate honeycomb mirrors from the Steward Observatory Mirror Lab which are being used in the MMT 6.5 m conversion and the Large Binocular Telescope. The technique is similar to previous work carried out for the 3.5 m Phillips Lab mirror support.

An image compression algorithm has ben developed that is well-suited to astronomical images. The method has 3 steps: an intensity mapping to generate an image that has roughly constant noise in each pixel, an orthonormal wavelet transform, and quadtree coding of the bit-planes of the wavelet coefficients. The quadtree values may be further compressed by an standard compression technique, such as Huffman or arithmetic coding. If the 2D Haar transform is used, the calculations can be carried out using integer arithmetic, and the method can be used for both lossy and lossless compression. The Haar transform basis functions are well-suited to most astronomical images because they are highly localized. The performance of the algorithm using smoother, longer range wavelets is also shown; they can give slightly better lossy compression at the cost of an increase in artifacts around point sources, but they are not effective for lossless compression using this scheme. This technique has also been used as the basis of a progressive image transmission system that can be used for either remote observing or access to remote image archives. After less than 1% of the data have been received, the image is visually similar to the original, so it is possible to assess the quality of images very quickly. If necessary, the entire compressed data set can be sent so that the original image is recovered exactly.

The NASA Infrared Telescope Facility is a 3 meter infrared optimized telescope available to the astronomical community based on a peer reviewed applications. Efforts underway for two years to reduce the telescope emission in the infrared have reduced the telescope emissivity by more than a factor of two. This paper will report on the present telescope emissivity, the method used to measure the telescope emissivity, and cleaning procedures.

Wavefront tilt variances have been measured directly at near infrared wavelengths with the IRCAM camera at the UK Infrared Telescope. Single aperture and differential tilt variances were obtain in the J (1.228 micrometers ), H (1.643 micrometers ), K (2.182 micrometers ) and L (3.42 micrometers ) photometric passbands using Hartmann masks containing, respectively, four and eight subapertures at a re- imaged pupil. Sampling runs at each wavelength comprised 200 short exposures taken over periods of about 35 seconds. The results have been analyzed by separating the effects of local (dome and telescope) seeing and telescope drive errors or wind shake from those of the external atmosphere. Although the measured wavefront tilt variances included contributions from these various sources, instead of the free atmosphere alone, in practice these are what currently limit the imaging performance of most astronomical telescopes at visible and infrared wavelengths.

The AURA Gemini Project Office proposed a mild steel closed cell structure to support the Primary mirror for the 8 meter Optical telescope. This paper deals the fabrication requirements, the method of construction, and the assembly of such a structure. The final assembly of all the actuators, other equipment including the testing of the complete mirror cell using a special concrete mirror is considered.

We describe a new telescope encoder that overcomes several traditional problems. The system uses a camera, a ruled scale, and a robust image processing algorithm to determine angular position. The ruled scale can be any of a variety of materials. The system is noncontacting, dirt insensitive, and has typical alignment tolerances of 0.25 mm.

The WIYN 3.5 Meter Telescope enclosure was designed to minimize the effects of dome seeing. A combination of strategies is being used to achieve this goal including a well ventilated telescope chamber, low thermal inertia construction, active ventilation using fans, and utilization of surface coatings to control radiation losses. This paper presents the design approach and preliminary thermal measurements of the facility.

The SUBARU telescope requires high image quality. To achieve this high image quality, the structure of high stiffness has been designed. Main drive system of high smoothness using direct drive motor of very low cogging torque and secondary drive system of high accuracy using unique mechanism configuration have been developed and tested. This paper describes the design of the structure and drive systems and the test results of the drive system.

The Japan National Large Telescope `SUBARU' will be completed on the summit of Mauna Kea by the end of the century. One of the major characteristics new to the SUBARU telescope is the active support system for its large monolithic primary mirror, which has 261 points of computer-controlled actuators to maintain a precise mirror figure. This paper describes the control principle, design concepts, results of engineering experiments and numerical simulations of the active support system to ensure the high imaging performance of this system.

We describe the solutions adopted for the most important mechanical subsystems of the Large Binocular Telescope (LBT, formerly Columbus Project), which is now in the phase of detailed design. We report in particular the design and the results of static and dynamic finite element analysis of the open telescope elevation structure, of the azimuth structure, of the cantilever swing arms supporting the auxiliary optics, and of the primary mirror cells.

The decision to use active optics for the Gemini 8-m Telescopes requires a re- evaluation of the control philosophy typically used in current telescope control systems. Rather than having a unique correspondence between error source, error detection, and error correction the Gemini control system must make choices about which control system to use to correct a given error. The basic problem is that motion in the focal plane of the telescope can result from movement of the optics or the mount. This paper will explain the Gemini control philosophy, present the hardware and software architecture proposed to support this philosophy, and estimate the pointing and tracking performance of the Gemini telescope based on this philosophy and architecture.

The real-time control system for the Gemini 8-m telescopes project will be formed from the integration of diverse work packages, each responsible for operation of separate physical systems. As each packages is created at distinct development sites, it is necessary to define a common interface to the core observatory control system.

The tracking simulation for the control system of the Gemini 8-M Telescopes is a nonlinear time-domain model with six degrees of freedom. Structural elements representing the telescope pier, mount, and tube have been obtained with the NASTRAN finite element analysis package and these results used to create a state-space description. The state-space matrices are used in the control package Matlab 4.0 to model the interaction of the telescope structure with linear and nonlinear elements such as bearing friction, encoder quantization, motor torque cogging and various noise sources. Line of sight image motion equations are used to produce an RMS image centroid error which is the metric by which performance is evaluated. The simulation includes the cassegrain rotator and the associated errors caused by spatial separation of the guide object and science object. A fourth-order tip-tilt secondary is modeled to show the effect of tip-tilt upon the image smear induced by other system components. While the rest of the simulation operates in the continuous domain, the tip-tilt controller is modeled in the discrete domain to include the errors and limitations associated with sampling at 200 Hz.

The European Southern Observatory is planning to construct its Very Large Telescope on Cerro Paranal, a site with superb astronomical observing conditions located in the Atacama Desert of Northern Chile some 100 km south of the town of Antofagasta and 16 km from the Pacific Ocean. This region of Chile is know for its rich mineral resources. One of these, Nitrates, are mined by surface, strip mining causing extensive area air pollution. To estimate the effects of a Nitrate mine which may be started 25 km SSE from Cerro Paranal a model for this pollution, based on estimates of aerosols resulting from similar mines north of Antofagasta, was developed. These estimates are based on measurements of the solar aureole brightness. The expected deteriorating effects on atmospheric extinction, sky thermal emissivity and night-time sky brightness appear acceptable.

Using simulations of time evolving speckle patterns we investigate the performance of three different wavefront sensors--a Shack-Hartmann sensor, a curvature sensor and an intensity moments based sensor. We compare the performance of these systems using detectors with two different levels of read noise--0 electronics read noise, corresponding to a photon counting detector and 5 electrons read noise, corresponding to a CCD. We also look at the effect of different source photon rates. For the UKIRT Upgrades program we will address the question which of the three wavefront sensors is optimal. We will also present a new simulation method for time-evolving speckle patterns using two turbulent layers.

A novel design for a chopping infrared secondary mirror using silicon carbide has been proposed which is compatible with a 0.1 arcsecond image quality, 8 meter class telescope. The image quality response to inputs such as changing gravity orientation, wind loading, thermal loading, active correction tilt and chopping profiles have been predicted using a combination of finite element analysis and optical design codes.

A parametric study was performed to optimize the support systems of a large primary mirror. In order to analyze the support pattern locations and the levels of the support forces, finite element models of the primary mirror were employed. Influence matrices for the axial defining supports were established by combining sets of unit load cases which were applied at each of the selected support points in the mirror models. A least square algorithm was utilized to satisfy the design requirements during the optimization process. In this paper, a description of an optimization scheme to define the axial support locations and the force directions of the lateral supports is presented. The optimization procedures were applied to the GEMINI primary mirrors and the optimized force sets were verified by finite element analysis.

Astronomers have traditionally monitored cloud cover visually. However, this technique is difficult under dark conditions, insensitive to thin cirrus, and impossible for remote telescope use. A sensitive camera operating in the thermal infrared escapes these problems. Unfortunately, commercially available cameras have smaller than desired fields of view, are not intended for continuous operation, and are expensive. Consequently, a single channel all- sky scanner has been constructed to continuously monitor observatory cloud cover and motions. Operating at a wavelength of 10 microns, it produces a 135 degree(s) X 135 degree(s) image with a resolution of 0.9 degrees. These data are suitable for returning to remote observers and for archiving with astronomical data.

Optimizing the performance of a telescope requires the ability to accurately measure and monitor the spatial variation of temperature in critical components. Surfaces near a telescope may warm or chill ambient air and cause image degradation. It is desirable to monitor the temperature of such systems. The design, fabrication and testing of a reliable, low-cost, multiplexed temperature measurement system with a resolution and stability approaching 0.01 degree(s)C is described. This system, with 176 temperature sensors, will be used to monitor the performance of a 3.5 m borosilicate mirror ventilation system. It is applicable to a broad range of telescope and telescope enclosure temperature measurement problems.

In the first part of this paper, we describe our strategy about how to deal with the problem of maintaining the reflective surface and how we use a portable scatterometer/reflectometer to monitor our results. The second part of this paper will describe and present preliminary results from an experiment designed to determine the optimum procedure for cleaning a front-surface metal coated mirror that is exposed at this high-altitude site.

The principle of replication of optical surfaces developed at OCA is given and positive results obtained in the double replication of a 1-m diameter concave master onto a concave substrate by means of an intermediate convex replica is presented. The effect of thermal cycles and humidity is addressed as well. The technology has potential applications which are not limited to serial production of small optical components but may ease the production of large convex mirrors, processing of lightweight substrates and possible exotic materials. Possible use of this technique for the VLT secondaries and/or Coude train will conclude.

A milliarcsecond x-ray telescope can image stars and can observe galaxies with resolution matching radio astronomy--opening the door to fundamental discoveries. The convenient milliarcsecond x-ray optic is a paraboloid mirror with a graze angle close to half degree and with 0.1 arc sec tolerance. For a reasonable plate scale, the cross sectional radius of the paraboloid should be about 5 - 10 meters. The glass substrate, looking near cylindrical, is prepared by cementing together 36 segments of 10 degree each. This is placed in the machine with its axis horizontal. Accurate positioning of the substrate is possible, using positron sensors and positioning pads at 5 locations. The substrate is supported all along its circumference, to compensate for the gravity effects. Along the axis, the focus point of the desired paraboloid is rigidly located for regular use throughout the prolonged period of mirror making. By 4 carefully chosen random motions of the tool and the substrate, it is possible to derive the desired paraboloid surface. The image at the focus is used to test the surface errors. For this, a test-parallel beam of light is generated using the same paraboloid mirror in combination with a flat reflector. At the end, the glass substrate is transformed into a vacuum chamber, and a multi-layer, thin film coating is deposited on the mirror surface.

Large, modern optical telescopes demand high performance pointing and tracking of the mount unless alternative methods of correcting the telescope `beam' are to be completely relied upon. This is rarely the case and `open-loop' specifications are still very demanding. The move from precision-geared to friction driven axes has excluded the use of gear-driven encoders while friction-driven encoders have not proved successful. Fiber and laser gyros are not sufficiently developed for use as a primary encoding system although they have useful inertial properties for inclusion in some systems. Tape encoders, which have been around for a very long time, are the major contender for today's applications. A commercially available inductive tape encoder system has been fitted to the 4.2 m William Herschel Telescope in order to properly evaluate its performance and hence its suitability for use with the 8 m Gemini telescopes. The encoder system and the method adopted for fitting it to an operational telescope is briefly described and the results from performance tests are presented. Subsequent investigations into sources of error and the desirability and methods of correcting them are discussed and future work is considered.

Coming Incorporated was awarded a contract in August 1991 by Mitsubishi Electric Corporation (Melco), Tokyo, Japan to fabricate an 8.31 meter diameter ULETMpnmary mirror blank for the Subaru (Japan National Large Telescope) Project. The project is sponsored by the National Astronomical Observatory of Japan. This paper reviews the progress and status of the project to date. Coming's history in producing mirror blanks for astronomy spans nearly 60 years beginning in 1936 with the manufacture of the 200 inch Hale Telescope blank made of Coming's borosilicate glass. Since that time, among the many mirror blanks made of Coming borosilicate, fused silica, and ULE glasses are such notable mirrors as the 3.6 meter ESO, 4.0 meter Canadian, 2.7 meter McDonald Observatory, 2.4 meter Hubble Space Telescope, and several 4.0 meter thin meniscus blanks. Coming ULE Ultra Low Expansion glass was the preferred material of choice for the Subaru primary mirror because of its highly homogeneous, zero thermal expansion coefficient.

This paper describes work done on the design of the thermal management system for the primary mirrors of the Gemini telescopes. The concept developed has a set of radiating plates behind the mirror, which can be used to heat or cool the mirror. In addition, there is a provision for heating the front surface of the mirror by passing a current through the reflective coating. It is shown that the heating and cooling together can be used to raise or lower the temperature of the surface of the mirror by about 1 degree per hour. Experiments and calculations are reported which show that the system can meet the target temperature range up to 90% of the time. The temperature gradients induced in the mirror have little effect on the optical performance. Experiments have shown that no degradation to the surface is caused by the current passing process. This approach potentially will allow thick mirrors of low thermal expansivity to follow rapid ambient air temperature changes, thereby avoiding mirror seeing.

An astronomical telescope employing a 2.7-meter diameter rotating liquid mercury mirror has recently begun operation at a site near Vancouver. The telescope achieves seeing-limited performance, and can detect galaxies as faint as 21st magnitude. Equipped with 2048 X 2048 pixel low-noise CCD detector, the telescope is now surveying a 20 arcmin wide strip of sky centered at +49 degree(s) declination. The CCD is operated in TDI mode, providing continuous imaging with a resolution of 0.6'/pixel and an integration time of 129 seconds. The primary scientific program of this instrument is to obtain spectral energy distributions of all objects in the survey area, by means of imaging through a series of 40 interference filters spanning the wavelength range 0.4 - 1.0 um. These data will then be used to identify and estimate redshifts for order X10⁵ galaxies and X10³ quasars.

One method of reducing the cost of a large terrestrial telescope is to fix its spherical primary mirror with respect to gravity and utilize a tracking mechanism revolved about the mirror's center of curvature to compensate for an astronomical image's ephemeral position and rate. To maximize throughput, hexagonal segments may be arrayed in a close-packed fashion on the primary sphere, but because regular hexagons do not tile a spherical surface, either the hexagons must be irregular, as in the Keck 10 m telescope, or they must be irregularly spaced. In either case, we can exploit the natural symmetry of the underlying geometry in order to define a truss generation scheme which is generally useful in both design and cost reduction.

The outlooks and abilities of creation of telescopes having passive lightweighted mirrors are analyzed. It is shown that on the base of newest achievements of technology, in particular, of silicon carbide application, the creation of diffraction quality mirrors of diameter not less than 3 meter is possible. The state-of-the-art and trends of works being performed in Vavilov State Optical Institute in the field of lightweighted beryllium, silicon, aluminum alloys, silicon carbide mirrors are printed.

Results from the design of a 28 m optical telescope with a spherical segmented primary are presented. The telescope is a four mirror configuration reimaging M1 on M4. The wish for a small and compact structure resulting in a need for controlling high order aspherical mirror coefficients has initiated development of a design procedure satisfying Fermat principle and Abbes Sine condition. Thus the only remaining point aberration will be astigmatism. For a given shape of M4, the design procedure delivers the Taylor expansion coefficients for the shapes of M3 and M4 to be directly used for optical analysis by software capable of handling the needed number of coefficients.

The actual efficiency of ground-based telescope results from the combination of atmospheric turbulence and transmissivity, diameter of collecting area and signal throughput, the latter being affected by the transmissivity of the telescope and by all phase errors generated by the telescope and its close environment. The optical quality requirement for the VLT unit telescopes is based on a formalism which allows a global assessment of performance, under taking into account all effects previously mentioned. The potential advantages of the method, in terms of the user needs as well as of design and manufacturing constraints will be reviewed. The expected performance will be presented, critical error sources identified and the underlying error budget detailed.

An important contributory factor to the overall quality of the images obtained with the VLT are random tilt errors caused by wind buffeting and the atmosphere. The techniques and strategy adopted to overcome these errors are described. Image position errors caused by wind buffeting are minimized, as far as possible, by optimizing the configuration of the telescope enclosure for the prevailing wind and observing conditions. Residual tilt errors are reduced to acceptable limits by the use of a secondary mirror tilt-tip mechanism in conjunction with the telescope axis control system. In situations where the ratio of D/r₀ is small, atmospheric tilt can contribute the dominant source of tilt error. In these cases the same correction procedure using the secondary mirror can even improve the image quality beyond the long integration atmospheric limit. Possible control schemes and results of simulations are presented to demonstrate the effectiveness of these solutions. The limitations of the techniques are discussed as well as their value compared to a full wavefront correction of the VLT adaptive optics system.

Star trails have been taken with a CCD-camera covering a field of approximately 10'. With the telescope movement stopped the sampling rate was of the order of 40 Hz. The correlations between the centroid variations of different trails, representing the image motion perpendicular to the trails, have been calculated. The power spectra of the centroid motions are used to identify their origin. The method turns out to be feasible for the measurement of the isoplanatic angle of image motion. The results show that the correlations of the image motions can be significantly enhanced, if the time delay due to the movement of the turbulence across the field of the telescope is taken into account.

A long term survey of airborne particles was initiated in 1992 at the VLT Observatory of Cerro Paranal to establish the cleanliness of the telescope area before the start of construction work. The results presented in this paper shows large variations with time of the density of inhalable particles (< 10 micrometers ), and a very low density of the larger aerosols when compared to clean room industrial standards. In parallel with the aerosol survey, an analysis of the damage caused to mirror coatings was conducted by periodically exposing sets of mirrors outdoors at 10 m above ground level for periods of two weeks. A follow-up of the evolution: with time and meteorological conditions of the reflectivity, diffusion and scattering coefficients gives insights on mirror maintenance requirements in modern telescopes using natural air flushing to right local seeing. From the study of damages caused during wind storms, some new constraints on operational limits are discussed, in particular with regards to the protection of an unbaffled secondary mirror.

The main structure is defined as the telescope mechanical structure including the drives, the encoder system, the hydrostatic bearing system and all those subsystems which make the system self standing safe and testable as an electromechanical system. Since the conceptual design started the main goal was to design a telescope structure with high first locked rotor eigenfrequencies and to eliminate all possible causes of non linear effects on the telescope motion (stick-slip effect in ball bearings and dragging of cable wraps). Moreover the previous experience had shown the superiority from the repeatability point of view of the direct coupled encoders. The paper describes the resulting final design performed by the Italian Consortium AES (Ansaldo GIE-Genova, EIE-Venezia and SOIMI-Milano) which could provide all the features necessary to meet the demanding requirements of ESO's technical specifications.

The altitude and azimuth axes of the VLT telescopes will be controlled by special designed servo controllers. These controllers are designed to minimize the effect of disturbances. The properties of all elements of these servo systems: mechanical system, motors, amplifiers, tachometer, encoder and finally the servo controller itself will be presented. Aspects like sensor noise, hysteresis, friction and bandwidth limits are described and the effect of each of them on the servo performance is evaluated. A systematic analysis of the effect of external disturbances to the servo system, mainly the wind, is included. The frequency spectrum of the wind that can be expected in the VLT enclosure is compared with disturbance sensitivity of the servo system. This analysis provides the power spectral density of the axial rotation. The reduction of this rotation with an autoguider and/or field stabilization (a fast servo system, which includes a tip/tilt of the secondary mirror) can then be judged. The data for the analysis has been derived from the present status of the VLT main structure design. The servo elements as described above are now in a well-defined state.

This paper describes the concepts of the VLT User Interface, and in particular the areas where standards in the design of control panels have been foreseen.

The ESO VLT control system is based on a distributed computer architecture, where a number of workstations are interconnected. They provide user interfaces and high level functions, while real-time control is handled by some 120 - 150 microprocessors, embedded in Local Control Units.

The VLT Control system is distributed on a number of different computing units. While the real time aspects is dealt with VME-based Local Control Units (LCU) running VxWorks, the high-level control is performed by several workstations running standard Unix. Each workstation will coordinate a group of LCUs logically related by the functions they control.

In this work, the problem of improving the transient response of a vibrating secondary mirror of IRAT with square input is considered. The secondary mirror held at the focus of the cassegrainian primary mirror of the telescope is vibrated at different frequencies so as to separate the signal from the background noise and other disturbances. However classical transient response problems put a limitation to the frequency with which the secondary mirror can be vibrated. Conventional PID tuning controllers are designed to minimize system transient errors, but it is not implemented due to the space limitation in the vicinity of the secondary mirror. Hence, another scheme, called Posicast control, which involves only modification of the input signal, is designed and implemented. The transient response is found to improve drastically allowing the output to reach steady-state in a much shorter time. The control has been designed and presented in a look-up table for all frequencies of vibration and for all amplitude levels, thereby effective tracking of distant celestial objects is made possible.

This paper presents technical data and potential of a Large Optical Test Facility `VERTICAL' being created in Russia. The facility is designed for investigations and testing large space telescopes under simulated space conditions.

The 10 m Keck project has demonstrated that very large segmented primaries can be actively controlled. But there are important technical and above all cost advantages accruing from a spherical primary. This presents, however, a serious problem for optical design solutions with adequate field performance, reasonable central obstruction and good baffling. Two new compact solutions are presented using 4 mirrors with optical power, the primary (f/1.5) and secondary both being spherical. Only the last mirror is strongly deformed (hyperbolic), the third being a weak hyperbola. Studies for the Gemini project and elsewhere have made it clear that technical solutions for improving the reflectivity of large optics will soon be available, making optical design solutions with 4 or more reflections acceptable. The f/no of the final image is about f/6 to f/7.3. The image is only slightly curved. The geometrical basis of the solutions is the use of a second optical axis at right angles, produced by a small folding flat.

We investigate the problem of defining the quality of a mirror from a global point of view, wherein both the low and high frequency errors are taken into account. We compute their effects on the MTF, EE and the PSF. It is found that there is a critical value of the rms wavefront error due to high frequency errors (about 30 nm), beyond which the EE profiles flatten out considerably. We conclude that the quality of a mirror has to be defined taking account the rms wavefront errors of the high frequency errors as well.

The telescope enclosure for the Sloan Digital Sky Survey 2.5 meter telescope is the first roll-off design used on a telescope of this size. The roll-off design eliminates dome-induced image degradation because the telescope enclosure is rolled downwind of the telescope during observations. Flow visualization techniques were used to study a 1/72 scale model of the enclosure in a water tunnel.

In March 1990 a contract was signed between the `Osservatorio Astronomico di Padova' and CARL ZEISS Oberkochen to fabricate the Galileo optic, consisting of a 3.6 m Primary F12.2, the corresponding secondary and the flat folding tertiary. All three components are made from low expansion glass ceramic zerodur. The intrinsic quality of the complete optical train is specified with respect to the encircled energy: E 80% within 0.15 arcsec (phi) . The paper presents the results of the final interferometric tests.

A continuously anamorph-changeable collimated anamorphotic prism system in spectrograph is presented here. It is the unique one which can change its own anamorph-ratio continuously and conveniently among all the non-collimated and collimated anamorphotic optical systems. An important advantage of this system is that we can manufacture in optics more easily, cheaply, precisely and fastly. It is more simple to get different anamorph-ratio than other systems needed to interchange different sets or optical elements. That is another important advantage.

The Space Infrared Telescope Facility (SIRTF), a cryogenic infrared observatory planned for launch around the turn of the century, will span the 3 - 180 micrometers spectral wavelength during its operation in an earth-trailing heliocentric orbit. SIRTF's environmental conditions and mass constraints present challenging requirements to the optical engineer. The SIRTF telescope will operate at near liquid helium temperatures after encountering a launch environment. The Optical Telescope Assembly, which incorporates the primary and secondary mirrors, baffles and support structure, must retain alignment after launch vibration, gravity release and cooldown. Additionally, telescope mass requirements indicate the use of lightweight primary mirror technologies for SIRTF. Candidate primary mirror substrates include fused silica, beryllium, and silicon carbide. Each material has its set of trade-offs. This paper presents these telescope engineering issues and how the SIRTF telescope technology plan addresses them in order to facilitate the final design.

REOSC has been awarded by the European Southern Observatory (E.S.O.), in July 1989, a contract to transport, grind, polish, figure and test the four 8.2m primary mirrors of the European Very Large Telescope (VLT). The following tasks are included in this contract: - the design and manufacture ofthe mirror handling tool and shipping container, the mirror blank transportation from SCHOTT plant in Mainz to REOSC optical shop, - the design, manufacture and bonding onto the mirror of the axial and lateral interfaces between the mirror and the actuators of the mirror cell lateral and axial supports, - the grinding, polishing, figuring and testing of the four mirrors, -the packing ofthe achieved mirror in the container. Furthermore, in the framework of this contract, a new optical shop specially adapted to mill, grind, polish and test 8m class mirrors has been designed and built. The first 8.2m mirror blank has been successfiuly transported last July from Mainz to REOSC's optical shop and the subject of this paper is to give a short description of this new plant and its equipment and the first results obtained by spherornetry after the first rough mirror grinding.

A technically simple modification of centrifugal moulding method for the solidifying and liquid very large telescope mirrors is described. The principal idea of this technique is the use of rotating containers floating freely in the intermedium dumping liquids (IDL). The results of testing experiments are discussed. The new type of telescope mount which is based on the IDL technique is presented.

The Anglo-Australian Telescope's horseshoe equatorial mount is driven from its northern end by a 3.6-meter diameter straight-spur gearwheel. The telescope's tracking performance depends critically on the accuracy of this gearwheel and on the gearbox which couples it to the hour-angle encoder system. Early tests on the telescope showed that the gear systems were very accurate, though some small errors were detected. Analysis of autoguider records obtained during the period 1990-93 have recently been used to calibrate these errors, and software to apply appropriate corrections have been added to the AAT control system. In tests of the new software, a 15-minute unguided CCD exposure showed circular 0.9 arcsec FWHM star images; a similar exposure with the corrections disabled had trailed images where the east-west FWHM had worsened to 1.5 arcsec.

This paper presents a solution to correct wind induced deformation on a 8.0 m thin meniscus mirror supported by a system of astatic active supports. The correction scheme is based on an active correction using the force actuators which support the mirror, and a passive rate dependent coupling of the mirror to the cell. The paper identifies the fundamental design parameters of the passive correction system and the active controller, and shows its wind attenuation capabilities. A 3D simulation verifies the good performance of the system for wind velocities of about 45 km/h. Furthermore, the influence of cell deflection on the mirror due to the passive coupling system is shown.

This paper presents the IAC (Instituto de Astrofisica de Canaries, Spain) proposal of a distributed control system intended for the active support of a 8 m mirror. The system incorporates a large number of compact `smart' force actuators, six force definers, and a mirror support computer (MSC) for interfacing with the telescope control system and for general housekeeping. We propose the use of a network for the interconnection of the actuators, definers and the MSC, which will minimize the physical complexity of the interface between the mirror support system and the MSC. The force actuator control electronics are described in detail, as is the system software architecture of the actuator and the MSC. As the network is a key point for the system, we also detail the evaluation of three candidates, before electing the CAN bus.

The cleaning of mirrors in large, remote telescopes is essential if these telescopes are to meet their performance goals, including low (2%) thermal emissivity in the infrared. Aluminum- coated mirror samples which were naturally or artificially contaminated with materials representative of those at observatories, were cleaned with a UV laser beam or CO₂ snow. Cleaning effectiveness was determined from studies of residual particle densities and size distributions (measured from low-magnification optical imaging). For exposures under two weeks both laser and CO₂ cleaning yield comparable results; however, for longer exposures (up to three months) UV laser cleaning is about twice as effective in our tests. At the laser energy densities required for effective cleaning, no surface changes or damage was observed even after cleaning the same spot 200 times. For 8-m class telescopes, the annual consumption cost of sufficiently pure CO₂ is comparable to the capital cost of a UV laser. Both methods clean such surfaces in less than one day. Two attractive features of laser cleaning are that the method can be fully automated and run frequently without significant cost of manpower or expendables, and that by focusing the laser beam tighter it can be used to strip old surface coatings prior to recoating.

Wave-front reconstruction from defocused stellar images has now been widely applied to the testing of ground-based optical telescopes. We describe here the latest improvements to the technique and discuss how to reach a maximum accuracy. Statistics are given on the aberrations observed over 10 different telescopes.

The telescope enclosure for the Sloan Digital Sky Survey 2.5 meter telescope includes a number of design features that reduce cost and enhance performance. These include: (1) A minimum volume enclosure that rolls away to expose the telescope. The building drive and door systems were chosen from commercially available components for low cost and high reliability. (2) The low profile reduces the visual impact on the environment. (3) The building projects horizontally into the prevailing wind from the top of the mountain. Access to the telescope and instruments is excellent. (4) A closely fitting telescope wind baffle doubles as a component of the telescope light baffling system and has excellent air flushing properties. (5) Attention to temperature control of floor and wind baffle surfaces reduces the production of chilled air near the telescope. (6) Wakes from the telescope and enclosure are small thereby reducing the impact on other telescopes sharing the same site.

A 26-cm diameter aspheric convex secondary mirror was successfully measured using a holographic test plate. This measurement demonstrates the viability of the holographic test plate method for measuring convex aspheres. An opticai writer was built and used to demonstrate the ability to write precise holograms on large, curved substrates. The hologram written for this test was fabricated using a thermochemical method that does not require the use of photoresist. The accuracy of the holographic test is demonstrated with a comparison with data from an independent Hindle test.

Short commnication is presented concerning the beginning of design of a Russian Large astronomical telescope. The principles of concept of telescope construction and its mounting are given.