The mechanical division of EADS-Astrium GmbH, Friedrichshafen is currently engaged with the development, manufacturing and testing of the advanced dimensionally stable composite reflectors for the ESA satellite borne telescope Planck. The objective of the ESA mission Planck is to analyse the first light that filled the universe, the cosmic microwave background radiation. Under contract of the Danish Space Research Institute and ESA EADS-Astrium GmbH is developing the all CFRP primary and secondary reflectors for the 1.5-metre telescope which is the main instrument of the Planck satellite. The operational frequency ranges from to 25 GHz to 1000 GHz. The demanding high contour accuracy and surface roughness requirements are met. The design provides the extreme dimensional stability required by the cryogenic operational environment at around 40 K. The elliptical off-axis reflectors display a classical lightweight sandwich design with CFRP core and facesheets. Isostatic mounts provide the interfaces to the telescope structure. Protected VDA provides the reflecting surface. The manufacturing is performed at the Friedrichshafen premises of EADS-Space Transportation GmbH, the former Dornier composite workshops. Advanced manufacturing technologies like true angle lay-up by CNC fibre placement and filament winding are utilized. The protected coating is applied at the CAHA facilities at the Calar Alto Observatory, Spain. The exhaustive environmental testing is performed at the facilities of IABG, Munich (mechanical testing) and for the cryo-optical tests at CSL Liege. The project is in advanced state with both Qualification Models being under environmental testing. The flight models will be delivered in 2004. The paper gives an overview over the requirements and the main structural features how these requirements are met. Special production aspects and available test results are reported.

The GAIA satellite, scheduled for launch in 2010, will make a highly accurate map of our Galaxy. It will measure the position of stars with an accuracy of 50 prad using two telescopes, which are positioned under a 'basic' angle between the the lines-of-sight of the telescopes of 106°. With a Basic Angle Monitoring system, variations of this angle will be measured with 5 prad accuracy, to correct for these variations on the measured position of stars. A conceptual design of the Basic Angle Monitoring system is presented. Two pairs of parallel laser bundles are sent to the telescopes, which create two interference patterns. If the basic angle varies, the interference patterns will shift. The optical design is such that the rotation of one pair of beams with respect to the other pair, does not affect the measured basic angle. The position stability requirement of the mirrors is a maximum shift of 1 pm in 6 hours. For material stability and good thermal and mechanical properties, Silicon Carbide has been chosen. The structural design is such that the design is as much monolithic as possible. The alignment is performed along the horizontal plane with external and removable alignment mechanisms. The components are locked by adhesives.

The design of the Primary Structure of the Mid Infra-Red Instrument (MIRI) onboard the NASA/ESA James Webb Space Telescope will be presented. The main design driver is the energy flow from the 35 K "hot" satellite interface to the 7 K "cold" MIRI interface. Carbon fibre reinforced plastic (CFRP) was chosen for this application due to the low thermal conductivity at cryogenic temperatures, high strength and low density. Details of the qualification program will be given.

MIRI is one of three focal plane instruments for the JWST covering the wavelengths region 5...28 μm. It is jointly developed by US and European institutes with the latter ones being responsible for the complete optical bench assembly, cryomechanisms, calibration source and the related electronics. MIRI is the combination of an imager with coronographic and low-resolution spectroscopic capabilities and a high-resolution integral-field spectrometer. These diverse options require several mechanisms to select a specific observing mode: (1) a filter wheel with bandpass filters, coronographic masks and a prism, (2) two grating/dichroic wheels with dispersing and order-sorting elements and (3) a flip mirror to direct the beam of an internal black body source into the spectrometer section. All mechanisms are required to operate under laboratory conditions (warm launch) as well as in the cryovacuum in space. The heat dissipation has to be small and the reliability and precision very high. Our low risk approach is the application of successfully qualified and flown components of the ISOPHOT (ISO) and PACS (HERSCHEL) instruments. We will report on the concept developed in phase B.

In 2011 NASA and ESA plan to launch the James Webb Space Telescope (JWST) as dignified successor of the Hubble Space Telescope. Three scientific instruments will cover the wavelength regions in the near-infrared (0.6-5μm, NIRCam and NIRSpec) and in the mid-infrared (5-28μm, MIRI), respectively. The ESA-led multi-object spectrograph NIRSpec as major European contribution is presently entering the detailed design phase in a collaboration between European space industries, scientific institutes, ESA and NASA. To allow for various operational modes in the instrument’s optical train several cryo-mechanisms are required, i.e. wheels for exchanging optical elements like filters and gratings as well as linear actuators on refocusing mirrors. We will give an overview on the detailed design, the prototyping and the testing of those mechanisms comprising highest reliability in the cryo-vacuum (~ 35K) combined with minimal power dissipation (~ 5mW on average), ultimate position accuracy (~ 0.5 - 1arcsec) combined with high launch vibration capability (ARIANE 5, ~ 60g) and a very long lifetime (~ 15 years) for ground tests and space operation under various environmental conditions. To reach this goal in a low cost and risk approach we rely on the heritage from ESA's earlier infrared missions, i.e. ISO and HERSCHEL.

Alt-azimuth mounted radio telescopes use since their beginning -- more than 50 years ago -- the wheel-on-track principle for the realization of the azimuth axis. For the very huge telescopes (as the Lovell telescope in Jodrell Bank, UK, 1956; the Effelsberg Telescope, Germany, 1969; and the Green Bank Telescope, USA 1996), the wheel-on-track system was and is always one of the most sensitive and maintenance consuming subsystems. On the other hand, the huge optical telescopes use since Mount Palomar (and earlier) hydrostatic axes mechanisms, which need also some maintenance efforts, but are very robust and reliable. The paper gives an update of the design approaches for the new built radio telescopes LMT Mexico, and SRT Sardinia, and compares them with hydrostatic solutions for optical telescopes.

Many antennas use wheel-on-track systems in which track segments consist of wear plates mounted on base plates. The hardened wear plates are typically connected to the base plates using bolts, and the base plates are supported on grout and anchored to the underlying concrete foundation. For some antennas, slip has been observed between the wear plate and base plate, and between the base plate and the grout, with migration in the wheel rolling direction. In addition, there has been wear at the wear plate/base plate interface. This paper describes the use of finite element models (FEM's) of the wheel, track, and foundation to understand the behavior of the wheel-on-track system, and to evaluate possible retrofit concepts. The FEM’s are capable of representing friction and slip, and the opening and closing of gaps at the interfaces between the wheel, wear plate, base plate, and grout. The FEM’s can capture the behavior of the components as the wheel rolls forward. The paper also describes a method to estimate the amount of wear at the wear plate/base plate interface based on the relative slip and contact pressure between the wear plate and base plate.

In a major effort to understand limitations on the performance of ground-based optical telescopes and gimbals, the Air Force Research Laboratory (AFRL) and a contractor support team measured the vibration and jitter of several large telescopes. These measurements were conducted over a ten-year period with high bandwidth accelerometers, angular position sensors, and seismometers attached to various locations on the telescopes, mounts, and bases. Measurements were taken over a 0.1 to 1000-Hz bandwidth and at angular tracking velocities between 0.001 and 18.5 degrees per second, single-axis. Jitter power spectral densities (PSD) and root-mean-square (rms) values and ranges were determined for both dynamic tracking and quiescent condition. Many telescopes exhibited near-noise-floor level jitter (i.e., about 10 nanoradians rms) under some conditions in certain bandwidth ranges. Under very high-speed single-axis tracking conditions (i.e., 10.5 - 18.5 degrees per second), jitter often rose to several micro-radians rms. This paper presents and discusses the overall mechanical jitter measurement results obtained for five telescopes with emphasis on two 3.5-meter aperture-class high-tracking rate instruments.

The paper presents the analysis results (in terms of settling time, bandwidth, and servo error in wind disturbances) of four control systems designed for the Large Millimeter Telescope (LMT). The first system, called PP, consists of the proportional and integral (PI) controllers in the rate and position loops, and is widely used in the antenna and radiotelesope industry. The analysis shows that the PP control system performance is remarkably good when compared to similar control systems applied to typical antennas. This performance is achieved because the LMT structure is exceptionally rigid, however, it does not meet the stringent LMT pointing requirements. The second system, called PL, consists of the PI controller in the rate loop, and the Linear-Quadratic-Gaussian (LQG) controller in the position loop. This type of controller is implemented in the NASA Deep Space Network antennas, where pointing accuracy is twice that of PP control system. The third system, called LP, consists of the LQG controller in the rate loop, and the proportional-integral-derivative (PID) controller in the position loop. This type of loop has not been yet implemented at known antennas or radiotelescopes, but the analysis shows that its pointing accuracy is the ten times better than PP control system. The fourth system, called LL, consists of the LQG controller in both the rate loop, and the position loop. It is the best of the four, with accuracy 250 better than the PP system, thus is worth further investigations, to identify implementation challenges for the telescopes of high pointing requirements.

In this paper we'll describe the active surface system that will be provided on the new Italian radiotelescope being in the phase of erection in the Sardinia Island. SRT (Sardinia Radiotelescope) will be a 64m shaped dish working up to 100GHz by exploiting the active surface facility designed by the authors. This facility will overcome the effects of gravity deformations on the antenna gain and will also be used to re-shape in a parabolic form the primary mirror, in order to avoid large phase error contribution on the antenna gain for the highest frequencies placed on the primary focus. Together with the description of the SRT system, a wide overview will be given regarding our previous installation of an active surface system, that can be seen like a prototype for SRT, mounted on the 32m dish of the Noto antenna.

In the fabrication of high-performance, low-cost secondary reflectors for radio telescopes, it is a significant challenge to avoid introduction of low-order surface errors such as astigmatism or coma. This arises primarily because low-order surface errors are easily induced by support structure placement or simple thermal variations in the manufacturing process. It is, of course, possible to bring these errors to within the required tolerance, but if an active primary reflector is present, it may be possible to relax the requirements on the secondary and perhaps lower its cost. In this paper, we take the Large Millimeter-wave Telescope (LMT/GTM) as an example system. We model the effects of correcting a deformed sub-reflector by using the existing segmented active primary. The sub-reflector deformation patterns employed are low-order (e.g., astigmatism or coma), but are allowed significant excursions from the nominal surface figure. For each case, we demonstrate the best theoretical performance, using the active primary to correct for the errors. Additionally, to determine whether such an approach would be practical, we also demonstrate the likely performance improvement that could be achieved using brief measurements on an astronomical source. In this approach, we introduce varying amounts of known low-order deformation patterns into the active primary and seek the combination that results in the maximum signal. Finally, we compare this result to the theoretical maximum and make recommendations on the practical utility of the approach.

Haystack, MIT's 37-m radio telescope, was built in the early 1960s. At the time considered to be a high-performance antenna, Haystack produced a number of outstanding scientific results. The antenna, originally designed to operate at 8-10 GHz, was upgraded at various times, notably in 1993 with the addition of a deformable subreflector to allow operations at 115 GHz. Planning is now underway for a major upgrade with the replacement of the entire elevation structure that is supported on the existing yoke and tower. The new antenna should be capable of operating at up to 325 GHz. In this paper, we will describe the limitations of the original design, the solutions used in the previous upgrades, and how the lessons learned led to the approach used in the planned upgrade. The major issues limiting the further upgrade of the existing telescope were in the elevation structure; these included fabrication tolerances and gravity sag of the reflector panels, thermal lag of a ring plate supporting the reflector panels, non-repeatable behavior of the sliding joint at the elevation bearing and shear pins, and the interaction of the steel yoke and the aluminum backstructure.

Optical telescope engineers use since more than 150 years iso-static supporting systems for their large primary mirrors. Radio telescope engineers use since about 50 years homologue principles for their large reflector backup structures. There is obviously some methodological affinity between both principles. Looking into the papers of the inventors and their disciples shows, that none of them did recognize the work of the neighborly discipline. The paper compares both principles by comparing the related structural analysis algorithms and gives hints for creative use of these insights for the design of new optical mirror or radio reflector supports.

AMiBA consists of a 90 GHz interferometric array telescope with dishes ranging in size from 0.3 to 2.4 meter in diameter, mounted on a 6-meter fully steerable platform. The dishes are attached to the receivers, which are mounted on a platform controlled by a six degree of freedom hexapod mount. The hexapod mount is a parallel connection manipulator also called Stewart Platform. The basic reference for this mechanism is a paper by Stewart. The Stewart Platform is a unique kinematically constrained work platform. It can be manipulated through the six degrees of freedom. The hexapod also provides better accuracy, rigidity, load to weight ratio and load distribution than a serial manipulator or traditional manipulator. The advantages of the hexapod shows that it is a great choice for the AMiBA project. Vertex Antennentechnik GmbH fabricates the hexapod. Testing has started in Germany. The telescope will be delivered in the summer of 2004. The 6m in diameter hexagonal platform is made of carbon fiber reinforced plastics (CFRP) and consists of seven pieces of three different unique types. The platform can be disassembled and fits in a container for transportation. The mounting plane flatness is an important issue for the platform assembly. The deflection angle of the mounting plane relative to any other mounting position must be less than 20 arcsec. Meanwhile, the platform must endure a loading of 3 tons. The platform has been built by Composite Mirror Applications, Inc. (CMA) in Tucson, and mounted on the Hexapod in Germany. This report describes the design and testing of platform and mount for the AMiBA telescope.

A concept design has been developed for the Giant Magellan Telescope (GMT). The project is a collaboration by a group of U.S. universities and research institutions to build a 21.5-meter equivalent aperture optical-infrared telescope in Chile. The segmented primary mirror consists of seven 8.4-meter diameter borosilicate honeycomb mirrors that will be cast by the Steward Observatory Mirror Laboratory. The fast primary optics allow the use of unusually compact telescope and enclosure structures. A wide range of secondary trusses has been considered for the alt-az mount. The chosen truss employs carbon fiber and steel and, due to its unique geometry, achieves high stiffness with minimal wind area and primary obscuration. The mount incorporates hydrostatic supports and a C-ring elevation structure similar in concept to those implemented on the Magellan 6.5-m and LBT dual 8.4-m telescopes. Extensive finite element analysis has been used to optimize the telescope structure, achieving a lowest telescope resonant frequency of ~5 Hz. The design allows for removal and replacement of any of the 7 subcells for off-telescope mirror coating with no risk to the other mirrors. A wide range of instruments can be used which mount to the top or underside of a large instrument platform below the primary mirror cells. Large instruments are interchanged during the day while small and medium-sized instruments can be enabled quickly during the night. The large Gregorian instruments will incorporate astatic supports to minimize flexure and hysteresis.

The Large Synoptic Survey Telescope (LSST) is an 8-meter class telescope with a proposed field of view between 3.0 and 3.5 degrees. The scientific goals of the survey establish a cadence that sets the telescope performance. The proposed cadence of the LSST telescope will typically require movements and settling of the telescope of approximately 3 degrees in 5 seconds. This dictates a high bandwidth to the telescope servo and thus a high locked rotor resonant frequency. In this study, the structure must accommodate three optical surfaces, the 8.4-meter primary, the 3-meter class secondary, and a 5-meter class tertiary in a long-tube configuration. The instrument must be accommodated in a "Trapped Focus" in the middle of the telescope. This imposes very stringent requirements on the structure and drives. This structure will require performance beyond the existing class of 8-meter telescopes. This can be achieved with the C-ring and azimuth platform concept demonstrated with the Large Binocular Telescope. The structure requires a low rotational inertia and a very high locked rotor resonant frequency at all angles of the sky. This is a challenging problem that can be overcome with this innovative solution.

The proposed science missions of the LSST require a telescope with an optical etendue of greater than 250 meters square degrees square. The current LSST Baseline Configuration has a field of view of 3.5 degrees and an optical etendue of 302 m²d². The etendue calculation includes the effect of gradual vignetting by the camera as the field angle increases. A current optical point design includes spun cast light-weighted borosilicate mirrors (primary and tertiary) of 8.4 and 5 m diameter respectively. Thermal control systems are needed to optimize telescope seeing and to minimize the thermal distortion of the mirrors. The goals of this study are to determine the airflow requirements for the specified ambient temperature rate of change, to identify thermal time constants and to predict the magnitude and form of thermal distortions that can be developed by environmental conditions. Operational data taken at the 6.5 m MMT (Multi-Mirror Telescope Observatory) and at the Magellan Observatory are presented for comparison with this study. Finally, the results from the thermal analysis were used to simulate the LSST focus control over one night of observation and to estimate the effect on the image quality for different correction frequencies.

The Chinese Future Giant Telescope (CFGT) has an aperture of 30 m with primary f-ratio of 1.2. Its primary mirror is segmented with 1020 annular submirrors arranged in 17 concentric annuluses. Each segment has average length and width both of 0.8 m. Apart from a conventional Cassegrain focus, four Nasmyth foci on two double-deck platforms on both sides are fundamental in CFGT. Still, there is a Coude focus provided for joining interferometric array. The comparatively small secondary mirror is 2.74 m in diameter, which will serve active corrections. Following optical system requirement, the preliminary structural configuration of CFGT is considered and presented in this paper, including related design schemes of driving systems, bearing systems and mirror support systems. Special consideration on overcoming dead gravitational and random thermal distortion has been taken into the support structure design of both primary and secondary mirrors. In order to evaluate the structural performance of the design, finite element analyses are performed for assemblies and full model of the CFGT.

The Discovery Channel Telescope (DCT) is a 4.2m aperture telescope with a unique front end pod assembly that incorporates a secondary mirror assembly on one end, and a prime focus corrector group with focal plane instrument on the other. By flipping the pod end-for-end, the DCT is quickly converted from an f/6.13 Ritchey-Chretien telescope with a 21 arcminute field of view, to an f/1.9 two degree wide field prime focus camera. This paper describes the conceptual opto-mechanical design and performance assessment of the Prime Focus Assembly (PFA), including the pod interfaces, structures, optic mounts and the functions and configurations of the various mechanisms within the pod.

The 6.5m Multiple Mirror Telescope Observatory (MMTO) installed a new f/5 secondary system in April 2003. We describe the design and performance of the mirror cell and supports for the 1.7 m diameter Zerodur mirror. Pneumatic actuators divided into one lateral and three axial zones support this 318 kg mirror. The control feedback for the high bandwidth pressure transducers for these four zones is obtained from six load cells attached to rigid positioning rods. The mirror cell includes thermal control, force limiters, passive supports, installation and handling, and alignment metrology. Optical test results are described and compared to the original design specifications.

The optical performance and pointing accuracy of the 1.5 m solar telescope GREGOR depend on the passive and active temperature control design features. Stringent thermal requirements are given in the technical specification for the passive thermal control of telescope structure and the main mirror air cooling system under operational conditions. The telescope structure has to be kept within a range of -0.5K through +0.2K against the ambient temperature. The main mirror temperature has to be kept up to a temperature difference of less than 2K under the ambient temperature with an accuracy of ±0.1K. The temperature difference across the main mirror surface has to be smaller than ±0.1K. Another thermal requirement asks for a main mirror safety cover to prevent the wandering of focused sunlight over the telescope structure in case of control system malfunction or power loss. Features of the chosen passive thermal concept for the telescope structure and the active main mirror air cooling system as well as the primary mirror safety cover system are presented together with the finite element analyses results and tests performed in order to find and verify the chosen design.

The commissioning experience of the facility for SALT is compared to the results of the analysis done during the design. A false steel floor incorporating forced ventilation that extends around the telescope azimuth pier is installed to prevent heat radiating from the concrete surfaces on nights when the ambient temperature drops to below room temperature. An infrared scan was done on this floor to verify that no heat is radiated into the telescope chamber from either the concrete or the warmer rooms underneath. The SALT site is windy all year round, and in order to utilize this natural resource and get better ventilation, adjustable louvers are used for natural ventilation. The control system automatically adjusts the louver openings depending on the wind speed and relative direction of the dome opening to achieve enough air changes during wind still nights. The louvers are throttled to limit windshake on the structure on windy nights. Results of the computational fluid dynamic analysis (CFD) and actual measurements are presented showing adequate temperature distribution at low wind speeds. The correlation between the CFD and actual measurements are discussed, with reference to the surface and air temperatures in the telescope chamber under different ambient conditions. The telescope chamber and dome are built out of insulation panels to limit energy losses during the day when the chamber is air conditioned. It also ensures thermal inertia of the building is low and consequently allows its temperature to react quickly to changes in external ambient temperature. The correlation between the expected and actual capacity of the air conditioners are also discussed.

In May of 2000, the construction progress of the Gemini South 8m telescope at Cerro Pachon in Chile was such that the telescope and dome were installed and able to move, but the primary mirror had not been installed. This provided a unique opportunity to make extensive tests of the structure in its nearly-completed state, including a modal impact test and simultaneous measurements of wind pressure and structural response. The testing was even more comprehensive because the Gemini dome design allows for a wide range of wind flow configurations, from nearly enclosed to almost fully exposed. In these tests, the operating response of 24 surface pressures on the primary mirror cell, 5 wind velocity channels (each with direction vector information), and more than 70 channels of accelerometers on the telescope structure were measured. The data were taken in a variety of wind loading configurations. While previous analysis efforts have focused on the wind velocity and pressure measurement, this paper investigates the dynamic behavior of the telescope structure itself. Specifically, the discussion includes the participation of the modes measured in the modal impact test as a function of wind loading configuration. Data that indicate the most important frequency ranges in the operating response of the telescope are also presented. Finally, the importance of the response of the enclosure on the structural vibration of the telescope structure is discussed.

The design of future large optical telescopes must take into account the wind-induced buffeting of the telescope structure caused by large-scale flow structures and turbulence inside the dome. However, estimating the resulting degradation in image quality is difficult due to our relatively poor understanding of the flow inside the dome. Data has been collected in a scaled wind-tunnel test of a telescope enclosure to understand the flow-field around the region near the dome opening where the secondary mirror and supporting structure would be subjected to wind loads. Digital particle image velocimetry (DPIV) data was collected in a vertical plane near the dome opening to obtain mean velocity and fluctuation kinetic energy. In addition, hotwire data was collected along the telescope axis to obtain temporal spectra of the velocity, and flow visualization was used to determine the general flow patterns. In addition to its direct use in telescope modeling and design, this data is of particular value in validation of computational fluid dynamic (CFD) analyses, so that CFD can be used with confidence in future design work.

While many telescopes employ some form of thermal monitoring and control to reach their surface accuracy and pointing specifications, such thermal systems are generally not available during field erection of the telescope structure. This presents a problem for large structures, because their size results in structural members with long time constants and substantial total deformations. These characteristics can potentially result in the development of significant thermal gradients across the structure or even across the width of a member. The resulting temperature variations complicate the alignment of critical features, including the main axes. As a result, it would be advantageous to monitor the thermal behavior of a large telescope structure during its construction. In the past, such monitoring was made difficult because of the size of the structure, continuing construction work, or rugged field conditions. However, with the advent of affordable field-ready thermal imaging systems, it is now possible to perform such monitoring. In this paper, we present thermal images of the alidade structure of the Large Millimeter-wave Telescope (LMT/GTM) at its site at 4600m on the top of Volcan Sierra Negra in central Mexico. We present images of typical thermal distributions for different times of day, and compare them with basic analytical models. Finally, we use the thermal imaging results to predict the effects of the temperature distribution on the location of the azimuth bogie connections and the elevation axis.

The authors have completed testing of a new class of structural actuators that demonstrate nanometer precision. The actuators can also incorporate a coarse stage for removal of several-millimeter-sized position errors (as in deployment errors of mirror segments in space or assembly errors and gravitational deflections in terrestrial instruments). One of the promises of these actuators has been performance at both room temperature (300°K) and cryogenic temperatures (30° K). The recent tests performed at both temperatures show that the behavior is nearly identical at both extremes. The paper will discuss the test methods and procedures as well as presenting the test results and some of the lessons learned in making nanometer-sized measurements at these temperature extremes.

The Research School of Astronomy and Astrophysics (RSAA) of the Australian National University (ANU) has designed and constructed The Gemini South Adaptive Optics Imager (GSAOI) that will be used with the Multi-Conjugate Adaptive Optics system on the Gemini South telescope in Chile. GSAOI contains three cryogenic mechanisms; two filter wheels and a utility wheel. An approach to the athermalization of cryogenic mechanisms is presented. The 280 mm diameter filter wheels are athermalized using bi-material conical bearing seats where the bearing preload is constant, irrespective of temperature. The lens mounting method is also described. Lenses up to 170 mm in diameter are mounted within precision cells such that radial clearances reduce to zero at operating temperature. The method used to derive the room-temperature lens and cell dimensions is described. Lenses are preloaded axially against conical seats using wave washers. This technique has been used successfully to mount lenses of ~100 mm diameter in the Gemini Near-infrared Integral Field Spectrograph (NIFS), also designed and constructed by RSAA.

Space interferometers consisting of several free flying telescopes, such as the planned Darwin mission, require a complex metrology system to make all the components operate as a single instrument. Our research focuses on one of its sub-systems that measures the absolute distance between two satellites with high accuracy. For Darwin the required accuracy would be in the order of 10 μm over 250 meter. To measure this absolute distance, we are currently exploring the frequency sweeping interferometry technique. Its measurement principle is to first measure a phase in the interferometer, sweep a tunable laser over a known frequency interval and finally measure a second phase. By also counting the number of fringes during the sweep it is possible to determine the absolute path length difference without ambiguities. The wavelength at the endpoints of the sweep is stabilized on a Fabry-Perot cavity. In this way the unknown distance is directly referenced to the length of the Fabry-Perot cavity.

Metrology will be an enabling technology for a new generation of astronomical missions having large and distributed apertures and delivering unprecedented performance. The x-ray interferometer Black Hole Imager, the UV interferometer Stellar Imager, and other missions will require measurements of incremental distance as accurate as 0.1 picometer, and absolute distance capability. Our Tracking Frequency laser distance Gauge (TFG) was developed a decade ago for a NASA-funded study of a spaceborne astronomical interferometer. It has achieved an accuracy of 10 picometer with a 0.1 second averaging time, and 2 picometer in 1 minute. The standard approach, the heterodyne gauge, displays nanometer-scale cyclic bias, whose mitigation has been the subject of much effort. Our approach is free of that bias and can measure absolute distance with little or no additional hardware. It provides the option of operation with a resonant optical cavity, which in many applications would provide increased accuracy of both incremental and absolute distance. We plan to develop a next-generation laser gauge based on our unique and successful architecture. We will improve incremental and absolute distance accuracy, increase sample and readout rates, and establish a capability for measuring long distances. The new TFG will use solid-state lasers and fiber-connected components in place of a HeNe laser and free-space beams. We expect that the new TFG will have low replication cost and be rugged, modular, and easy to set up. It will employ components that are likely to be space qualifiable.

This paper presents a preliminary study dedicated to a spatial three-degree-of-freedom (3-DoF) parallel mechanism intended to be used as the active reflector surface of a large spherical radio telescope. The forward and inverse kinematic problems of the mechanism are addressed, and solved in analytical form. Since the mechanism has only three degrees of freedom, constraint equations describing the inter-relationship between the six Cartesian coordinates are derived. Furthermore, the Jacobian matrix of the mechanism is obtained. Finally, the dexterity of the mechanism and the parasitic motion are discussed. A simulation result is reported.

The SOAR Telescope developed by NOAO and sited on Cerro Pachon, Chile is a 4.1-meter Ritchey-Chretien design incorporating active optics (AO). The AO system is composed of PC-hosted control software, a solid primary mirror supported by 120 electro-mechanical actuators, a lightweighted 600 mm secondary mirror supported by a six degree-of-freedom hexapod mechanism, and a lightweighted 600 mm tertiary mirror controllable over a range of ±100 μrad in two axes with a bandwidth of 50 Hz. The tertiary mirror assembly is in turn mounted on an azimuthal bearing that allows the output bundle of the telescope to be directed to one of five science instruments located at nasymth and bent-cassegrain foci. This paper discusses the active tertiary mirror assembly from the perspective of a control system designer. After a brief overview to establish the tertiary mirror's place in the overall AO system architecture, the paper presents the requirements that drove the design and some of the design’s salient electrical and mechanical features. A model representing electrical and mechanical aspects of the mirror and controller is presented and observed performance metrics such as frequency response, NEA, and measures of servo robustness are compared with values predicted by this model. The paper discusses a number of the design challenges which arose from the requirement to control a massive load with great precision and over a relatively large bandwidth and concludes with the "lessons learned."

A wide range of positioning technologies has been exploited to flexibly configure fiber ends on the focal surfaces of telescopes. The earliest instruments used manual plugging, or glued buttons on the focal plane. Later instruments have used robotic fisherman-round-the-pond probes and articulated armsto position fibres, each probe or arm operated by its own motors, or buttons on fiber ends moved by pick-and-place robotic positioners. A positioner using fiber spines incorporating individual actuators operating over limited patrol areas is currently being manufactured and a derivative proposed for future large telescopes. Other techniques, using independent agents carrying the fiber ends about the focal plane have been prototyped. We describe these various fiber positioning techniques and compare them, listing the issues associated with their implementation, and consider the factors which make each of them suitable for a given situation. Factors considered include: robot geometries; costs; inherent limits to the number of fibers; clustering of targets; serial and parallel positioning and reconfiguration times; adaptability to curved focal surfaces; the virtues of on-telescope versus off-telescope configuration of the field, and suitability for the various telescope foci. The design issues include selection of actuators and encoding systems, counterbalancing, configuration of fiber buttons and their associated grippers, interchanging field plates, and the need for fiber retractors. Finally we consider the competing technologies: fiber and reflective image slicer IFUs, multislit masks and reconfigurable slits.

TNO TPD, in cooperation with Micromega-Dynamics, SRON, Dutch Space and CSL, has designed a compact breadboard cryogenic delay line for use in future space interferometry missions. The work is performed under ESA contract in preparation for the DARWIN mission. The breadboard (BB) delay line is representative of a future flight mechanism, with all materials and processes used being flight representative. The delay line has a single stage voice coil actuator for Optical Path Difference (OPD) control, driving a two-mirror cat's eye. Magnetic bearings provide frictionless and wear free operation with zero-hysteresis. Overall power consumption is below the ESA specification of 2.5 W. The power dissipated on the optical bench at 40 K is considerably less than the maximum allowable 25 mW. The BB delay line will be built in the second half of 2004. The manufacturing and assembly phase is followed by a comprehensive test program, including functional testing at 40 K in 2005. The tests will be carried out by Alcatel Space and SAGEIS-CSO.

TNO TPD, in cooperation with Micromega-Dynamics and Dutch Space, has designed an advanced Optical Delay Line (ODL) for use in future ground based and space interferometry missions. The work is performed under NIVR contract in preparation for GENIE and DARWIN. Using the ESO PRIMA DDL requirements as a baseline, the delay line can be used for PRIMA and GENIE without any modifications. The delay line design is modular and flexible, which makes scaling for other applications a relatively easy task. The ODL has a single linear motor actuator for Optical Path Difference (OPD) control, driving a two-mirror cat’s eye with SiC mirrors and CFRP structure. Magnetic bearings provide frictionless and wear free operation with zerohysteresis. The delay line is currently being assembled and will be subjected to a comprehensive test program in the second half of 2004.

Producing extreme light weighted structures by combining a new design concept with the most recent production machines and production software tools. Weight reductions of up to 50% compared to the traditional techniques are feasible with the same stiffness performance. Suitable for standard materials like aluminium and steel, for single construction parts out of mono material and with a single production process. Astronomical instruments for space applications and ground-based applications require more and more extreme light and extreme stiff structures. The traditional technique like 3-axis or multisided machining of metal parts seems limited and not suitable for the next generation instruments. New materials with new production technologies are used more and more with all their specialties and restrictions. ASTRON developed a new structural design of traditional materials with heritage optimized for production with the most recent milling machines. The structural shapes are closely linked to the extremes of 5-axis simultaneous milling. The design and production process is patented and now free for publication.

This paper describes the basic structural design of the Terrestrial Planet Finder (TPF) Structurally Connected Interferometer concept developed within the Jet Propulsion Laboratory design team. Descriptions of the key structural components, optical elements, and basic load paths are included. Key structural requirements related to launch loads and on-orbit stability and alignment are identified. The analysis results for the baseline design are shown for both launch configuration and the deployed, on-orbit configuration. The finite element models are described with preliminary results shown. Excitation of the structure and the optical train caused by assumed external disturbances are shown for a preliminary analysis. Future work is identified.

Design parameters and considerations for the successful construction of ground base telescope axes drives will be examined. A brief evaluation of different types of drive systems will be offered. Tolerances and heat-treating issues will be described. Article will close with a review of the effects of insect infestations on the Sloan Digital Sky Survey telescope drives.

Developments in computer hardware and software have made analysis techniques that were formerly too expensive within the reach of most project budgets. Foremost among these has been seismic response spectrum analysis. This method yields much more accurate results than the equivalent static approach. The problem with using response spectrum analysis exclusively in the design of deflection controlled structures, such as astronomical telescopes, is that the nature of the structure minimizes the benefits of the approach. A typical response spectrum from Eurocode 8 deals with a range of natural periods between 0.1 and 5 seconds. These correspond to a frequency range of 0.2 to 10 Hz. The typical telescope structure has a minimum frequency of around 10 Hz. or greater. The result is that the response spectrum analysis involves only a narrow band of frequencies and accelerations. This result could be reliably obtained using an equivalent static analysis approach.

The 4.1-meter SOuthern Astrophysical Research (SOAR) Telescope mount and drive systems have been commissioned and are in routine operation. The telescope mount, the structure and its full drive systems, was fully erected and tested at the factory prior to reassembly and commissioning at the observatory. This successful approach enabled complete integration, from a concrete pier to a pointing and tracking telescope, on the mountain, in a rapid 3-month period. The telescope mount with its high instrument payload and demanding efficiency requirements is an important component for the success of the SOAR scientific mission. The SOAR mount utilizes rolling element bearings for both azimuth and elevation support, counter torqued sets of gear motors on azimuth and two frameless torque motors built into the elevation axles. Tracking jitter and its associated spectra, pointing errors and their sources, bearing friction and servo performances are critical criteria for this mount concept and are important factors in achieving the mission. This paper addresses the performance results obtained during the integration, commissioning, and first light periods of the telescope mount system.

The behavior of future LAMOST mount tracking is one of crucial issues for the telescope's overall performance. In order to demonstrate and to sense the real situation to some degree, the LAMOST team has set up a model mount at the camps of Nanjing Institute of Astronomical Optics & Technology. Painstaking effort has been made during the course of the interim outdoor test to improve the accuracy of the model mount tracking. The major test progress, starting from scratch to date, has been recorded in this paper, such as the anti-disturbance measures taken, the cascaded feedback application, the two-motor-differential drive till the overhaul of the model mount in its drive mechanism, etc. The tracking accuracy has been dramatically improved up to 0.35"-0.42" RMS, promising the future LAMOST tracking requirement will be met given more reliable mount and sophisticated control system.

In the past years, friction drive was developed to overcome the inherent deficiencies in both worm drive and gear drive. No periodical error and free of backlash are the main advantages of friction drive. With the trend towards bigger and bigger aperture of the optical telescopes, there are some reports about friction drive employed to drive the telescopes. However friction drive has its own deficiencies, such as slippage and creepage. This report here describes the study on the friction drive finished in an experiment arranged by LAMOST project. It comprises three main parts. First, it introduces the experiment apparatus and proposes a new kind of measurement and adjustment mechanisms. Secondly, the report gives the analysis of friction drive characteristics theoretically, such as slippage, creepage and gives the results of corresponding experiments. The experiment shows that the lowest stable speed reaches 0.05"/s with precision of 0.009"(RMS), the preload has little influence on the drive precision in the case of constant velocity and the variable velocity when the angle acceleration is less than 5"/s² with close loop control and the creepage velocity of this experiment system is 1.47"/s. Lastly, the analysis in the second section lists some measures to improve the precision and stability further. These measures have been actually conducted in the testing system and proved to be reliable.

The increasing demand to improve focusing accuracy and to accommodate higher frequencies in space communications and radio astronomy has created significant challenges for improving the capability of the constituent systems in radio antennas and telescopes. One important system is the radio antenna/telescope backup structure connections. The backup structure is a key element in providing a stable, precise and rigid support for the reflective surface. The ideal connection for these types of structures is rigid and concentric resulting in minimal deformation with stress/strain curves that are linear, repeatable and exhibiting no hysteresis over the entire service load range. Conceivably such a connection could be designed so that the stress/strain curve mimics the stress/strain characteristics of the connecting member in both tension and compression. When this is achieved then such joints can be said to be "invisible" in the global behavior of the backup structure. At that point, overall reflector deflection becomes more linear and highly predictable. In conjunction with this advantage, optimized backup structure geometries, adaptive reflectors and compensating algorithms can best be applied in producing an instrument of unparalleled performance. This paper introduces Co-Axial Joint (CAJ) technology as the practical and economical means to produce an invisible connection.

The holography program to measure and set the surfaces of the antennas of the Submillmeter Array (SMA) has been very successful, with the best antenna meeting the stringent 12 μm rms specification. The surfaces of the 6-meter diameter antennas of the 8 element array have been set to accuracies of 12-25 μm, and are under constant improvement. This allows efficient operation in the 660 GHz band, currently the highest frequency band of observations. The system used to make routine near-field holographic measurements at 232.4 GHz -- the primary method of obtaining surface error maps -- is now fully integrated into the SMA. The measurements are carried out remotely from Cambridge. A sequence of upto 4 rounds of measurements and adjustments is needed to achieve the design specification of 12 μm rms starting typically from 65 μm rms. The last sets of adjustments incorporate corrections for panel flexures, allowed by the 4 points of adjustment for most of the panels, and the high spatial resolution (~ 8 cm) of the surface error maps. Repeat measurements indicate a surface stability time scale of ~ 1 year including antenna transport between stations. Celestial holography to characterize gravitational deformations and careful efficiency measurements to validate the holographic measurements are in progress.

This paper will examine the advantages of using aluminum castings for large astronomical instruments for ground-based telescopes. The applications of such fabrication methods for the Sloan Digital Sky Survey telescope will be reviewed.

The Large Binocular Telescope has two 8.4 meter mirrors, one of which is now in the telescope. Handling and moving the 8.4-meter honeycomb mirrors calls for moving 16 metric ton mirrors while maintaining very low stresses. We have now handled the first LBT mirror off the furnace, turned on edge, cleaned out, turned upside down, on the grinder, turned again, put on a polishing cell, moved under the polishing machine, lifted with a vacuum lifting fixture, moved to the telescope cell, to a transportation box, down the highway, onto a multi-axle trailer on edge, up Mount Graham, into the telescope building, back into the telescope cell and up through a hatch onto the telescope itself. The second LBT mirror is in the polishing stage. We have designed and manufactured many pieces of specialized equipment to handle the task. This equipment must be able to handle the mirrors without exceeding 0.7 MPa (100 psi) stress in the glass.

A common problem in the large telescopes is the accurate positioning of the optical elements. In the Salt Telescope, there is no secondary mirror, but a light collection system mounted on an optical payload, which is dynamically positioned above the primary mirror. Fogale Nanotech has developed an optical ranging system, the LISE LS40-LD, which is derived from a commercial product, to measure the distance from the payload to the primary mirror, and adjust their relative position. The LISE system is a fibre optic low-coherence interferometre illuminated by an infrared broadband source. It consists in a control unit placed under the telescope and a small collimator mounted on the payload, which focuses the measurement beam on the mirror and collects the reflected light. The collimator is linked to the control unit by a 50 m long optic fibre. It measures a true absolute distance. The distance to measure is 13.505 m, the measuring range is +/-20 mm and the required accuracy is +/- 10 μm. The system has been installed on the Salt Telescope on July, 2003. The results shows that the specifications are met.

The current LSST Baseline Configuration has a field of view of 3.5 degrees and an optical etendue of 302 meters square degrees square. The etendue calculation includes the effect of gradual vignetting by the camera as the field angle increases. A current optical point design includes an 8.4 m spun cast light-weighted borosilicate primary mirror, a 3.2 m secondary mirror and a 5.0 m tertiary mirror. The goal of this study is to determine if these mirrors can be actively supported and retain figure control over elevation angles without closed-loop control based on wave-front measurement. Support systems for the tertiary and primary mirrors are adapted from proven systems utilized on 6.5 and 8.4 m class primaries developed by the University of Arizona's Mirror Laboratory. The number and locations of axial and lateral supports is determined for each mirror and the gravitational and support induced surface distortions are calculated and are shown to be within budgeted limits. The support components and their performance are described and it is demonstrated that predicted mirror distortion attributable to the support system is consistent with the known performance of the support components.

The GTC Acquisition Cameras and Wavefront Sensors are based on a modular design with remote, low-profile and lightweight CCD heads and a compact CCD controller. The cameras employ E2V Technologies Peltier cooled CCD47-20 and CCD39-01 detectors, which achieve 1Hz and 200Hz full frame readouts, respectively. The CCD controller is a modified version of the Magellan CCD controller (Greg Burley - OCIW), which is linked to the GTC control system. We present the detailed design and first performance results of the cameras.

The thirty-seven one meter class mirror segments which comprise the LAMOST primary mirror need precision support and cell to meet the demand of optics surface figure. Because true ZERODUR glass mirror has had the final design drawing that has been slight different from the former one, a modified sub-mirror finite element model and re-analysis have considered many new factors including the anti-drop groove, center blind hole and invar pad. At the same time, After a sub-cell prototype have been designed and manufactured, with a similar K9 glass mirror segment, the experiment of mirror figure testing is being conduct to verify sub-cell's performance and modify some detail. The support truss of primary mirror also has new scheme. This paper introduces the sub-cell prototype experiment, analysis of sub-mirror and new support truss.

This paper summarizes the main aspects of the design and qualification test results of the secondary mirror mechanism for the 10.4-m Gran Telescopio Canarias (GTC). The design of the M2DS consists of a two stage mechanism, a hexapod for alignment using six linear actuators and a compensated tilt/chop stage with three voice coils taking its base on the hexapod mobile plate. The system has been tested after servos adjustment and calibration and the latest results are presented, which illustrate the quality and accuracy of this mechanism in both alignment and chop performances. Finally, the results and experiences are summarized in order to provide useful information for new developments of such systems.

After 6.5 years of development, the telescope of the Stratospheric Observatory For Infrared Astronomy, SOFIA has been integrated into the aircraft and is awaiting the final touch-up before first test observations will start. Due to its rather unique environment in the open port of a Boeing 747SP, the telescope optics of SOFIA is exposed to extreme aero-acoustic excitations. The telescope pointing system is equipped with several design features, such as a vibration isolation system, a flexible body control system and -- potentially -- active mass dampers, to handle excitations in different frequency ranges. Final performance features of these systems will only be available after the first test flights, which will happen in the second half of 2004. Here we present a progress report and describe the recent achievements as well as the status of the telescope, and give an update of the SOFIA pointing system, and the planned commissioning tests.

We are developing an ultra-lightweight and inexpensive mid-sized telescope on an alt-azimuth mount. Utilizing commercially available truss elements and compact high precision bearings we are able to achieve significant weight and cost savings while maintaining mechanical strength and stability. The design features a structure integrating the mirror-cell and the altitude bearing's arc-rails coupled to a short-armed fork through R-Guide bearings. The fork sits on a turn-table providing rotation about the azimuth axis. The telescope-tube connects to an extension of the mirror-cell. The mount can carry a mirror of up to 3m in diameter, yet is only 5m from the bottom of the base to the top ring and weights only 5000kg without the primary mirror. Assembly of the mount was performed by three students in two weeks and within the budget of $80,000. We measured the altitude bearing's motion accuracy to be 0".07 (rms) at a constant speed of 2".2/s for 15 minutes in both forward and backward motion without any sign of hysteresis, stick-slip, or backlash.

This paper describes the conceptual design of the optics support structures of a telescope with a primary mirror of 8.4 m, the same size as a Large Binocular Telescope (LBT) primary mirror. The design goal is to achieve a structure for supporting the primary and secondary mirrors and keeping them joined as rigid as possible. With this purpose an optimization with several models was done. This iterative design process includes: specifications development, concepts generation and evaluation. Process included Finite Element Analysis (FEA) as well as other analytical calculations. Quality Function Deployment (QFD) matrix was used to obtain telescope tube and spider specifications. Eight spiders and eleven tubes geometric concepts were proposed. They were compared in decision matrixes using performance indicators and parameters. Tubes and spiders went under an iterative optimization process. The best tubes and spiders concepts were assembled together. All assemblies were compared and ranked according to their performance.

The Euro50 is a European Extremely Large Telescope. Its enclosure will be among the largest buildings of the world. Determining the maximum wind load is crucial for the survival of the structure, and local forces have an impact on the detailed design such as cladding. Pressure variations on the primary mirror and the wind load on the telescope are important for the development of active optics and segment control systems. To obtain data for the survival wind load as well as for typical observing conditions, the airflow pattern has been studied both with a wind tunnel model and computational fluid dynamics (CFD). Special attention has been given to determination of pressures on the primary mirror. Results are compared for the two methods and also with data available from previous studies and from measurements on existing telescopes. Finally, a typical wind load envelope is defined for the integrated telescope model.

For decades designers of dish antennas and radio telescopes have known the aerodynamic properties of parabolic reflectors. However, site planners and end users are not necessarily versed in their properties, and so can place them on sites or use them in such a manner that the wind causes a maximum of disruption of the pointing and tracking performance. Parabolic reflectors make excellent airfoils, and as such act like an airplane wing in many respects. Having some knowledge of sensitive wind directions relative to the Line Of Sight (LOS) can lead a user to change his site selection or operating procedures to achieve the optimum pointing and tracking performance for most observations. This knowledge can also contribute information to help specify the necessary performance characteristics. This paper discusses the aerodynamic properties of parabolic reflectors so the reader can get a ready grasp of the issues.

The optical path of LAMOST is 60-meter long with its main optical axis fixed and lying in close to horizontal orientation, that causes much more serious dome seeing problem than the one in conventional 4-meter class telescopes. The temperature gradients generated by many thermal sources in the dome induce the seeing problem. It is necessary to control of thermal distribution inside the enclosure to keep a good dome seeing. In this paper we introduce our computation and experiments. Through analysis we have obtained a method dealing with design of the cooling system to remove the effect of local heat source and reduce temperature gradients. The methods are outlined as follows: Cool the main heat sources with the cooling air with a temperature of 5°C lower than the ambient temperature. Because of the temperature difference between summer and winter, we have developed two sets of cooling systems to deal with respectively, particularly to keep the system workable in wintertime. We have designed a special structure for removing heat resources inside the enclosure. There is enough ventilation to keep the wind velocity at 0.5~1m/s in the optical path, and special fans generating low velocity wind provide good ventilation and avoid vibration noises. Temperature sensors feeding signals back to the computer, which adjusts the control loop to maintain a good thermal distribution in optical path and to minimize the dome seeing problem.

To realize the full performance potential of the new generation of ground based telescopes, whether optical or IR, enclosure seeing control and telescope temperature control must be optimized. While the techniques for optimizing seeing are well known, EOS' IceStorm Enclosures, specifically developed for interferometric applications, introduced a number of novel physical implementations of these known techniques. A 9.5m prototype IceStorm enclosure was assembled and extensively tested in 2002, including independent testing of thermal performance. Four enclosures have since been completed, one of which was installed at the EOS Space Center on Mt Stromlo, Australia in 2003. Design principles for the IceStorm enclosure and thermal test results are described, and the application of these principles to larger enclosures is discussed.

The next generation of large telescope in China will be 30m class. Based on the studies of LAMOST enclosure, we carry on feasibility studies for CFGT enclosure and conclude that the dimension of enclosure will be in the range of 50-60 m. The design should be featured with optimum dome seeing performance, good wind buffet capability and reasonable cost effectiveness. All the structure frames will be made of compact space trusses, optimized in dimension and minimized in cost. Two preliminary plans for enclosure adopting "open-air" and "foldaway" approach are proposed. The folded enclosure located on a large circular air-cushion can rotate to the wind direction and act as wind screen to mitigate the effects of wind-buffeting on the telescope. A new kind of crane-robot crane will be employed into the service of telescope. The enclosure control system based on Fieldbus network controls and monitors all the devices in the enclosure, including robot crane, driving motors and sensors. The sensors send relevant parameters to computer and the computer controls the corresponding devices to ensure better performance of the enclosure. This paper describes the preliminary design for the CFGT enclosure and discusses the possible technologies to build enclosure.

Several approaches for design optimization in control-structure interaction problems for precision reflectors and structure are discussed. Applications for shape control and active vibration damping show the need to treat this interaction simultaneously or to use properly defined iterative decomposition.

The measuring system of fiber position of LAMOST (Large Area Multi-Object Fiber Spectroscope Telescope) is introduced in this paper. A method of photometric gravity's center is discussed. A laboratorial system for measuring position of a few of fibers in a small scale has been finished. The results of the experiment are satisfactory. Based on the experience of laboratorial experiments, we are now designing the measuring system of fiber position of LAMOST. A conceptual design is described in this paper.

We propose a new robotic system for positioning payloads such as pickoff mirrors, fibres or deployable IFUs on telescope focal planes. Based on a combination of concepts used in existing fibre positioning systems, the proposed system retains the advantages of each type of existing positioner, while eliminating many of the disadvantages. It employs micro-robotic actuators to independently and simultaneously position an arbitrary number of small payloads accurately on an arbitrarily large field plate and offers a cost-effective and multiply-redundant design for payload positioning systems suitable for use at Cassegrain or Nasmyth foci of large telescopes. Operation in cryogenic environments, positioning accuracies of a few microns, simultaneous movement of arbitrary numbers of positioners and the removal of many movement constraints are some of the advantages offered. We demonstrate a prototype positioner for the system.

EMIR is a NIR multiobject spectrograph with imaging capabilities to be used at the GTC. A general description of instrument performances, as well as the updated optical and mechanical layouts, can be found elsewhere on these proceedings (reference documents 4, 6 and 7). After the successful results of the Preliminary Design Review in March 2003, EMIR optical design is now complete. Some specific features of the optical components make it particularly difficult to mount them in the instrument. For example, the first collimator lens in EMIR is one of the largest Fused Silica lenses ever mounted to work under cryogenic conditions, and some other lenses in the system present features such as aspheric surfaces, tight centering tolerances etc. The analysis of the testing being done in order to validate three different lens mounting design concepts is presented here, as well as the detailed status of the lens mounting design solutions adopted.

We present case studies on the application of passive compensation in two large astronomical instruments: the Gemini Near Infrared Spectrograph (GNIRS), including actual performance, and the NOAO Extremely Wide Field Infrared Mosaic (NEWFIRM) camera. Image motion due to gravity flexure is a problem in large astronomical instruments. We present solutions for two different cases using passive mechanical compensation of the optical train. For the Gemini Near Infrared Spectrograph (GNIRS), articulation of a single sensitive optic is used. Adjustable cantilevered weights, designed to respond to specific gravity components, are employed to drive tilt flexures connected to the collimator mirror. An additional requirement is that cryocooler vibration must not dynamically excite this mirror. Performance testing of the complete instrument shows that image motion has been satisfactorily compensated. Some image blur due to dynamic excitation by the cryocoolers was noted. A successful damping scheme has been developed experimentally. For the NOAO Extremely Wide Field Infrared Mosaic camera (NEWFIRM), the entire optical support structure is mechanically tuned to deflect and rotate precisely as a rigid body relative to the telescope focal plane. This causes the optical train to remain pointed at a fixed position in the focal plane, minimizing image motion on the science detector. This instrument is still in fabrication.

In the last few years, astronomical instruments with infrared detectors have become increasingly important. These detectors as well as the mechanical mechanisms inside the instruments are operated in high vacuum at cryogenic temperature. Since ready-for-use cryogenic actuators are often not available from stock, the Max-Planck-Institut fur Astronomie (MPIA) in Heidelberg has developed actuators for both linear and circular movement. Information about the use of materials, dry film lubricants, and components like motors, micro switches and resolvers for this temperature region is hard to find in literature. Thus, large-scale experiments and tests were made to gain experience and to qualify the actuators for their use at cryogenic temperatures.

Multi-integral-field spectrographs for near-infrared observations require a large number of complex cryogenic mechanisms to select source images in the telescope field of view which are then re-format on the spectrograph entrance slit. Source selection can be achieved in several ways, but the two methods most adequate for large fields and cryogenic environment are positioning of an optical element in the telescope field to pick off the source image, or steering a mirror located in a pupil image to deflect the light from a source into relay optics. The first solution permits high flexibility in source selection at the cost of large mechanical travels. The second solution limits the source selection to one per pre-defined sub-field, but gets by with small mirror tilts. Higher flexibility can be regained for the second solution by assigning different sub-field sizes to the steering mirrors in the central and in the peripheral areas of the field of view. We present a solution for a cryogenic steering mirror unit with a mirror diameter of about 20 mm and tilt angles of a few degrees, appropriate for source selection in a 1 arc minute field of an 8 m class telescope. The gimbaled mirror can be tilted about two perpendicular axes in the tangential plane of the mirror apex. The mirror is driven by two Nanomotors, and the motor strokes are measured by LVDTs. Motors and sensors are specified for operation at LHe temperatures.

We have a plan to install a micro-crack alert system for the primary mirror of Subaru Telescope based on the monitoring of the acoustic emission from any incident events. We report the results of our preliminary experiment for characterizing the acoustic properties of actual Subaru primary mirror. The attenuation of acoustic wave was confirmed to be small enough to allow detection of such events at any locations of the mirror. The position of incident events that might lead to the generation of possible micro-cracks can be identified within less than 3 cm accuracy by placing seven acoustic sensors along the circumference of the primary mirror.

There are currently several projects for giant telescopes with segmented mirrors under way. These future telescopes will have their primary mirror made of several thousand segments. The main advantage of segmentation is that it enables the active control of the whole mirror, so as to suppress the deformations of the support structure due to the wind, gravity, thermal inhomogeneities etc. ..., thus getting the best possible stigmatism. However, providing active control of segmented mirrors requires numerous accurate edges sensors. It is acknowledged that capacitance-based technology nowadays offers the best metrological performances-to-cost ratio. As the leader in capacitive technology, FOGALE nanotech offers an original concept which reduces the cost of instrumentation, sensors and electronics, while keeping a very high level of performances with a manufacturing process completely industrialised. We present here the sensors developed for the Segment Alignment Measurement System (SAMS) of the Southern African Large Telescope (SALT). This patented solution represents an important improvement in terms of cost, to market the Position Sensors for Segmented Mirrors of ELTs, whilst maintaining a very high performance level. We present here the concept, the laboratory qualification, and the first trials on the 7 central segments of SALT. The laboratory results are good, and we are now working on the on-site implementation to improve the immunity of the sensors to environment.

For currently market available levelmeter can not meet the requirement for measuring the azimuth mounting of the LAMOST, this paper presents a novel design scheme of an opto-electronic levelmeter with needed high precision. The levelmeter is essentially a combination of an optical front end and a computer aided measuring back end. The light from a point source is firstly turned to be parallel and reflected by a tip-tilt mirror which keeps pointing to the zenith and then imaged onto a CCD target through optical system, afterwards, the position of the image spot is processed by computer software to give measurement results. By rotating the LAMOST mounting about azimuth axis with the levelmeter on it, the axis system is measured, and if the measured azimuth axis is not perpendicular enough, the image spot on CCD target is to offset some distance by which the tilt angle of the axis can be evaluated. The design principle and data processing of the levelmeter are detailed systematically in this paper. Experiment results confirmed that the accuracy of the levermeter is up to 0.043" beyond that required by the technical specification of the LAMOST. Also, the novel levermeter is applicable to measuring azimuth axes of other telescopes.

The photogrammetry technique is widely used for measuring 3-D shape in diverse industries thanks to its easy implementation and straightforward algorithm. However, most measurements done by this technique are using a single camera with multi-exposure and the results are derived after a period of time. Along with the development of the CCD and computer technology, it is now possible to use multi-camera, real-time photogrammetric measurements and the results could be derived without much time delay. A multi-camera system avoids the error caused by time-delayed exposure. So the new technique could be widely used in telescope surface or position measurement and control. In this paper, the pertinent formulations, implementation and application details are discussed before a general conclusion is drawn.