The Large Synoptic Survey Telescope (LSST) has a 10 degrees square field of view which is achieved through a 3 mirror optical system comprised of an 8.4 meter primary, 3.5 meter secondary (M2) and a 5 meter tertiary mirror. The M2 is a 100mm thick meniscus convex asphere. The mirror surface is actively controlled by 72 axial electromechanical actuators (axial actuators). Transverse support is provided by 6 active tangential electromechanical actuators (tangent links). The final design has been completed by Harris Corporation. They are also providing the fabrication, integration and testing of the mirror cell assembly, as well as the figuring of the mirror. The final optical surface will be produced by ion figuring. All the actuators will experience 1 year of simulated life testing to ensure that they can withstand the rigorous demands produced by the LSST survey mission. Harris Corporation is providing optical surface metrology to demonstrate both the quality of the optical surface and the correctablility produced by the axial actuators.
The Large Synoptic Survey Telescope (LSST) is a large (8.4 meter) wide-field (3.5 degree) survey telescope, which will be located on the Cerro Pachón summit in Chile. Both the Secondary Mirror (M2) Cell Assembly and Camera utilize hexapods to facilitate optical positioning relative to the Primary/Tertiary (M1M3) Mirror. A rotator resides between the Camera and its hexapod to facilitate tracking. The final design of the hexapods and rotator has been completed by Moog CSA, who are also providing the fabrication and integration and testing. Geometric considerations preclude the use of a conventional hexapod arrangement for the M2 Hexapod. To produce a more structurally efficient configuration the camera hexapod and camera rotator will be produced as a single unit. The requirements of the M2 Hexapod and Camera Hexapod are very similar; consequently to facilitate maintainability both hexapods will utilize identical actuators. The open loop operation of the optical system imposes strict requirements on allowable hysteresis. This requires that the hexapod actuators use flexures rather than more traditional end joints. Operation of the LSST requires high natural frequencies, consequently, to reduce the mass relative to the stiffness, a unique THK rail and carriage system is utilized rather than the more traditional slew bearing. This system utilizes two concentric tracks and 18 carriages.
Over the past two years, the New York Astronomical Corporation (NYAC), the business arm of the Astronomical Society of New York (ASNY), has continued planning and technical studies toward construction of a 12-meter class optical telescope for the use of all New York universities and research institutions. Four significant technical studies have been performed investigating design opportunities for the facility, the dome, the telescope optics, and the telescope mount. The studies were funded by NYAC and performed by companies who have provided these subsystems for large astronomical telescopes in the past. In each case, innovative and cost effective approaches were identified, developed, analyzed, and initial cost estimates developed. As a group, the studies show promise that this telescope could be built at historically low prices. As the project continues forward, NYAC intends to broaden the collaboration, pursue funding, to continue to develop the telescope and instrument designs, and to further define the scientific mission. The vision of a historically large telescope dedicated to all New York institutions continues to grow and find new adherents.
Corning’s ULE® is an ultra-low expansion glass used for machine tool blocks to astronomical mirrors. Its primary
alternative is a glass ceramic. In many applications, ion beam milling is used for final surface figuring. Ion milling
removes material at an atomic level and is typically a slow, expensive process. Experiments have determined the upper
limits of removal rate for ion beam milling during optical figuring. The goal was to increase the power density of the ion
beam during figuring to achieve higher removal rates with no negative effects on surface properties. Testing shows that
the removal rate on ULE® is about 50% higher than on glass ceramics under the same conditions. With an increase in
material removal rate, both ULE® and the glass ceramic show an increase in surface roughness. Average birefringence
of both materials increases slightly after milling; however the level of birefringence in the glass ceramic is seven times
larger than for ULE®. Therefore using higher ion milling power densities, the surface figuring of ULE® can be
accelerated to produce shorter processing times without adverse effects on surface properties. This can help lower the
cost for manufacture of ULE® optics.
The Astronomical Corporation of New York has commissioned a study of a 12-meter class telescope to be
developed by a group of NY universities. The telescope concept builds on the basic principles established by the
Keck telescopes; segmented primary mirror, Ritchey Chretien Nasmyth instrument layout, and light weight
structures. New, lightweight, and low cost approaches are proposed for the primary mirror architecture, dome
structure and mechanisms, telescope mount approach, and adaptive optics. Work on the design is supported by
several NY based corporations and universities. The design offers a substantially larger aperture than any existing
Visible/IR wavelength telescope at historically low cost. The concept employs an adaptive secondary mirror and
laser guide star adaptive optics. Two First Light instruments are proposed; A High resolution near infrared
spectrograph and a near infrared Integral field spectrograph/imager.
CCAT will be a 25 m diameter, submillimeter-wave telescope. It will be located on Cerro Chajnantor in the
Atacama Desert, near ALMA. CCAT will be an on-axis, Ritchey-Chrétien design with an active primary to
compensate gravitational deformations. The primary mirror will have 162 segments, each with ~0.5 × 0.5 m
reflecting tiles on a ~2×2 m, insulated, carbon-fiber-reinforced-plastic subframe. CCAT will be equipped with
wide-field, multi-color cameras and multi-object spectrometers at its Nasmyth foci. These instruments will cover
all the atmospheric windows in the λ = 0.2 to 2 mm range. The field of view at the Nasmyth foci will be 1°,
so CCAT will be able to support cameras with a few ×104 detectors (spaced 2 beamwidths) at λ = 1 mm to
a few ×106 detectors (spaced half a beamwidth) at λ = 350 μm. Single instruments of this size are probably
impractical, so we will break the field into smaller pieces, with a separate sub-field camera for each piece. The
cameras will require some relay optics to couple the fairly slow beam from the telescope to the detectors. A
reflective relay for 1° field of view is too large to be practical, so we plan to use a compact, cold, refractive relay
in each sub-field camera.
CCAT will be a 25 m diameter submillimeter-wave telescope that will operate inside a dome located on Cerro
Chajnantor in the Atacama Desert. The telescope must have high aperture efficiency at a wavelength of 350 microns
and good performance out to a wavelength of 200 microns. A conceptual design for a carbon fiber reinforced plastic
(CFRP) truss and primary reflector support truss has been developed. This design yields a telescope with a net ½ wave
front error of <10 microns using a lookup table to adjust the segment actuators to compensate for gravitational
deflections. Minor corrections may be required to compensate for the expected 20 C temperature excursions. These can
be handled using a coarse lookup table.
The CCAT Project is an effort to construct a 25 meter aperture telescope above 5600 meters altitude operating down to
wavelengths as short as 200 μm. CCAT has developed some new and innovative approaches to telescope and optics
design, added new partners to the project, and has plans for substantially increased activities over the next two years.
Begun by Cornell University and the California Institute of Technology, CCAT currently has six national and university
partners. Funding has been increased and significant technical activities are underway to investigate the key enabling
technologies. Areas of development include telescope optical design, mount design, application of CFRP materials to
the telescope, sensing and control of primary mirror segments, and control system architecture.
Schedules and budgets for the Project have been updated and an overall approach leading to first light in 2016-2017 has
been developed. CCAT promises to have a significant scientific impact on submillimeter astronomy and the prospects
for success has never looked better.
Technologies of modern optical telescopes with large primary mirror are based on adaptive optics. These telescopes
operate with many small mirror segments, so that all the segments work as a large piece of a reflective curved plate, i.e.
a paraboloid. Each mirror segment is independently attached to a support structure via adjustable warping harnesses. A
support structure is required to be extremely rigid in order to maintain the reflective surface. This paper describes the
conceptual approach for the design of such support structures. A system proven to fulfill these requirements with
efficient structural material use is a node-and-bar system,
so-called space frame. The rules for geometry of space frame
structures are based on the system of the five 'platonic solids': The edges of the conceptually assembled solids can be
replaced by the bar members of a space frame to achieve maximum stiffness. This conceptual approach is demonstrated
with examples in the paper, by illustrating the determination of the geometry and examining the deformation due to the
telescope rotations during operation. This paper also demonstrates design solutions for other issues relevant to space
frame geometry, such as effects of gradient thermal load and redundancy of the structures.
To meet the 10 µm RMS half wavefront error requirement for the 25 m diameter Cornell Caltech Atacama Telescope
(CCAT), active control of the approximately 200 primary mirror panels is required. The CCAT baseline design includes
carbon fiber aluminum honeycomb sandwich mirror panels. Distortions of the panels due to thermal gradients, gravity
and the mounting scheme need to be taken into consideration in the control system design. We have modeled the
primary mirror surface as both flat and curved surfaces and have investigated mirror controllability with a variety of
sensor types and positions.
To study different mirror segmentation schemes and find acceptable sensor configurations, we have created a software
package that supports multiple segment shapes and reconfigurable panel sizing and orientation. It includes extensible
sensor types and flexible positioning. Inclusion of panel and truss deformations allows modeling the effects of thermal
and gravity distortions on mirror controllability.
Flat mirrors and curved mirrors with the correct prescription give similar results for controlled modes, but show
significant differences in the unsensed flat mirror modes. Both flat and curved mirror models show that sensing
schemes that work well with rigid, thermally stable panels will not control a mirror with deformable panels. Sensors
external to the mirror surface such as absolute distance measurement systems or Shack-Hartmann type sensors are
required to deal with panel deformations. Using a combination of segment based sensors and external sensors we have
created a promising prototype control system for the CCAT telescope.
Five partners have currently joined a Consortium to develop the Cornell Caltech Atacama Telescope (CCAT.) Included
are Cornell University, the California Institute of Technology (Caltech), the University of Colorado at Boulder, the
United Kingdom as represented by the Astronomy Technology Centre (ATC), and Canada as represented by the
Universities of British Columbia and Waterloo. This consortium has continued work toward the design of the telescope
and instrumentation, pursued fund raising, and further developed the science case for CCAT. An Engineering Design
Phase is being planned for 2009-2011 with construction planned to begin shortly thereafter. CCAT continues as a wide
field (20 arc min) FOV telescope operating from a shortest wavelength of 200µ. Testing has continued near the summit
of Cerro Chajnantor in the Atacama Region of Chile above 5600 meters altitude and data indicates significantly lower
water vapor in the seeing column than measured at the ALMA site on the plateau below. Work over the past two years
has included research on manufacturing methods for optical segments, extensive study of mirror alignment sensing and
control techniques, additional concepts for major structures, and further development of instrumentation.
The 25 m aperture Cornell Caltech Atacama Telescope (CCAT) will be the first segmented telescope of its size and precision. A new technology was required to be able to economically manufacture the segments for the primary mirror. This technology had to be a low cost, low risk, volume manufacturing process in addition to meeting all of the optical and mechanical requirements. The segments had to be lightweight (10-15 kg/m2), have high specific stiffness and be thermally stable. The segments had to have sufficient robustness for practical transport and use and be compatible with high-reflectivity coatings. ITT has designed a replicated, lightweight glass mirror solution to these manufacturing problems. This technology can be used to fabricate segments for CCAT. It can be used to fabricate segments for visible wavelength segmented telescopes or any other application requiring lightweight optics in large quantities. This technology enables the fabrication of large, lightweight mirror segments in a few weeks to a couple of months, depending on the figure requirements. This paper discusses the design of these mirrors and presents demonstrated results to date, including a 0.5 m diameter, 8 kg/m2 borosilicate mirror blank and 0.2 m diameter replicated borosilicate mirrors.
Cornell, California Institute of Technology (Caltech), and Jet Propulsion Lab (JPL) have joined together to study development of a 25 meter sub-millimeter telescope (CCAT) on a high peak in the Atacama region of northern Chile, where the atmosphere is so dry as to permit observation at wavelengths as short as 200 μm. The telescope is designed to deliver high efficiency images at that wavelength with a total one-half wavefront error of about 10 μm. With a 20 arc min field of view, CCAT will be able to accommodate large format bolometer arrays and will excel at carrying out surveys as well as resolving structures to the 2 arc sec resolution level. The telescope will be an ideal complement to ALMA. Initial instrumentation will include both a wide field bolometer camera and a medium resolution spectrograph. Studies of the major telescope subsystems have been performed as part of an initial Feasibility Concept Study. Novel aspects of the telescope design include kinematic mounting and active positioning of primary mirror segments, high bandwidth secondary mirror segment motion control for chopping, a Calotte style dome of 50 meter diameter, a mount capable of efficient scanning modes of operation, and some new approaches to panel manufacture. Analysis of telescope performance and of key subsystems will be presented to illustrate the technical feasibility and pragmatic cost of CCAT. Project plans include an Engineering Concept Design phase followed by detailed design and development. First Light is planned for early 2012.
The Cornell Caltech Atacama Telescope (CCAT) is a 25m far infrared telescope in the conceptual design phase. Its primary mirror is composed of a set of panels supported by a space truss. The primary and secondary mirror arrangement resembles the reflector and quadrapod arrangement seen in many radio telescopes, but with shallower primary mirror geometry. In addition, the optical layout calls for a close spacing between the tertiary mirror and the Nasmyth and bent Cassegrain instruments. The mount design is driven by the spacing of the optical elements, the presence of the Nasmyth and bent Cassegrain ports, and the size of the primary mirror truss. This paper examines the mechanical and control system design solutions provided in response to the challenges posed by the optical requirements. These solutions include tradeoffs in structure, drive, and control system design.
The Cornell Caltech Atacama Telescope (CCAT) is a joint project to design and construct a 25-meter class submillimeter telescope in the Atacama region of northern Chile. The conceptual design and cost analysis done by M3 Engineering and Technology Corporation (M3) incorporates a cost trade-off of three optional sites and a baseline design of the facilities at Cerro Chajnantor. The details covered in this paper provide the final concept design of the CCAT facility, the decisions and rationale of the design process as well as a critical risk assessment of building at high altitude sites.
Cornell University and California Institute of Technology are currently studying the feasibility of constructing a 25
meter telescope to operate down to 200 micron wavelength to be sited on a high peak in the Atacama region of Chile.
An enclosure dome is required to protect the telescope from wind, solar heating, snow, and dust. A diameter of 50
meters at the equator is anticipated, larger than any existing opening telescope enclosure. A review of various
approaches indicates that a "calotte" type design, which uses two rotational axes to achieve full sky pointing, is
structurally and dynamically superior to other large enclosure approaches. The calotte design is balanced about both axes of rotation and features a circular aperture which provides optimal isolation from the wind. The nearly continuous
spherical shell lends itself to efficient space frame type structural form. An initial conceptual design was developed,
including structures, bearings, and drive systems. Analysis of these components was performed which illustrates the
feasibility of the chosen approach and provides indications of areas of critical risk in further development.
The Discovery Channel Telescope (DCT) is a 4.2 meter telescope that will provide a two degree diameter well-corrected field of view at prime focus with wavelength coverage across the groundbased ultraviolet and optical range. The design of the telescope and the prime focus corrector are described in other papers at this conference. The prime focus of the DCT will be occupied by a CCD camera similar in scope to the SAO Megacam for the MMT, the CFHT MegaCam, and the Kepler focal plane, but with differences in detail. It will be used for a variety of planetary science and astrophysics observing programs, the most demanding technically being searches for near-Earth and Kuiper Belt objects. This paper describes the design requirements, major systems issues, current design, and expected performance of the prime focus camera for the DCT.
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.
Development of the 4.1 meter SOuthern Astrophysical Research (SOAR) Telescope is now complete. All baseline systems are in place and extensive commissioning activities have been performed with and without the primary optics installed in the telescope. The facility and dome have been under observatory operations and TCS control for a year of testing and tuning. The altitude over azimuth telescope mount was integrated on the mountain in a rapid 3-month period due to the complete assembly and testing performed at the factory prior to delivery. Early mount testing and successful integration into the Telescope Control System (TCS) without the optical system was accomplished on the sky through use of two separate small aperture telescopes fixed to the structure. One of these, the "feed telescope" was also pivotal in early testing of the calibration wavefront sensor and SOAR optical imager by directing focused light to these separate instruments. The SOAR optical system, with its 4.1 meter clear aperture, 100 cm thick, ULEtm primary mirror, its lightweight ULEtm secondary, and its fast tip tilt ULEtm tertiary has been delivered and installed in the telescope. This system was also assembled as an electrically connected system and individually optically tested under a visible interferometer at the factory enabling rapid integration and a short commissioning period on telescope. In this paper we present the project status, a summary of the commissioning period, and the performance data for the completed telescope and its major components.
The Discovery Channel Telescope (DCT) is a project of Lowell Observatory in conjuction with the Discovery Channel to design and construct a 4.2-meter clear aperture telescope and support facility on a site approximately 40 miles southeast of Flagstaff, Arizona, USA. The site is an undisturbed mountain top at an altitude of 7800-feet. Design and construction of the telescope enclosure includes all of the site utilities, access road to the site along with the fixed base enclosure, telescope pier and rotating dome structure.
The details covered in this paper are the decisions and rationale of the DCT enclosure conceptual design completed by M3 Engineering & Technology Corporation (M3).
This paper describes the design and summarizes the performance of the recently completed SOAR telescope Active Optical System (AOS). This system is unique in that it uses a thin, solid 4.3-meter diameter ULE lightweight meniscus primary mirror only 100 mm thick. The figure of the primary mirror surface is controlled with 120 electro-mechanical actuators that are force feedback controlled. The telescope is calibrated against the sky using a calibration wave-front sensor; as this calibration progresses, feedback forces, initially set from finite element analysis predictions, are replaced with sky database look-up tables. The system also includes a 0.6-meter diameter secondary mirror articulated by a hexapod for real-time optical alignment of the telescope, a 0.6-meter class tertiary mirror that also works as a 50 Hz tip tilt corrector to compensate for atmospheric turbulence and a rotary turret mechanism for directing the light to either of two nasmyth or three-bent cassegrain instrument ports. An operation control system interfaces with the telescope control system and each of the hardware assemblies.
The paper provides an overview of the design of each assembly as well as summarizes results of performance testing the system.
Lowell Observatory has initiated the development of a four meter class optical telescope with significant capabilities for solar system and broad spectrum astronomical research. Key to the Discovery Channel Telescope (DCT) is the ability to rapidly switch between 2 degree FOV imaging via a prime focus camera to 30 arc min FOV instrumentation at Ritchey-Chretien (RC) focus. The telescope is to be constructed at approximately 7700 feet altitude, Southeast of Flagstaff, Arizona at a site which has exhibited 0.6 arc sec best quartile seeing. The telescope will feature active optics and alignment capability and the Prime Focus Instrument will feature a Mosaic Focal Plane array of 40 2k x 4k CCDs. The RC instrument payload will be approximately 5000 lbs, allowing either large instruments or suites of co-mounted instruments. This telescope is being developed in partnership with Discovery Communications, Inc. (DCI), who will utilize the DCT and the association with Lowell Observatory to develop educational programming about astronomy and technology. The telescope will be a substantial enhancement to the current capabilities of Lowell Observatory.
The 4.2 m Discovery Channel Telescope requirements create interesting challenges for the Mount mechanical and control system design. The wide field of view survey telescope incorporates two operational foci: prime focus and cassegrain, either one must be available during any night's observing. The mission for observing requires fast slewing / offsets between each exposure with fast settling times to maintain the mission requirements. The prime focus arrangement includes a dedicated camera on the spider assembly and the cassegrain configuration includes a secondary mirror at the spider assembly with a dedicated instrument located at the cassegrain focus. This requirement challenges the design team to incorporate a prime focus / secondary mirror flipping mechanism within the secondary spider. The configuration requires a substantial prime focus and cassegrain payload with long focal distances creating a large inertia on the altitude axis. These are a few of the interesting challenges that are presented in this paper along with the design, trade-offs of different solutions, and the recommended design for the telescope Mount.
The Discovery Channel Telescope control system incorporates very demanding requirements regarding fast serviceability and remote operation of the telescope itself as well as facility management tools and security systems. All system capabilities are accessible from a central user interface anywhere, anytime. Although the mature stage of telescope control technology allows focusing more on science rather than on telescope operation, the time and effort needed to integrate a large suite of software modules still impose a challenge to which reusing existing software is one of the answers, especially for advanced subsystems with distributed collaborative development teams. DCT's large CCD camera presents enormous computational problems due to the overwhelming amount of generated data. Properly implemented preventive maintenance and reliability aspects of telescope operation call for historical and real time data in order to determine behavioral trends and permit early detection of failure factors. In this new approach utility monitoring and power conditioning and management are integral parts of the control system. Proposed real time spectral analysis system of sound and vibration of key mount components allows tracking mechanical component deterioration that could lead to performance degradation. Survival control cells and unmanned operation systems are other options being explored for operation in harsh climatic conditions.
Adaptive Optics Associates has designed and built the Southern Astrophysical Research (SOAR) telescope primary mirror calibration wavefront sensor. It will be used to monitor the figure of the active primary mirror during observations. The package also includes an acquisition camera subsystem. The sensor uses many commercial components to control cost while meeting the desired technical specifications. We describe the wavefront sensor system and present results of performance testing obtained in the laboratory.
We discuss plans for the construction of a 15-m class telescope located in the high Atacama desert of Northern Chile. The baseline concept is a segmented mirror telescope optimized for operation at wavelengths longer than 3.5 microns but capable of working at shorter wavelengths. An adaptive secondary will be used to achieve diffraction limited imaging while maintaining low emissivity. The facility will be designed for eventual remote/robotic operation and include a number of instruments designed to take advantage of the low precipitable water vapor and good seeing conditions.
Development of the SOuthern Astrophysical Research (SOAR) Telescope is nearing completion atop Cerro Pachón in Chile. The facility and many accessory systems have been completed and are operational. The dome is installed and in the final stages of debugging, the telescope mount is being assembled on site after a successful trial integration and complete test at the contractor's facility, and the optical system is well on its way to completion later this year. Many instruments are under development with one in the final phases of integration and laboratory testing. This paper summarizes the status of the major subsystems, provides measured performance parameters where available, and outlines the remaining plans for the telescope development and subsequent commissioning.
The SOAR Telescope Project has developed a highly integrated Telescope and Observatory Control System, written in the LabVIEW graphical "G" language. A "plug-in" architecture and the ease of developing GUIs in LabVIEW has lead to a design and implementation that gives the operators flexibility, ease of use and a clear visual insight into the complex interactions of the many subsystems of a modern telescope. Care has been taken to abstract the many complexities into displays and controls that allows the operators to concentrate more on the functional operation of the telescope and observatory, and less on the intricacies of the various subsystems hardware. The User Interface includes many innovative features to make the operator?s job easier. Our process methodology for developing the TCS/OCS and continuous peer review/revision are enabling us to exceed SOAR's requirements and create a TCS/OCS that can easily be applied to other telescopes.
Development of the SOAR telescope is currently underway. Project plans include many tactics for smooth assembly, integration, and validation of this new facility to be located on Cerro Pachon at the Cerro Tololo Inter-American Observatory in Chile. A small project team has been established to manage and engineer the development of the major subsystems that are combined in this high image quality 4.2-meter diameter telescope. The status and plans for the development of the 28m telescope are discussed. A modest-sized facility building is under construction by CTIO, the organization appointed to operate the facility for the SOAR partners. Each telescope subsystem is contracted on a firm fixed price basis and will include complete performance testing at the contractor's facility before acceptance and shipment to the site. To ensure seamless integration, representatives of each contractor will come to the site for assembly and testing in place. They join personnel from the Project Office, the new operations staff, and the CTIO maintenance organization to form integrated product teams for subsystem integration, SOAR eases integration by using and mandating common, commercial control software. The contractors, the SOAR team, and the instrument buildings are making extensive use of LabVIEW/BridgeVIEW running under Linux (with real-time extensions as necessary) on compactPCI chassis. The telescope will include sufficient instrumentation, including a possible adaptive optics system, to allow system testing and optimization. An exceptionally large instrument payload ensures that instruments can remain in place upon the telescope as they are delivered and brought on line.
The Hobby-Eberly Telescope (HET) has been examined as a prototype for an Extremely Large Telescope (ELT) with a 33- meter diameter primary mirror. In this paper we examine the feasibility of scaling the HET/ELT up to 100-meters in diameter. In this 100-meter telescope design (called ELTX) the advantages of the tilted Arecibo concept seem to emerge even more strongly. For example the whole primary mirror is below grade and extremely well shielded from wind shake and the Stewart platform which carries the spherical aberration corrector and the instruments is capable of being scaled up to this massive size without any serious problems. Such a design is on track for probable science missions in the next half century.
The Hobby Eberly Telescope features a unique eleven-meter spherical primary mirror consisting of a single steel truss populated with 91 ZerodurTM mirror segments. The 1 meter hexagonal segments are fabricated to 0.033 micron RMS spherical surfaces with matched radii to 0.5 mm. Silver coatings are applied to meet reflectance criteria for wavelengths from 0.35 to 2.5 micron. To support the primary spectroscopic uses of the telescope the mirror must provide a 0.52 arc sec FWHM point spread function. Mirror segments are co-aligned to within 0.0625 ar sec and held to 25 microns of piston envelope using a segment positioning system that consists of 273 actuators (3 per mirror), a distributed population of controllers, and custom developed software. A common path polarization shearing interferometer was developed to provide alignment sensing of the entire array from the primary mirror's center of curvature. Performance of the array is being tested with an emphasis on alignment stability. Distributed temperature measurements throughout the truss are correlated to pointing variances of the individual mirror segments over extended periods of time. Results are very encouraging and indicate that this mirror system approach will prove to be a cost-effective solution for large optical collecting apertures.
The Hobby-Eberly telescope (HET) is a recently completed 9- meter telescope designed to specialize in spectroscopy. It saw first light in December 1996 and during July 1997, it underwent its first end-to-end testing acquiring its first spectra of target objects. We review the basic design of the HET. In addition we summarize the performance of the telescope used with a commissioning spherical aberration correlator and spectrograph, the status of science operations and plans for the implementation of the final spherical aberration corrector and facility class instruments.
Should the astronomical community pursue development of telescopes 10 times larger than the 8 and 10 meter individual and arrayed telescopes currently under development or recently commissioned? The question devolves into two parts: Is construction of such a telescope feasible from an engineering and cost standpoint? Does the scientific benefit justify the probable cost of such development? An Extremely Large Telescope (ELT) has previously been proposed based on the Arecibo type design employed in the recently completed Hobby Eberly Telescope. Analysis of the performance and scientific viability of the ELT shows that it can have an important role in near and IR spectroscopy for cosmology providing that stringent image and background performance requirements are met. Further development of engineering design and interaction with the manufacturing community conclusively shows that not only is such a telescope feasible, but that the entire observatory can be constructed for of order $DLR250 million at a site likely to provide optimal optical seeing. It remains an issue for the scientific community to judge whether such capability provides benefits commensurate with the costs.
The 10-meter class Hobby-Eberly telescope (HET), now nearing completion, provides technology for optical Arecibo-type telescopes which can be extrapolated to even larger apertures. Utilizing a fixed elevation angle and a spherical segmented primary mirror provides cost effective and pragmatic solutions to mirror mounting and fabrication. Arecibo-type tracking implies a greatly reduced tracking mass and no change to the gravity vector for the primary mirror. Such a telescope can address 70 percent of the available sky and exhibit optical quality easily sufficient for effective spectroscopy and photometry. The extremely large telescope takes advantage of several key engineering approaches demonstrated by the HET project to achieve a cost comparable to similarly-sized radio rather than optical telescopes. These engineering approaches include: bolted pre-manufactured primary mirror truss, factory manufactured geodesic enclosure dome, air bearing rotation of primary mirror, tracker, and dome systems directly on concrete piers, and tracking via a hexapod system. Current estimates put the cost of the ELT at $200 million for a 25-meter aperture utilizing a 33-meter primary mirror array. Construction of the ELT would provide the astronomy community with an optical telescope nearly an order of magnitude larger than even the largest telescopes in operation or under construction today.
The Hobby-Eberly Telescope, nearing completion at McDonald Observatory in west Texas is an optical Arecibo-type telescope utilizing an 11-meter primary mirror and a 9.2-meter effective aperture. Innovative approaches have been employed to provide this large modern telescope at a total cost of $13.5 million. A joint project of the University of Texas, The Pennsylvania State University, Stanford University, the University of Munich, and the University of Goettingen, the telescope will be completed in mid 1997. First light is expected in mid 1996.
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.
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.