Proceedings Volume 3352

Advanced Technology Optical/IR Telescopes VI

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Proceedings Volume 3352

Advanced Technology Optical/IR Telescopes VI

View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 25 August 1998
Contents: 12 Sessions, 87 Papers, 0 Presentations
Conference: Astronomical Telescopes and Instrumentation 1998
Volume Number: 3352

Table of Contents

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Table of Contents

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  • Major Projects
  • Advanced Telescopes of Medium Aperture
  • Mirror Fabrication, Polishing, and Handling
  • Major Projects
  • Mirror Fabrication, Polishing, and Handling
  • Major Projects
  • Mirror Support and Active Optics
  • Major Projects
  • Mirror Coating
  • Telescope Enclosures and Thermal Control
  • New Concepts and Approaches
  • Chopping Secondary Mirror Systems
  • Poster Papers on Chopping Secondary Mirror Systems and Next-Generation Concepts
  • Next-Generation Concepts
  • Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
  • Next-Generation Concepts
  • Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
  • Next-Generation Concepts
  • Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
  • Mirror Fabrication, Polishing, and Handling
  • Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
  • Poster Papers on Mirror Support, Active Optics, Coatings, and Telescope Enclosures
  • New Concepts and Approaches
  • Poster Papers on Mirror Support, Active Optics, Coatings, and Telescope Enclosures
  • Mirror Support and Active Optics
  • Poster Papers on Chopping Secondary Mirror Systems and Next-Generation Concepts
  • New Concepts and Approaches
  • Next-Generation Concepts
  • Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
  • Poster Papers on Chopping Secondary Mirror Systems and Next-Generation Concepts
  • Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
  • Poster Papers on Chopping Secondary Mirror Systems and Next-Generation Concepts
  • Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
  • Major Projects
  • Poster Papers on Mirror Support, Active Optics, Coatings, and Telescope Enclosures
  • New Concepts and Approaches
  • Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
  • Poster Papers on Mirror Support, Active Optics, Coatings, and Telescope Enclosures
  • Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
  • Poster Papers on Chopping Secondary Mirror Systems and Next-Generation Concepts
Major Projects
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Gemini 8-m telescopes
The paper briefly reviews the scientific rational and scientific systems approach used by the Gemini 8M Telescopes Project in the design, construction and operations of the Gemini telescopes. We report on the progress of the telescopes on both Mauna Kea, Hawaii and Cerro Pachon Chile, give an updated schedule and describe some of the science operations concepts and planning that is going into the Gemini Observatory. In addition, at the end of this paper we give a full bibliography of Gemini papers presented at the numerous sessions of the 1998 SPIE meeting in Kona.
Subaru Telescope
Norio Kaifu
Subaru Telescope is an 8.2-m optical-IR telescope being constructed by NAOJ at the 4,130-m altitude site on Mauna Kea, Hawaii. The 7th financial year of the 9-year project will end in this March. The construction in the assembling/testing phase, and mechanical data obtained so far suggest that the performance of the telescope structure will achieve our design target. The Subaru Telescope Hilo base facility was formally established in April 1998. We expect to obtain the first light by the end of the year. Here we report the general status and perspective of the Subaru project. Some details of individual parts of the construction will be described by other authors in this symposium.
Large Binocular Telescope Project
The Large Binocular Telescope (LBT) Project is a collaboration between institutions in Arizona, Germany, Italy, and Ohio. With the addition of the partners from Ohio State and Germany in February 1997, the Large Binocular Telescope Corporation has the funding required to build the full telescope populated with both 8.4 meter optical trans. The first of two 8.4 meter borosilicate honeycomb primary mirrors for LBT was cast at the Steward Observatory Mirror Lab in 1997. The baseline optical configuration of LBT includes adaptive infrared secondaries of a Gregorian design. The F/15 secondaries are undersized to provide a low thermal background focal plane. The interferometric focus combining the light from the two 8.4 meter primaries will reimage the two folded Gregorian focal planes to three central locations. The telescope elevation structure accommodates swing arms which allow rapid interchange of the various secondary and tertiary mirrors. Maximum stiffness and minimal thermal disturbance were important drivers for the design of the telescope in order to provide the best possible images for interferometric observations. The telescope structure accommodates installation of a vacuum bell jar for aluminizing the primary mirrors in-situ on the telescope. The detailed design of the telescope structure was completed in 1997 by ADS Italia (Lecco) and European Industrial Engineering (Mestre). A series of contracts for the fabrication and machining of the telescope structure had been placed at the end of 1997. The final enclosure design was completed at M3 Engineering & Technology (Tucson), EIE and ADS Italia. During 1997, the telescope pier and the concrete ring wall for the rotating enclosure were completed along with the steel structure of the fixed portion of the enclosure. The erection of the steel structure for the rotating portion of the enclosure will begin in the Spring of 1998.
Early performance and present status of the Hobby-Eberly Telescope
Lawrence W. Ramsey, Mark T. Adams, Thomas G. Barnes III, et al.
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.
Upgraded UKIRT
Timothy G. Hawarden, Nicholas P. Rees, Charles P. Cavedoni, et al.
The 3.8 m UK Infrared Telescope has been the focus of a program of upgrades intended to deliver images which are as close as possible to the diffraction limit at (lambda) equals 2.2 micrometers (FWHM equals 0.'12). This program is almost complete and many benefits are being seen. A high-bandwidth tip-tilt secondary mirror driven by a Fast Guider sampling at <EQ 100 Hz effectively eliminates image movement as long as a guide star with R < 16.m5 is available within +/- 3.'5 of the target. Low-order active control of the primary mirror and precision positioning of the secondary, using simple lookup tables, provide telescope optics which are already almost diffraction limited at (lambda) equals 2 micrometers . To reduce facility seeing the dome has been equipped with sixteen closable apertures to permit natural wind flushing, assisted in low winds by the building ventilation system. The primary mirror will soon be actively cooled and the concrete dome floor may be thermally insulated against daytime heating if fire safety concerns can be resolved. Delivered images in the K band now have FWHM which is usually <EQ 0.'8, frequently <EQ 0.'6 and quite often approximately 0.'3. Examples of the latter are shown: these approximate the resolution achieved by NICMOS on the HST. We estimate that the productivity of the telescope has approximately doubled, while its oversubscription factor has increased to > 4.
Gran Telescopio Canarias: project status
The Grain Telescopio CANARIAS (GTC) is a high performance 10-m class telescope which is being built by the IAC (Instituto de Astrofisica de Canarias). It will be installed at the Roque de los Muchachos Observatory, on the island of La Palma. First light is planned for end-2002. Image quality, operational efficiency and reliability are emphasized in the Conceptual Design which was finished in mid-97. Preliminary design is now proceeding on all the aspects of the project. The GTC Project is presently funded at the 50% level by Spain. Other partners are encouraged to participate in this challenging project. The scientific drives behind the GTC project are described here, as well as the current technical, managerial, and operational baseline.
Advanced Telescopes of Medium Aperture
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Concept and design of the 2.0-m NGAT: the new generation of astronomical telescopes
The Royal Greenwich Observatory and Liverpool John Moores University, United Kingdom, have joined in a collaboration to produce high quality, ground based robotic telescopes (2.0 to 5.0 m), for use with optical, infrared and interferometric astronomy. This venture has taken the form of a commercial company, Telescope Technologies Limited, to produce the range of Alt-azimuth telescopes. The reliability of the low cost, advanced technology, telescope design will enable remote observing over the Internet. The first two telescopes, currently under production, will see first light in La Palma and India in 1999. This paper covers the concept, design and capability range of the NGAT telescopes.
MAGNUM (multicolor active galactic nuclei monitoring) Project
Yukiyasu Kobayashi, Yuzuru Yoshii, Bruce A. Peterson, et al.
The MAGNUM Project is designed to carry out multi color monitoring observations of hundreds of AGNs over several years in order to measure the distance of these far away objects using simple physical principles and thereby determine cosmic parameters. The project has been funded by the Research Center of Early Universe. This project started in 1995 and observations are planned to begin in 1998. For the project, we are building a new remote controlled observatory with a 2 m automated telescope as well as new infrared and optical instruments. The telescope is optimized for infrared observations and for obtaining monitoring observations over many years. Our plane is to operate the observatory at the Haleakala summit on the Island of Maui, a suitable place for long time monitoring observations. The telescope is 2 m in diameter and has an alt-azimuth mount. The observatory will be equipped with such facilities as an automated instrument changer, weather monitor, environmental monitor and cloud cover monitor, making it easier to operate the telescope automatically and remotely. Observations will be carried out using an on-site scheduler, which will be commanded through a networked remote computer. Two observatory instruments are being built for the MAGNUM Project. The first is an infrared and optical imaging photometer which incorporates a dichroic beam-splitter and has an imaging capability over a wide wavelength range from 0.3 micrometers to 4 micrometers . It will be primarily used for AGN monitoring. The other is a wide field (33' field of view) 8K X 8K mosaic CCD camera.
Mirror Fabrication, Polishing, and Handling
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ZERODUR for lightweight secondary/tertiary mirrors
Hans F. Morian, Reiner H. Mauch
ZERODUR, the glass ceramic, with CTE values near zero, has excellent performance up to 8.2 m primary mirrors as well as for lightweight (LW) mirrors for Secondaries and Tertiaries. The high homogeneity of major properties of ZERODUR is an important prerequisite for the LW production and application. The production of CNC grinding and subsequent lightening via acid etching for additional weight reduction will be discussed. Results of Secondary and Tertiary LW mirrors for advanced technology telescopes like 8 m, 6.5 m and 3.5 m telescopes will be addressed. This paper presents also some examples of space based satellites with LW Zerodur mirrors in use since several years.
Ultralightweight galvanic mirrors
The high precision mirror replication technology with electro-formed Nickel was substantially optimized within projects such as JET-X and XMM. Based on this experience demonstrated on several hundred mirror shells with optical surface areas up to 1.3 m2 a new ultra lightweight mirror technology has been developed, enabling the production of low cost, isotropic, precision meniscus like reflectors highlighting excellent optical performance. In principle, any reflector thickness (typically 200 mm) can be electro-formed with the desired curvature and surface characteristics which is close to the optical quality of the mandrel (master). Any spherical or flat shape, including even offset elliptical reflectors, can be produced. No honeycomb or alternative stiffening structure is envisaged since the objective is to achieve a lightweight, perfectly isotropic reflector. A specific bonding between reflector meniscus and supporting structure made from Nickel is provided which avoids the introduction of local internal stress concentrations due to the final quasi-monolithic configuration. This technology can cover mirror dimensions up to several meters for astronomical, spaceborne and ground based telescopes (e.g. FIRST primary mirror) and radio antennas in the (sub)millimeter wave length range.
Major Projects
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Large-sky-area multiobject fiber spectroscopic telescope (LAMOST) and its key technology
Ding-qiang Su, Xiangqun Cui, Ya-nan Wang, et al.
The optical system of LAMOST is a special reflecting Schmidt system. It has an aperture of 4 m, f ratio of 5 and a 5 degree(s) field of view. The main optical axis is fixed on the meridian plane and tilted 25 degree(s) to the horizontal from south to north. The celestial objects were observed for 1.5 hours when they pass through the meridian. The reflecting Schmidt plane, MA, and the spherical mirror, MB, are segmented mirrors. MB is fixed on the foundation. The shape of the reflecting Schmidt plate has to be changed with different declination (delta) and in the tracking process. This is achieved with active optics. About 4000 optical fibers are planed to put on the focal surface. Some key technologies for LAMOST have been studied and presented in this paper: the calculation and testing method for thin mirror and segmented mirror active optics; the preliminary calculation and the experiment for active optics; the simulated calculation for optical image quality in 1.5 hours observation; a preliminary design of the alt-azimuth mounting and tracking system of MA.
Mirror Fabrication, Polishing, and Handling
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Comparison of shipping, handling, and shock instrumentation results for two 3.5-m-class primary mirrors
Don H. Killpatrick, James W. Mayo III
Packing, shipping, and handling procedures employed during several transportation activities for two large telescope primary mirrors are presented along with detailed shock recording results. Operations monitored included craning, forklifting, and shipping by air, sea, and land during all phases of manufacture and installation. The mirrors monitored were the SOR 3.5-m Telescope spun cast borosilicate primary mirror and the AEOS 3.67-m Telescope Zerodur thin meniscus primary mirror. Shock recording instrumentation included 2-, 5-, and 10-g Omni-GTM impact indicators, 10-g Impact o-graphTM 3-axis recording accelerometers, and high-resolution 3-axis accelerometers with Astromed Dash 8 eight-channel chart recorders and audio indicators. Shock results for some operations were monitored to the 0.01-g level. In-shipment temperature data are also presented and discussed. Effects of lifting operations, road conditions via truck, flight conditions via C-5B aircraft, and transportation via sea- going barge are discussed. Data are presented for three different crate designs and configurations and, in some cases, include mirror-in-cell shipping data. Shock results were observed from as low as a few hundredths-g to over 3- g's during various operations.
Advances in beryllium optical technology utilizing spherical powder
Beryllium has been used as an optical and structural material for astronomical/IR telescope applications for the past 20 years. Some of the most recent applications have been for the VLT (Very Large Telescope) M2 secondary mirror, SIRTF (Space Infrared Telescope Facility), plus many other space based IR sensors. The traditional forms and optical grades of the material, I-70H and I-220H, are well characterized from a mechanical and thermal standpoint over a wide range of temperatures. Beryllium's limiting factor's for astronomical and/or IR telescopes has been traditionally two fold: cryogenic stability and perceived higher cost than some of the other material options, such as glass, silicon carbide, and some composites. To address those two factors, Brush Wellman has developed a new optical grade of Beryllium produced by gas atomization (spherical powder) called O-30. This paper will detail the development of this grade of Beryllium, with emphasis on the cryogenic properties of the material from a thermal and mechanical view. It will also report on the results of the optical polishing/thermal cycling work done under government sponsored contracts. Finally the paper will describe the process of producing Near Net Shape parts utilizing O-30 spherical powder in order to reduce manufacturing cost and schedule.
Total quality manufacturing: the success of Corning's 8-m mirror blank programs
Corning Incorporated has successfully completed the manufacture of three 8-meter class mirror blanks, the Subaru 8.3-meter and the twin Gemini 8.1-meter blanks. These mirrors, constructed of ULETM zero expansion glass by the hex seal process, are nearing completion at the optical polishers and will soon be installed in their respective telescopes. This paper describes the manufacturing process for the mirror blanks, summarizes key engineering studies performed during these projects, and gives quality results for the three blanks. In all cases, the 8-meter mirror blanks met both the customer's quality requirements as well as schedule timelines. Other applications of this process are also given.
Fabrication and measured quality of the MMT primary mirror
Hubert M. Martin, R. G. Allen, James Roger P. Angel, et al.
The primary mirror for the Multiple Mirror Telescope Conversion is the first 6.5 m honeycomb sandwich mirror cast and polished by the Steward Observatory Mirror Lab. We describe the optical fabrication and testing of the f/1.25 paraboloid, and present the final measurements of figure accuracy and inferred image quality. Figuring was performed with a 1.2 m stressed lap--which bends under active control to match the local curvature of the optical surface--and a variety of small passive tools. The mirror was pressurized to compensate for polishing loads and thereby eliminate print-through of the honeycomb structure. The net result is a smoother surface on scales of 5 - 20 cm than has been achieved on previous honeycomb sandwich mirrors. The figure was measured with IR and visible interferometers, using refractive null correctors to compensate 810 microns of aspheric departure. The final measurements were used to calculate synthetic stellar images in a variety of seeing conditions.
Gemini 8.2-m primary mirror no. 1 polishing
Marc Cayrel, P. Beraud, Jacques Paseri, et al.
The 8-m class primary mirrors of the GEMINI Telescopes are thin ULE menisci actively supported. The two mirror blanks are produced by CORNING, the optical figuring, manufacturing and assembling of interfaces are done by REOSC. REOSC is as well in charge of the transportation of the mirror blanks from CORNING to REOSC, and of the shipment of the finished optics to Hawaii and to Chile. The mirror assembly requirements are summarized, the manufacturing and testing methods are addressed. REOSC had to design and manufacture a dedicated active supporting system, representative of the one used at the telescope level. Its design and performance are presented. The manufacturing steps undertaken at REOSC and the results achieved are then detailed: mirror blank surface generating and grinding, polishing, testing. The current status of the mirrors is finally presented.
Handling 20 tons of honeycomb mirror with a very gentle touch
Warren B. Davison, Joseph T. Williams, John M. Hill
The 6.5 meter and 8.4 meter mirrors being produced at Steward Observatory have to be lifted, turned, ground, polished, shipped and installed without exceeding 0.7 MPa (100 psi) stress in the glass. Many pieces of specialized equipment and some innovations are required to do this on a tight budget. We have developed lifting fixtures that are either glued on or held by vacuum. We have also designed turning rings that fill our lab, and transportation boxes to hold the mirror horizontal, vertical or in a ship. The sheer size and mass of the mirrors and equipment, plus the very stringent constraints makes the solutions interesting. This may not be the part of telescope design and construction that attracts the most attention, unless...
Major Projects
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Commissioning of the Italian National Telescope Galileo
The commissioning phase of the Telescopio Nazionale Galileo is started during the first half of 97. Large parts of the drive, the optical and the control system have been mounted at the telescope in site (LaPalma, Canary Islands). The telescope is expected to be ready for the technical first- light during February - March 98 while the instrumentation first-light is expected for mid 98. On this review of the commissioning operations we will describe the problems encountered and the results achieved integrating the main telescope subsystems.
Mirror Support and Active Optics
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Support structure of LAMOST Schmidt plate MA
The Large Sky Area Multiobject Fiber Spectroscopic Telescope (LAMOST) is a very special designed reflecting Schmidt telescope. It will have both the large field of view (5 degrees) and the large aperture (4 meters). The reflecting Schmidt plate MA will not only serve as the correcting plate for eliminating the spherical aberration of the primary mirror MB, but also will serve as the tracking mirror simultaneously during the observation with an alt- azimuth mounting. The support structure of MA, corresponding to its 24 hexagonal submirrors, is composed of 24 hexagonal submirror cells and a MA primary cell which is a space truss structure. This paper presents the design and the analysis for the MA support structure which mainly aided by the finite element method. It includes analyses of gravitational influence, the thermal deformation and the wind load influence. The dimension of the whole support structure is about 6 X 5 X 3 meters. The total mass of the structure is about 2 tons and the lowest natural frequency is 27.3 Hz. During 1.5 hours observation, the maximum image spread due to the gravitational deflection is less than 0.13 arcseconds. While the temperature gradient along the structure in vertical direction is 1 degree(s)C, the image spread due to the thermal deformation of the structure is less than 0.06 arcseconds.
Image quality and active optics for the Gran Telescopio Canarias
Javier Castro, C. Delores Bello, Lotti Jochum, et al.
The conceptual design of the Gran Telescopio Canarias (GTC) has been completed. One of the challenges facing the GTC Project is to obtain excellent image quality using a segmented primary mirror. The segmentation will introduce a physical effect that contributes to significant degradation of image quality. The image quality requirement imposes the use of an active optical system to correct figure instabilities of the optical surfaces and part of the unavoidable fabrication figure errors. The active correlation includes the capability of 5-axis motion of the secondary mirror, 3-axis motion of each primary mirror segment, and 6 active degrees of freedom to deform each segment. A mixed strategy of closed- and open-loop control of the active correction will be implemented. This paper discusses the expected wavefront errors of the GTC, how they are corrected by active optics, and the expected image quality performance in FWHM, (theta) 80 and Central Intensity ratio.
AEOS 3.67-m telescope primary mirror active control system
James E. Kimbrell, David Greenwald
The Advanced Electro Optical System 3.67 m Telescope recently installed at the Air Force Maui Optical Station is the government's newest and largest satellite tracking telescope. The primary mirror is a 23:1 meniscus type with an 84 point actively controlled back support system. Each back support contains a microprocessor and electromechanical actuator to control the figure of the mirror based on the primary mirror reflected wavefront. The primary and secondary mirrors are servo positioned to actively control the telescope alignment. The support hardware, the control implementation, and the performance achieved during factory testing and initial integration are discussed.
Active supports and force optimization for the MMT primary mirror
We describe the active support system and optimization of support forces for the 6.5 m primary mirror for the Multiple Mirror Telescope Conversion. The mirror was figured to an accuracy of 26 nm rms surface error, excluding certain flexible bending modes that will be controlled by support forces in the telescope. On installation of the mirror into its telescope support cell, an initial optimization of support forces is needed because of minor differences between the support used during fabrication and that in the telescope cell. The optimization is based on figure measurements made interferometrically in the vibration- isolated test tower of the Steward Observatory Mirror Lab. Actuator influence functions were determined by finite- element analysis and verified by measurement. The optimization is performed by singular value decomposition of the influence functions into normal modes. Preliminary results give a wavefront accuracy better than that of the atmosphere in 0.11 arcsecond seeing.
Design and tests of the VLT M1 mirror passive and active supporting system
Thierry Hovsepian, Jean Luc Michelin, Stefano Stanghellini
Keywords : Very Large Telescope, Active force control, Hydraulic supporting system design, M I mirror position loop, M 1 supporting dynamic behaviour
Development and performance of Hobby-Eberly Telescope 11-m segmented mirror
Victor L. Krabbendam, Thomas A. Sebring, Frank B. Ray, et al.
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.
Major Projects
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Magellan 6.5-m telescopes project: status report
Construction of the Magellan Project 6.5 meter telescopes is in progress. The enclosure for the first telescope is complete along with the auxiliary building that will house the coatings plant. The mount for the first telescope was shipped to Las Campanas Observatory in October 1997. Currently the telescope is being installed in its enclosure and will be cabled up and under test in the first half of 1998. Primary mirror #1 is being polished by the University of Arizona and will tested on its supports in the telescope cell prior to shipping in Chile in early 1999. The work of re-building the oven for casting the second Magellan mirror has just started. The development of the first suite of science instruments that will include visible band and 1 - 2.5 micron infrared multi-object spectrometers is underway. A two channel Echelle spectrometer is also planned. This paper summarizes the progress to date.
Mirror Coating
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Status of the coating facility of the Subaru Telescope
Saeko S. Hayashi, Yukiko Kamata, Tomio Kanzawa, et al.
One of the major problems to retain the efficiency of a telescope is to achieve and maintain high reflectivity in the wide wavelengths of the coatings of the telescope optics. For coating the large mirrors of Subaru Telescope, we employed the conventional evaporation scheme, in the expectation of uniform coverage of the film. In this paper, we will report the installation and the performance verification of the coating facility. This facility consists of a washing tower for stripping off the old coating, an evaporation coating chamber, two trolleys and a scissors- like lifter for handling the primary mirror. To supply a large number of filaments loaded with uniform quality molten metal, the practical solution is to pre-wet the filaments with the agent metal and keep them in a controlled manner before the evaporation. The aluminum film deposit on the test samples in the 8.3 m coating chamber proved the film thickness uniformity matching with the specification. Reflectivity of the fresh surface was over 90% at visible wavelength. In September 1997, we re-aluminized 1.6 m and 1.3 m mirrors for the first time (at least for ourselves) application to the real astronomical telescopes. The resultant surface reflectivity confirmed the feasibility of using pre-wetted filaments.
Scatter and reflectivity measurements on large telescope mirror coatings
James W. Mayo III, Don H. Killpatrick
The reflectivity and scatter of several large telescope mirrors have been measured immediately after coating and after various use times. Mirrors evaluated inside those of the AEOS 3.67-m Telescope and the SOR 3.5-m Telescope. Reflectivity and scatter measurements were made on the actual mirrors and witness samples using a (mu) ScanTM reflectometer/scatterometer, a Minolta 2002 hand-held spectro-reflectometer, and laboratory spectro-photometers and scatterometers. The reflectivity measurements made on coating witness samples were compared to measurements in round-robin tests by nationally recognized optical measurement laboratories. From the results of the round- robin measurements, correction factors were determined for the hand-held instruments and used to establish actual reflectivities of the large mirrors as a function of location on the mirror and time after coating deposition. Measurements of the reflectivity and scatter of the SOR 3.5- m primary mirror taken immediately after coating, nine months after coating, and 39 months after coating, and measurements of the reflectivity and scatter of the AEOS 3.67-m telescope primary mirror immediately after coating are presented and discussed. Correlations with coating data taken on other large mirrors are also presented. Depending on coating selection, initial coating quality, operational conditions, and cleaning procedures, coating lifetimes may vary from less than one to more than five years.
Development of silver coating options for the Gemini 8-m telescopes project
Michael Ray Jacobson, Ruth C. Kneale, Fred C. Gillett, et al.
Acting as a prime contractor to the Gemini Project, the Optical Data Associates (ODA), with its major subcontractors, BOC Coating Technologies (BOCCT) and Deposition Sciences, Inc. (DSI), developed options for depositing protected silver coatings on the 8-M primary mirrors. The project began with a study that identified sputtering as the preferred deposition technique, defined a set of candidate adhesor and protective coatings for the silver layer, and simulated stack performance. The next phase involved pilot magnetron sputtering studies by BOCCT and DSI of designs involving silicon nitride and hafnia, respectively. ODA also developed mid-IR reflectance standards at (lambda) equals 10.6 micrometers to control the silver coating measurements in the critical 8 - 12 micrometers atmospheric window. The study results were successful, with both BOCCT and DSI producing Ag coatings with R equals 0.9920 +/- 0.0001 and protected Ag coatings with R equals 0.9910 +/- 0.0001. The Gemini Project coating plants are designed to sputter bare and protected Al and Ag coatings.
Telescope Enclosures and Thermal Control
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Present status of the GTC installations design
German R. Pescador, Javier Serrano
The maximum exploitation of the excellent astronomical quality of the `Roque de los Muchachos' Observatory is one of the most important design criteria for the GTC Project. The installations of the GTC are being designed in such a way as to minimize local seeing degradation while providing adequate support for the operation and maintenance of the facility. The paper will firstly describe the options that have been adopted as a solution for specific subsystems of the installations, such as: the dome, the auxiliary service areas or the thermal control, with special attention to the environmental conditions in the telescope chamber. Other subjects discussed are: the solutions proposed to minimize the possibility of turbulent surface-layer air negatively affecting astronomical observations; the minimization of wind-induced telescope vibrations and transmission of vibrations between the separate foundations of the buildings and the telescope pier through the ground; the guidelines to carry out the tasks of operating and maintaining the GTC and of course, the solutions for protection against adverse weather conditions.
Conversion of the MMT to a 6.5-m telescope: the optics support structure and the enclosure
Joseph Antebi, Donald O. Dusenberry, Atis A. Liepins
The Multiple Mirror Telescope, located on Mt. Hopkins AZ, previously used an array of six telescopes on a single mount to achieve an effective aperture of 4.5 m. It is now being converted to a 6.5-m telescope with a single primary. The design for the converted telescope includes provision for three interchangeable secondaries. Difficulties and risk associated with handling large mirrors dictated that the realuminizing of the 6.5-m primary be performed in-place. This requires the use of an onboard vacuum chamber. The observing chamber in the rotating observatory building had to be enlarged to accommodate the longer telescope, and sliding doors were built into the rear wall to improve air circulation through the chamber. Described are the objectives, constraints, and the development of the design, from concept to implementation, of the Optics Support Structure for the 6.5-m telescope. Also described are the modifications to the observatory building.
New Concepts and Approaches
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Design of a large low-scattered light telescope for solar observations
This paper is an interim report of a feasibility study which is in progress for a large 400 cm aperture solar telescope (`CLEAR'). Unlike other large solar telescopes constructed in the last three decades, CLEAR does not use the concept of evacuated telescopes to eliminate internal seeing. The requirement for full access to the far infrared spectral region (> 2.5 micrometers ), and for low scattered light, eliminates the use of the entrance window which evacuated telescopes require. Instead, CLEAR avoids internal seeing by carefully controlling the internal thermal environment of the telescope by a number of means: (1) thermal control of the primary mirror; (2) flow of ambient air over the primary mirror surface and in the telescope; (3) locating the primary focus outside the telescope beam and enclosure where the heating resulting in concentrated sunlight can be managed better (this requires the use of an off-axis primary mirror); and (4) the use of a prime focus heat stop/absorber. In addition to controlling the internal seeing, such a configuration produces a telescope with very low scattered light characteristics, allowing quality observation of regions outside the solar limb and of sunspots. By eliminating the need for a large entrance window, the CLEAR concept therefore opens up the possibility of larger aperture solar telescopes. Notwithstanding its off-axis configuration, the Gregorian telescope produces excellent images (< 0.1 arcsec) over a 5 arcminute diameter field-of-view at the f/130 Gregorian focus. In addition to the four instrumentation stations near the Gregorian focus (i.e., direct Gregorian, Nasmyth, two `folded Gregorian'), the design provides for extensive instrumentation locations in a coude area. By means of a 3- level rotating coude platform, large instruments can be located at respectively f/30, f/45 and f/60 foci.
Advances in ground-based and space-based reflecting coronagraph designs
Raymond N. Smartt, Serge L. Koutchmy
A program to develop ground-based emission-line solar coronagraphs based on super-polished primary objective mirrors has been underway at the National Solar Observatory/Sacramento Peak over the last several years. The fundamental design requirements for effective coronagraph performance are discussed. The instrument currently under development has a 60-cm diameter objective with a 700-cm primary focal length. This design has been refined with an emphasis on achieving an extremely low level of instrumental stray light, suitable for both visible and IR operation and for carrying out high-precision polarimetry. It is seen as a possible prototype for a new generation of much larger aperture, low-scattered-light solar telescopes. Special applications include measurements of the signatures of waves in coronal loops, transient events such as coronal loop interactions, the spatial variation of magnetic fields in prominences allowing computation of electric currents, and the determination of the magnitude of coronal magnetic fields. For applications in space, a white-light reflecting coronagraph has been developed under the USAF program, SWATH (Space Weather and Terrestrial Hazards). This novel catadiopric design has a 10-cm diameter superpolished primary objective mirror, and a unique external occulter. It has a significantly higher throughput and resolution as compared with conventional space-borne lens-objective coronagraphs. Two basic modifications of this design are proposed that result in an in-line symmetric configuration, with a decrease in mass, while improving the scattered-light performance. Some special applications are described.
Sloan digital sky survey 2.5-m telescope: light baffles
Walter A. Siegmund, Siriluk Limmongkol, Charles L. Hull, et al.
The Sloan Digital Sky Survey 2.5-m telescope is unique with a 3 degree(s) field of view at f/5. The two-mirror optical design includes two transmitting correcting elements. To avoid excessive central obstruction of the entrance pupil, a conical baffle is necessary in addition to the usual primary and secondary baffles. This conical baffle is suspended approximately midway between the primary and secondary mirrors. In addition, an exterior close-fitting wind and light baffle, not found on most modern telescopes, blocks rays at large angles from the field of view. The baffle design was analyzed using stray light analysis software. Scattered light as a function of source angle was calculated and the dominant scattering surfaces were identified. For sources near the field of view, the uniformity of scattered light over the focal surface was determined.
Chopping Secondary Mirror Systems
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Extremely compact secondary mirror unit for the SOFIA Telescope capable of 6-degree-of-freedom alignment plus chopping
Lorenzo Zago, Pierre M. Genequand, Joseph Moerschell
SOFIA is a 2.5-m telescope to be carried on a special Boeing 747 for airborne observations at about 15'000 m. The paper describes the main features of the secondary mirror unit. The SOFIA secondary mirror needs active control for alignment along five degrees of freedom as well as for very fast chopping with a frequency up to 20 Hz. Moreover the general optical concept and the housing of the telescope inside a Boeing 747 have required the design of a very compact mechanism: indeed while the secondary mirror has a diameter of 350 mm the entire height of the secondary mirror unit (including the mirror) cannot be greater than 300 mm, which makes the SOFIA design much more compact than any other similar project. The objective is achieved by a very tight integration between a novel hexapod mechanism, in charge of tilt offsets and alignment along 3 axes, and a fast chopping mechanism based on advanced flexure structure technology. In the hexapod mechanism (which is in fact capable of 6-dof), the six linear actuators are arranged in an original geometry in order to leave as much space as possible to the overlying chopping system. Also, the actuators' `hinges' are here materialized by flexure elements. Three motorized levers are linked by flexure elements to the mirror isostatic interface as well as to a reaction ring for compensating angular momentum, which is mechanically driven together with the mirror. This a major difference from other designs (e.g. Keck or VLT) where the compensation mass is driven and controlled separately. The SOFIA solution obtains thus various advantages in term of used volume and has a simpler control system. Various details of the chopping mechanism are provided in the paper. Simulation preliminary results are also given.
VLT secondary mirror unit performance and test results
Reiner Barho, Stefano Stanghellini, Georg Jander
This paper explains the overall mechanical functions of the Very Large Telescope Secondary Mirror Unit (VLT-M2 Unit), and the method of their verification by tests inside two different test adapters. The cinematic mechanisms of the M2 Unit are the Focusing Stage, Centering Stage and the Chopping Assembly which enables to adjust the position of the secondary mirror along 5 degrees of freedom. All these mechanisms are characterized by very high stiffness and accuracy, long lifetime and high reliability. The presentation of the finally achieved performance test results of the M2 Unit, realized by their high precision mechanisms, are the major points of this presentation.
Design of the infrared fast steering mirror chopping control system for the Keck II Telescope
Jean-Noel Aubrun, Kenneth R. Lorell, Gregory J. Feher, et al.
The Keck 2 ten meter telescope will utilize an advanced chopping secondary mirror in order to enhance observations in the infrared. The Infrared Fast Steering Mirror (IFSM) can execute a square-wave chop at frequencies as high as 25 Hz with an accuracy of +/- 0.1 arcsec. Chopping can be synchronized by focal plane instruments, and the system can simultaneously perform high-performance chopping as well as beam-steering (for atmospheric correction), providing the Keck telescope with greatly enhanced capability. Details of design, testing, and performance of the Keck 2 IFSM are presented in this paper. The mirror is controlled by three voice coil actuators. Reaction forces generated by the actuators are absorbed by a reaction mass suspended from the main IFSM structure. Motor driven springs are used to minimize power dissipation in the actuators. The IFSM all- digital control system uses a unique adaptive algorithm that forces the mirror to precisely follow the commanded chop waveform. Tests use various computerized instruments: a three-axis laser interferometer for calibration and stability, a 6-axis dynamometer to evaluate reaction forces transmitted to the telescope. In addition to specifics of the design, performance, and testing, a video illustrating details of the IFSM hardware and showing it in operation will be presented.
Test and performance evaluation of the Gemini secondary mirror chopper and position control system
Kenneth R. Lorell, Jean-Noel Aubrun, Ernesto O. Perez, et al.
This paper describes the testing and performance evaluation of the Gemini secondary mirror control system. The mechanism and control system have demanding requirements for dynamic performance, precision position control throughout the mirror chop cycle, and the virtual elimination of residual forces and torques. The test procedures and hardware required to measure and verify the system performance were specifically developed for this application. The tests utilize a special computer-controlled laser interferometer to calibrate the mirror position sensors. Dynamic chopping performance of the system is also tested and verified. A range of chop waveform parameters; amplitude, frequency, and duty cycle, is employed to fully exercise the control system and electromechanical hardware. Measurements of angular stability and repeatability under dynamic chop conditions are made to verify performance. Effectiveness of the active force cancellation system is evaluated using a six-axis digital dynamometer.
Design and manufacture of ULE glass chopping secondary mirrors
W. Scott Smith, Mary J. Edwards
ULETM titania-silica binary glass is being used for the primary, secondary, and tertiary mirrors for the Subaru 8- meter Japanese National Large Telescope. The primary 8.3- meter mirror blank was made by Corning Incorporated in 1994, and is in the final stages of optical polishing at Contraves Brashear Systems. Corning has also manufactured two of three lightweight secondary mirror blanks to date, and has delivered two lightweight tertiary blanks. These lightweight mirrors were designed and analyzed by Mitsubishi Electric Corporation and Contraves Brashear Systems to meet performance requirements. This paper describes key design criteria for the chopping secondary mirrors, including finite element analysis results. The manufacturing process used by Corning is also described.
VLT beryllium secondary mirror no. 1: performance review
Marc Cayrel
The four Very Large Telescope secondary mirrors are 1.2-m Beryllium lightweight convex mirrors. REOSC has been selected for the design and manufacturing of the optics and of their supporting system. The first mirror unit has been delivered in September, 1997. Operating from visible to near infrared, the mirror defines the telescope aperture stop and may be chopped during observation. The optical requirements are tight and a high stiffness, low weight and inertia are requested as well. Using beryllium is a technical challenge for such a large optic manufacturing, in particular regarding its stability. The requirements and design are presented, we review the mirror manufacturing steps: blank production, machining, grinding, Nickel plating, polishing, integration and testing. The optical quality control method, a problem for large convex mirrors control, is detailed. The results of acceptance testing of mirror No. 1 are summarized, we present conclusions about the mirror figure stability. The status of the three additional mirrors manufacturing is presented to conclude.
Poster Papers on Chopping Secondary Mirror Systems and Next-Generation Concepts
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Possible design and fabrication approach for the optics of the 25-m telescope
The optical prescription of the primary mirror of an astronomical telescope is an essential design driver, with major implications on cost and complexity. The sheer size of an alt-az 25-m class telescope emphasizes the need for a short focal ratio of the primary mirror. The optical solution for the 25-m telescope is that of a 4-mirror axial design. Cost and fabrication constraints require the segmented primary mirror to be spherical; on similar grounds, we conclude that the same statement most likely applies to the secondary mirror as well. This makes the correction of spherical aberration and field aberrations comparatively more complex than with a 2-mirror Ritchey- Chretien design, and the very high asphericity of the fourth mirror will constrain the minimal achievable focal ratio of the primary mirror. A parametric study will be presented, showing the available field of view and the difficulty of fabrication of the optics of the telescope as a function of the f/ratio of the primary mirror. The peculiar properties of the design will be reviewed, and it will be shown that the f/ratio of the primary mirror shall most likely fall in the f/1.20 to 1.40 range, as a result of fabrication, testing and alignment constraints. Novel optical testing methods will be proposed as well.
Next-Generation Concepts
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Diffraction-limited 8- to 20-m telescope with an active and adaptive tertiary
Active Optics Methods are extremely performing to obtain highly aspherical mirrors. The development of these methods is underlined with the presently proposed 4 to 5 mirror large telescope in the 8 - 20 m class. In this design, a particular emphasis has been placed to achieve the following main features: diffraction limited images over a restricted 1 or 2 arcmin field of view at f/30 for separate use with 16 Mpx detectors, diffraction limited images over a 2 to 5 arcsec field of view at the interferometric Mersenne focus, fast primary mirror of spherical shape, next mirrors the smallest as possible, low asphericity secondary and a tertiary spherically polished. It was found from optimization process with, for instance a 8 m primary at f/1.75, that, of the four mirrors required to achieve a diffraction limited afocal beam to a Mersenne focus, the spherical tertiary is the only mirror to be actively aspherized. Thus, due to a small aperture of this mirror (0.7 m), the vase form already developed from elasticity analysis would allow accurate aspherization by active optics. Similarly to our previous proposal TEMOS, the tertiary figure at f/6 can be achieved in situ, from a spherical polishing, by air depressure inside the mirror. The Sphe3 deformation sag of this mirror is quite large since is congruent to 1.3 mm ptv. Further, slight variations of the loading intensity allow a full optimization of the system as a function of the selected spectral range. The telescope design includes a pupil transfer on the tertiary realized by a doublet-lens. Thus, an adaptive optics system can be co- added to the same mirror. Finally, a concept for an active- adaptive tertiary is proposed.
Future of filled aperture telescopes: is a 100-m feasible?
Roberto Gilmozzi, Bernard Delabre, Philippe Dierickx, et al.
We explore the scientific case and the conceptual feasibility of giant filled aperture telescopes, in the light of science goals needing an order of magnitude increase in aperture size, and investigate the requirements (and challenges) these imply for possible technical options in the case of a 100 m telescope. The 100-m f/6.4 telescope optical concept is of a four mirror design with segmented, spherical primary and secondary mirrors, and 8-m class aspheric tertiary and quaternary mirrors, providing a 3 arc minutes field of view. Building on the experience of the VLT and other large telescope projects, we investigate mirror fabrication issues, a possible mechanical solution, the requirements for the absolutely essential adaptive optics system and for the instrumentation package, and the implications for budget and schedule.
Extremely large telescope: further adventures in feasibility
Thomas A. Sebring, Frank N. Bash, Frank B. Ray, et al.
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.
Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
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Large Zenith Telescope project: a 6-m mercury-mirror telescope
Paul Hickson, Ermanno F. Borra, Remi Cabanac, et al.
The Large Zenith Telescope is a zenith-pointing telescope with a 6-meter diameter rotating mercury mirror. Located in mountains near Vancouver, Canada, it is expected to see first light in 1998. Equipped with a low-noise drift- scanning CCD camera, the telescope will survey a 17-arcmin- wide strip of sky using a set of medium-band filters. The data are expected to provide spectral energy distributions and photometric redshifts for over a million galaxies which will form a base for studies of galaxy evolution and large- scale structure.
Next-Generation Concepts
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SOLIS: a modern facility for synoptic solar observations
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite of instruments that will modernize and greatly improve synoptic solar observations carried out by the National Solar Observatory. It will provide fundamental data necessary to understand the solar activity cycle, sudden energy releases in the solar atmosphere, and solar spectral irradiance changes. State-of-the-art instrumentation and data collection techniques will be employed to enhance both the quality and quantity of data. A high degree of automation and remote control will provide faster user access to data and flexible interaction with the data- collection process. The instruments include a vector spectromagnetograph that will measure the magnetic field strength and direction over the full solar disk in 15 minutes, a full disk patrol delivering digital images in various spectral lines at a high cadence, and a Sun-as-a- star precision spectrometer to measure changes in many spectral lines.
Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
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SUMMIT: the Sydney University Multiple Mirror Telescope
Anna Marie Moore, John Davis
High resolution spectra are required to complement the angular dimensions of pulsating stars and binary systems determined with the Sydney University Stellar Interferometer. To meet this need a multiple mirror telescope feeding a spectrograph via optical fibers is being developed. The design has been inspired by the Multi- Telescope Telescope project at Georgia State University but much of the design of the Sydney University Multiple Mirror Telescope is new. In particular, an adjustable mirror cell has been designed to allow the accurate alignment of the starlight on the input end of the fiber in its focal plane. Alignment tests carried out on a prototype cell are reported.
Intranet approach to the GTC configuration management
Alvaro Medina
Due to its size and complexity the GTC Project represents a challenge for configuration management. On the one hand there is a need to record and store the enormous amount of configuration documentation generated during the different phases of the project (technical specifications, engineering drawings, interface control documents, acceptance tests, etc.), so that it can be easily accessed on-line by an Project Office member. Moreover, this documentation has to be properly related to the different configuration elements and interfaces composing the GTC's Product Tree. On the other hand, the need arises to simplify all the configuration control procedures established by System Engineering and their associated work flow (document and baseline approval, configuration change requests and evaluation, approval or rejection of those changes, etc.). Intranet technology has proven to be an excellent and innovative tool for satisfying both needs in an integrated manner. This integrated philosophy of the GTC intranet application helps each member of the Project Office fulfill his or her own responsibilities, reduces paper work and minimizes errors. It also assists in locating and accessing any configuration information with a single tool (any standard Internet navigator) regardless of the original format of such information. This paper presents the main features of this intranet application, as well as the intended future extension to other management disciplines (project management, quality management, etc.).
Fabrication of the secondary mirrors for the W.M. Keck telescopes
Joseph S. Miller, David F. Hilyard, Terry S. Mast
Fabrication of the secondary mirrors for the W. M. Keck Telescopes required advances in techniques and tools for grinding, polishing, and testing. We describe the development and performance of those techniques and tools.
Status of the secondary mirrors (M2) for the Gemini 8-m telescopes
Ernst-Dieter Knohl, Armin Schoeppach, Michael A. Pickering
The 1-m diameter lightweight secondary mirrors (M2) for the Gemini 8-m telescopes will be the largest CVD-SiC mirrors ever produced. The design and manufacture of these mirrors is a very challenging task. In this paper we will discuss the mirror design, structural and mechanical analysis, and the CVD manufacturing process used to produce the mirror blanks. The lightweight design consist of a thin faceplate (4-mm) and triangular backstructure cells with ribs of varying heights. The main drivers in the design were weight (40 kg) and manufacturing limitations imposed on the backstructure cells and mirror mounts. Finite element modeling predicts that the mirror design will meet all of the Gemini M2 requirements for weight, mechanical integrity, resonances, and optical performance. Special design considerations were necessary to avoid stress concentration in the mounting areas and to meet the requirement that the mirror survive an 8-g earthquake. The highest risk step in the mirror blank manufacturing process is the near-net-shape CVD deposition of the thin, curved faceplate. Special tooling and procedures had to be developed to produce faceplates free of fractures, cracks, and stress during the cool-down from deposition temperature (1350 C) to room temperature. Due to time delay with the CVD manufacturing process in the meantime a backup solution from Zerodur has been started. This mirror is now in the advanced polishing process. Because the design of both mirrors is very similar an excellent comparison of both solutions is possible.
Next-Generation Concepts
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Sloan digital sky survey 2.5-meter telescope: optics
Patrick G. Waddell, Edward J. Mannery, James E. Gunn, et al.
The Sloan Digital Sky Survey 2.5-meter telescope optical design is optimized for wide field (3 degree(s)), broadband (300 nm to 1060 nm) CCD imaging and multi-fiber spectroscopy. The system has very low distortion, required for time-delay-and- integrate imaging, and chromatic aberration control, demanded for both imaging and spectroscopy. The Cassegrain telescope optics include a transmissive corrector consisting of two aspheric fused quartz optical elements in each configuration. The details of the fabrication of these elements are discussed. Included is the design, development and performance of custom optical coatings applied to these optics.
Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
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Long-term reliability of large ULE mirror blanks
Suresh T. Gulati, Mary J. Edwards
ULETM, the titania-silica binary glass with zero expansion coefficient, is an ideal material for large telescope mirror blanks due to the unique combination of its optical, thermal and mechanical properties--together with the ease of fabrication--which help meet performance and durability requirements in a cost-effective manner. Indeed, the 8m class Subaru and Gemini telescope mirror blanks have been fabricated successfully from ULE glass and will be in full operation in the not too distant future. This paper will focus on the stringent reliability requirements which the mirror blank must meet during fabrication, transportation, installation and operation atop high mountains with extreme environmental fluctuations. In particular, the paper will present strength and fatigue data for ULE glass as a function of surface finish. Such data are critical for selecting the appropriate surface finish to ensure mechanical reliability of the mirror blank at various stages of fabrication and during transportation. The use of Weibull statistical distribution for surface flaws combined with Power Law fatigue model helps arrive at a safe stress level which should not be exceeded to ensure the mechanical reliability of the mirror blanks. The safe stress level is verified through independent static fatigue tests on ULE discs with surface finish identical to that of the mirror blank. In this manner the mechanical reliability of large ULE mirror blanks can be ascertained at extremely low failure probabilities. The successful application of reliability model to both Subaru and Gemini mirror blanks will be illustrated.
Mirror Fabrication, Polishing, and Handling
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Casting the first 8.4-m borosilicate honeycomb mirror for the Large Binocular Telescope
John M. Hill, James Roger P. Angel, Randall D. Lutz, et al.
We report on the casting of the first 8.4 meter diameter borosilicate honeycomb mirror at the Steward Observatory Mirror Laboratory. This blank will become the world's largest monolithic glass telescope mirror, and is the first of two mirrors for the large Binocular Telescope Project. The honeycomb 8.4 meter mirror was cast from 21 tons of E6 borosilicate glass manufactured by Ohara. This glass is melted into a mold constructed of aluminosilicate fiber to produce a honeycomb structure with roughly 20% of solid density. The 1662 hexagonal voids that form the honeycomb structure are produced by ceramic fiber boxes bolted to the bottom of the mold with SiC bolts. The furnace rotates at 6.8 rpm during the casting process to produce the F/1.14 paraboloid on the front surface. This shaping minimizes the amount of glass which must be removed during the grinding process. The front faceplate of the mirror will be 28 mm thick after generating and the back faceplate will be 25 mm. The overall thickness of the finished honeycomb blank is 89 cm at the outer edge and 44 cm at the central hole. The first 8.4 meter mirror blank was cast in January 1997. During the casting, two tons of glass leaked from the mold inside the spinning furnace. After a three month annealing cycle the furnace was opened for inspection. As a result of the leakage about 2 square meters of the faceplate near one edge of the mirror was too thin to be polished. In April 1997, an additional two tons of glass was loaded on top of the intact honeycomb structure. In June 1997, after heating slowly back to the annealing temperature, this extra glass was flash melted onto the front of the blank to assure that the faceplate was of sufficient thickness. After a further three month annealing cycle, the furnace was re-opened to reveal a superb casting with low bubble content and little trace of the fusion boundary. The blank has been removed from the furnace using a fixture glued to the upper surface of the blank. It will soon be stripped of its mold material in preparation for polishing.
Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
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Large-size laser collimator with phase conjugation compensation for primary mirror distortions
Michael V. Vasil'ev, Alexey Leshchev, Pavel A. Sergeev, et al.
The novel scheme of laser collimator for optics testing and evaluation purposes is proposed, using the principles of the bypass telescopes with the nonlinear-optical compensation for the primary mirror distortions.
Poster Papers on Mirror Support, Active Optics, Coatings, and Telescope Enclosures
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Coating experiment with 1.6-m vacuum evaporation chamber
Yukiko Kamata, Saeko S. Hayashi, Takeshi Noguchi, et al.
We have conducted a series of coating experiments using the newly installed 1.6 m evaporation chamber at the Advanced Technology Center (ATC) of the National Astronomical Observatory of Japan. The main task of this chamber is to re-aluminize the 1.6 m mirror of the Infrared Simulator at the ATC. The design concept of the 1.6 m chamber is basically the same with the 8.3 m coating facility for Subaru Telescope. Therefore, we could utilize this chamber to evaluate the fundamental performance of the larger chamber. The extensive coating experiments were done in the spring, autumn of 1996, and autumn of 1997. Reduction of the number of the filaments has lead to the increase in their size, which caused difficulty in the annealing process. Attempts are focused on securing the sufficient metal loads on the filaments. Then the filaments are fired to measure the spray pattern of a single filament exposure, or the uniformity pattern resulted from the full setup of filament arrays. Using small slide glasses, the important parameters of the resultant reflecting film that are the thickness, the uniformity of the thickness, and the spectroscopic reflectance are measured. The absolute value of the reflectivity is estimated to be around 91% immediately after opening the chamber. In order to cover a wide range of observing wavelengths for the Infrared Simulator, and eventually for the optical-IR Subaru Telescope, it is necessary to seek after a higher evaporation rate with these chambers.
Novel kinematic equatorial primary mirror mount
Robert E. Parks, R. Kent Honeycutt
Although there is great interest in mounts for very large telescopes, there are still many new telescopes in the I to 3 m range that require careftilly engineered mounts to achieve diffraction limited performance. In addition, the very large telescopes have associated large instruments with optics in the I to 2 mrange that need careful mounting. This paper addresses mounting schemes for optics in this size range. .. Becauseall materials are so relatively flimsy ifthey are expected to hold their shape to a few nanometers, all but the smallest optics must have mounts that are well conceived. This means the design must be based on the kinematic principle of not over-constraining the optic. Even the modestly large optics ofprojects like the National Ignition Facility (NIF) can benefit from following kinematic principles. The problem is exacerbated in most telescopes because they operate over such a large temperature range and cost prohibits using structural materials that closely match the coefficients of expansion of most mirror substrate materials. Given that there is the need to mount moderately large optics and that the mounting must be well designed, there should be an approach to the problem that does not require zero based engineering for every new mount. The approach should also use commercially available components wherever possible and be lightweight. All these aspects ofthis approach to mount design should be useful in reducing costs. By utilizing a systematic approach to kinematic mount design that is built around readily available components, we show there is a rather universal solution to mounting mid-size mirrors. We outline such a design strategy and illustrate it using the new indiana University 1 .25 m telescope intended for unattended synoptic spectroscopy. The design is applied to both the primary and secondary mirrors, lightweight, gas-fusion bonded sandwich structures made by Hextek Corp.'
Mirror cell and active support system of the Mexican infrared optical telescope (TIM)
Elfego Ruiz Schneider, Erika Sohn, Irene Cruz-Gonzales, et al.
We present the conceptual design of the primary mirror support system of the 7.8 m Mexican Infrared-Optical Telescope. The primary mirror consists of 19 hexagonal off- axis parabolic Zerodur segments, which are carried by a tubular, lightweight and high stiffness cell structure. Each segment is actively supported by 19 pneumatic actuators, that cover the whole back area and provide a uniform force distribution. The array of actuators will be able to correct for high order aberrations. Each of these actuators contains a hydraulic damping system to provide a stiff coupling to the tubular cell to sustain the wind buffeting. The tip/tilt and piston control of each segment will be done through three axial, nanometer resolution position defining actuators. The lateral positioning of each segment is performed through 3 independent electro-mechanical actuators. With the combination of the whole set of actuators and differential positioning sensors, the phasing or coherent superposition of images of the segments, will be more feasible. The whole system will be cost effective, since several subsystems have already been tested on our 2.1 m telescope.
New Concepts and Approaches
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Gran Telescopio Canarias: current status of its optical design and optomechanical support system
The conceptual design of the GTC optical system was completed in summer of 1997. It will be a Richey Cretien type telescope with a flat tertiary mirror to feed Nasmyth and folded Cassegrain focal stations. The telescope will have a segmented primary mirror and an entrance pupil area equivalent to a circular aperture of 10 m diameter. The details of its optical- and opto-mechanical design have been chosen to meet the scientific requirements such as excellent image quality in a broad wavelength range (0.3 microns < (lambda) < 15 microns) and maximum operational efficiency. To achieve the required specifications, all optical elements, in particular the primary mirror segments need to be equipped with a high performance opto-mechanical support system. In this paper we present the results of the optical- and opto-mechanical design of the GTC.
Phasing the Keck Telescope with out-of-focus images in the infrared
We describe a method for phasing segmented optics which makes use of a novel variation of the established technique of curvature sensing. In traditional curvature sensing, the difference between inside-of-focus and outside-of-focus images provides a direct measure of the curvature of a relatively smooth wavefront. We illustrate how this approach can be extended to enable one to measure and correct the discontinuous wavefronts associated with segmented mirrors. A detailed algorithm, based not on curvature measurement, but on correlation of the `difference image' with theoretical images or templates, is presented. In a series of tests of this `Phase Discontinuity Sensing' or PDS algorithm at the Keck 1 Telescope, at a wavelength of 3.3 microns, the RMS piston error (averaged over the 36 primary mirror segments) was repeatedly reduced from about 230 nm to 40 nm or less. Furthermore the PDS phasing solution was shown to be consistent with our previous `phasing camera' results (to within 66 nm RMS), providing strong independent confirmation of this earlier approach.
Poster Papers on Mirror Support, Active Optics, Coatings, and Telescope Enclosures
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Dual-wavelength random-phase-shift interferometer for phasing large segmented primaries
Kjetil Dohlen, Frederique Decortiat, Francois Fresneau, et al.
For active phasing of large segmented primary mirrors with thin, actively shaped controlled segments, we propose the use of a new, random phase-shift, dual wavelength, interferometric technique. We have considered a Michelson- type interferometer with diode lasers placed in the center of curvature of the primary, but another version employing a Mach-Zehnder interferometer with a pinhole in one arm placed in the telescope focal plane and using real or artificial star sources may also be considered. Results from a demonstration experiment are presented and discussed.
Tracking system of LAMOST telescope
Zheng qiu Yao, Guoping Li
The Large Sky Area Multiobject Fiber Spectroscopic Telescope (LAMOST) has an extraordinary structure, its spherical primary mirror MB is fixed. The optical axis from MA to MB is tilted 25 degree(s) from horizontal direction. This structure has the advantage of compact, light, low cost, it is similar to a horizontal Meridian Circle, but different in many structure details. The MA has the alt-azimuth mounting. While observing, MA should be rotating in altitude and azimuth direction to tracking the celestial objects, at the same time the focal plane should be rotating for certain angle. For observing different sky area the focal plane will tilt a small angle. When the temperature changes, the focal plane can focusing. This paper describes the preliminary design of the tracking system for LAMOST telescope. That include the Schmidt corrector MA system and focal plane system. For the MA system there are driving and supporting mechanism of altitude axis, diving and supporting mechanism of azimuth axis. For the focal plane system, there are five subsystems: base, frame, field rotator, focusing mechanism and tilting mechanism. The analyses for the tracking system are presented in this paper.
Mirror Support and Active Optics
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Active optics at UKIRT
Antonio C. Chrysostomou, Nicholas P. Rees, Timothy G. Hawarden, et al.
The 3.8 m United Kingdom Infrared Telescope (UKIRT) has recently installed active control of the primary mirror figure, taking advantage of aspects of the original mirror design, which permits the correction of low order aberrations. In this paper, we present results from a campaign of all-sky wavefront sensing carried out UKIRT. As a result of the campaign, a lookup table is being used to correct for attitude dependent astigmatism, while fixed corrections are applied to trefoil and spherical aberrations. Coma is removed by secondary mirror alignment. A continuous, model based, correction of focus for thermal and elastic effects is also applied. Accurate focus is now maintained throughout an observing night.
Poster Papers on Chopping Secondary Mirror Systems and Next-Generation Concepts
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In-situ cleaning of the primary mirror of the Subaru Telescope
Yasuo Torii, Saeko S. Hayashi, Masahiro Toda
The factors degrading optical performance of the telescope optics are the deterioration of the reflecting coating itself on the surface, and the accumulation of the contamination on the mirror surface. We consider that fine cinders at the Mauna Kea summit are blown into the enclosure by wind and get stuck on the optical elements, particularly on the primary mirror as it has large area and it will be looking up the sky for long hours during its operation. Contamination on the primary mirror surface decreases its reflectivity and increases its emissivity and scattering. These will affect the observational efficiency of the Subaru telescope, for it will be used in the wide range of the wavelength. Not only the decrease in reflectivity but also the increase in the emissivity are of major concern in the infrared region. In order to prevent the accumulation of the contaminant particles, which cleaning technique could be applicable for the large telescope optics? Several cleaning methods to replace freon washing are devised to clean silicon wafer in its production process in the semiconductor industries. We adopted basic concept from such techniques and made experiments in the hope of using for the preventive maintenance of the telescope mirrors.
Conjugate Cassegrain telescopes for thermal source FTIR spectral radiometric calibration
Martin Wolk, Robert J. McGillicuddy, Joseph E. Zurlinden
Two Cassegrain telescopes were constructed to function as sender and receiver for an FTIR spectrometer primarily for the purpose of obtaining spectral data for analysis of military night vision emission targets, and spectral calibration of external variable temperature thermal radiation sources, utilizing freezing-point type blackbodies for primary radiation temperature standards. The sender and receiver telescopes, F/7 and F/5, respectively, each employ 0.30 m (12 in) diameter primary and 0.15 m (6 in) diameter secondary, protected Ag coated Zerodur mirrors. In operation, a thermal target image formed by the sender, whose optical axis is aligned with that of the receiver and spectrometer, is transmitted to and brought to a focus at the spectrometer entrance aperture by the receiver telescope. With (lambda) /8 p-v optical surface accuracy at 633 nm, telescope system tests indicate near diffraction- limited performance in the visible, and 2.81 mrad (full) FOV with further reduction achieved with field stops. Wavelength range capability of the commercially available FTIR instrument employed is approximately 0.22 micrometers (55000 cm-1) to 22 micrometers (450 cm-1) with wavenumber resolution of about 0.013 cm-1 in the IR to 0.769 micrometers (13000 cm-1). In this paper, the techniques and tests employed for the telescope mirror construction are described. An innovative technique for secondary alignment for Hindle's tests of a Cassegrain utilizing a He-Ne laser is presented. Telescope mountings for positioning and alignment with the FTIR are briefly discussed, as well as radiometric and calibration parameters for the integrated system.
Novel polarimetric sensors for remote sensing and astronomical observation
Blair A. Barbour, Michael W. Jones
Nichols Research Corporation is currently developing innovative imaging polarimetric sensors for a number of applications such as mine and minefield detection, aircraft ice detection, and remote sensing. The wave bands in which the various sensors operate include the visible, mid-wave infrared (IR), and long-wave IR bands. This paper will summarize the current research that Nichols is conducting in the field of remote sensing using imaging polarimetric cameras. The polarization signatures of various targets, acquired from ranges up to 10 kilometers, will be presented for all three wave-bands. The benefits obtained using polarimetric imaging will be discussed along with potential applications for this innovative technology including possible astronomical observation applications.
Parallel controllable optical fiber positioning system for LAMOST
Xiaozheng Xing, Chao Zhai, Huasheng Du, et al.
In present paper we have proposed an optical fiber positioning system for LAMOST (Large Sky Area Multi-Object Fiber Spectroscopy Telescope). In this system, the convex focal plate of the telescope (Its diameter is 1.75 m) is divided into about 4000 individual domains. Each domain contains a controllable unit which is named X-unit. Each unit is specified with the position of its domain with polar coordinates (the angle and the radius distance from the center) by using its center point as reference. Each X-unit can be moved smoothly by verifying the two parameters of polar coordinates. An optical fiber is hold with X-unit and introduced to spectroscope, thus the optical fiber can be moved anywhere within its designated circular domain at the convex focal surface. In addition, the circular domain are overlapped in a `honeycomb-like' arrangement to ensure that there are no blind spots on the convex focal surface, such as that the radius distance of the units is 25 mm, and the positioning range of the optical fibers is 30 mm, thus there will be no `blind areas'. The structural parameters of the X-unit could also be optimized to limit the directional error of the axis in a certain range. Each unit is driven by two stepping motors and mobilized by computer in a way to ensure that the units do not collide with each other. It is possible that very high or very low star densities in areas of the field of view may reduce the observational efficiency of the telescope. This should not a significant problem by using optimization of the observation program.
Tip-tilt and chopping mechanism for the Subaru Telescope IR secondary mirror unit
Noboru Itoh, Yasushi Horiuti, Kouki Asari, et al.
The Subaru Telescope introduces a tip-tilt and chopping mechanism for an IR secondary mirror, which is of ULE light weight type, 1.3 m in diameter and 180 kg in weight. Performance targets of the tip-tilt and chopping mechanism are 30 Hz control band width with 0.01' resolution for tip- tilt and 30' amplitude with frequency of 5 Hz for chopping. To archive these targets, a system combining 6-point dynamic drive mechanism and 15-point passive support mechanism against for gravity are developed and compact actuators of electric magnet type for the drive mechanism are employed. Test with a dummy mirror shows that the performance target are achieved. This paper describes the design and test results of the tip-tilt and chopping mechanism.
New Concepts and Approaches
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4-m off-axis telescope
Gilberto Moretto, Jeff R. Kuhn
We describe here a possible off-axis configuration for a 4.0 m class telescope. The improvements in the scattered light performance of such a telescope in comparison to a conventional concentric design are described. We consider a modified Ritchey-Chretien (RC) design for the off-axis configuration which allows three possible instrument focal planes: a wide field (15') f/10, a `bare' polarimetric and coronagraphic narrow field f/10, and a moderate field (7') f/16 focal plane. The design uses a primary mirror that is an off-axis section of a conic quasi-paraboloid parent mirror that produces a 4.0 m off-axis section optimized for the f/16 configurations. Using this same primary we proceed with the f/10 optimization using a second dedicated secondary and a three element powered Fused-Silica lens corrector for the visible (U-J) that yields a near- diffraction limited 15 arcmin FOV. The bare, two reflection, f/10 focus provides comparable encircled energy performance over a 10' field if the secondary is not tilted and translated, or if it is translated (and tilted) by approximately 4 cm it achieves comparable core energy concentration over a 3' field. Mirror polish, optical mount and enclosure cost data suggest that the off-axis telescope would cost approximately 10% more to build than a conventional RC concentric 4 m telescope. The construction cost normalized by the effective aperture of an on- and off- axis telescope are approximately equal.
Next-Generation Concepts
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Choice of aperture for large telescopes for optical wavelengths
For a very large telescope for optical and adjacent wavelengths, we have studied a number of parameters that influence choice of aperture size. These are defined by scientific drivers, aspects of segmentation, f-ratio considerations, provisions for interferometry, and structural limitations as well as cost. Alignment systems and adaptive optics operation have been studied. Primary mirror layout and provisions for interferometric operation are commented. Cost estimates have been made based on a number of selected parameters. We conclude that a range of 25 - 50 m seems most interesting for the diameter of the primary mirror. Tentatively, we opt for a 40 m telescope with fully adaptive operation and with optional non-adaptive wide-field operation. The primary mirror is composed of 2 m super-segments, divided into 15 cm adaptive segments.
Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
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IDL fabricating technique: further developments for the liquid and replicated mirrors of very large ground-based and space telescopes
Results of advanced experiments in centrifugal molding of the mercury parabolic mirrors and convex replicable matrix are presented and discussed. The stationary ground-based installation was erected following the IDL fabrication technique and put into action. A number of important technical peculiarities of the installation are described along with some theoretical background for its dynamics. The experiments done prove well the IDL-technique prospects in building zenith telescopes employing liquid paraboloids as working mirrors beyond 10-m in diameters, as well as thin superlightweight replicated mirrors intended for the NGST- like projects.
Poster Papers on Chopping Secondary Mirror Systems and Next-Generation Concepts
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Site survey for a large solar telescope
In 1996 we started a seeing survey of a number of existing and potential solar observing sites using solar scintillometry. This paper reports the result of the first year of that survey. It confirms earlier reports about the superior observing conditions of lake sites. I also describe the first results of atmospheric structure constant (Cn2) probing using a solar scintillometer array.
Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
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Selection of the GTC dome and support areas configuration
Javier Serrano, German R. Pescador
Several options for the GTC (Gran Telescopio Canarias) installations have been evaluated. In particular, different dome types and distribution schemes for the support areas to be incorporated in the installation have been assessed. In order to select the dome type, a technical-economic study has been carried out, in which the most important parameters that determine the efficiency of the dome during its life- cycle have been qualitatively and in some cases quantitatively evaluated. This paper presents a compilation of the evaluation process followed for the selection of the GTC dome and the establishment of a general layout for the location of the support areas.
Quality management in the GTC project
Fernando Manso Cuellar
The purpose of this paper is to describe the Quality System that has been implemented in the Gran Telescopio Canarias (GTC) project to assure the quality of the GTC Telescope. This project is a high-tech scientific-engineering program and therefore it is important, as well as in other programs, that a common working methodology is set up among the participants. The vehicle used to meet this goal has been an Quality System consisting of the Quality Manual and the Operative Procedures. The first step in this effort has been to define what Quality is for the GTC. There are many definitions of Quality, and it was decided to adopt the following definition: `Quality is the fulfillment of the GTC requirements within the Project Schedule and Budget'. The present paper summarizes the process followed to set up the Quality System and the structure of this system, including the Quality Manual and the Operative Procedures.
Residual misalignments of the Keck Telescope primary mirror segments: classification of modes and implications for adaptive optics
Mitchell Troy, Gary A. Chanan, Edwin Sirko, et al.
Because of limitations in the alignment process resulting from the telescope active control system and from atmospheric turbulence, the segments of the Keck Telescope primary mirror are never perfectly aligned. We describe a scheme for classifying these misalignments in terms of modes of the primary mirror. We describe these modes in terms of noise propagation, symmetry (specifically their resemblance to Zernike polynomials), and how discontinuous they are. Modal spectra of typical mirror misalignments are presented. The edge discontinuities which result from these misalignments are significantly smaller than what one would expect from random, uncorrelated tip/tilt errors of the same size, a fact which may have important implications for adaptive optics systems on segmented mirror telescopes.
Safe handling of 8-m mirror blanks during manufacture: equipment designs based on finite element analyses of stresses
William R. Powell, Stephen R. Ormsby, Mary J. Edwards
The manufacturing of 8-meter mirror blanks by Corning's hex seal process requires several handling steps, including lifting and turnover of both plano and contoured configurations. Special equipment was designed and built to lift and turn these 35-ton, 27-foot diameter pieces of glass. Finite Element Analysis was used to evaluate the lifting and turnover equipment designs and the handling processes. The analyses predicted the stress and distortion in the glass. The results showed that the equipment designs and processes would be safe; that is they would not stress the glass beyond the safe design stress limits.
Fast tip-tilt secondary for the Blanco 4-m Telescope
Gabriel Perez, Richard Elston
A very low cost fast tip-tilt secondary mirror has been implemented on the NOAO Blanco 4m Telescope located on Cerro Tololo, Chile. The secondary mirror is an 800-mm diameter Cervit mirror, lightweighted to 92 kg and polished to match the optical quality of the existing 4.0-m primary. The final focal ratio is 14.5. The secondary mirror is mounted in a cell which provided high torsional inertia and stiffness. This mirror cell drew heavily from an existing secondary mirror cell, which had been built for the Blanco Telescope many years ago but never used. Modifying an existing mirror cell provided significant cost savings. The mirror is axially attached to this cell via three piezo electric actuators, which are computer driven to provide the tip-tilt corrections required. A mercury band provides radial support. Unlike most tip-tilt systems, this secondary is not momentum compensated. Tests have shown that the telescope is unaffected by the reaction forces induced by motion of the secondary mirror. Eliminating the momentum compensating system makes the mechanical design simple, reliable and inexpensive. The system will normally operate at frequencies in the range of 10 to 40 Hz with only very small amplitudes at the higher frequencies. When operated as part of a closed loop feedback system using a fast CCD camera operating at 240 Hz the RMS image motion of stars is reduced to 0.02 arcseconds. All image motion at frequencies lower than 25 - 30 Hz is removed.
Managment and construction of the Large Binocular Telescope enclosure: Meeting unusual challenges with a competitive discipline
James H. Slagle, John M. Hill, Warren B. Davison, et al.
Planning, estimating, and building a telescope and its enclosure within a budget is a challenge to any project staff. The Large Binocular Telescope (LBT) project office goal has been to break every phase of the project into small packages and competitively bid the packages. In this way the project office can minimize costs and keep the project budget from escalating out of control. This paper will discuss both the unique and common problems associated with the building of telescopes into the next millennium. The discussion is centered on the planning and execution phases of construction for the LBT, located on Mt. Graham in Arizona. The paper will discuss the effects of delays on the actual start of the telescope due to environmental issues and the impact the delays had on design and budget. The paper will provide the solutions that have been incorporated by the LBT project office to maximize the quality of construction while holding costs to a minimum. The use of a team approach by the contractors, engineers, and the project office has been successful in maintaining quality construction at a reasonable cost.
Poster Papers on Chopping Secondary Mirror Systems and Next-Generation Concepts
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Hardware implementation of the primary mirror surface heating system for the Gemini 8-m telescopes
Brian J. Perona, Eric R. Hansen, David Hagelbarger, et al.
The Gemini 8m Telescopes Project is implementing two cooperating systems for thermal control of the monolithic primary mirror. The first system consists of temperature controlled radiation panels, behind the primary mirror, that are used to control the bulk temperature of the primary. The second system is the Surface Heating System which allows for adjustment of the optical surface temperature independent of the bulk mirror temperature. By heating just the reflective coating of the primary, a quicker thermal adjustment of the optical surface temperature can be achieved. The result is a minimizing of mirror seeing effects by better tracking of fluctuations in nighttime ambient temperature. A development effort and subscale prototype testing of this technique was completed by the Gemini staff in 1996 and presented at the SPIE Landskrona Conference. This paper reviews the detailed hardware design and implementation of a Surface Heating System being installed on the Gemini 8-meter primary mirror.
Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
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Performance of the active M1 cell for the 2.1-m telescope of the Observatorio Astronomico Nacional de Mexico
Optical testing of the 2.1-m telescope in San Pedro Martir, Observatorio Astronomico Nacional de Mexico, by the methods of wavefront curvature sensing and bi-Ronchi analysis, has shown that the telescope suffered of large amounts of astigmatism. We identified these as due to improper primary mirror support and developed an active control system to correct for it. The number and position of the actuators were decided in accordance to the flexural modes that needed to be corrected, resulting in a system of 18 pressure controlled pneumatic actuators, with an outer loop that verifies the load at three hard points. A PID algorithm and matrix inversion are fundamental parts of this outer loop, that guarantees that the M1 mirror is tilted as a rigid body to maintain it properly aligned. The successful performance of the system to correct low order aberrations is reported.
Novelty mount for M2 with five degrees of freedom
Esteban Luna-Aguilar, Gerardo Sierra, Victor M. Garcia, et al.
In this paper, the characteristics of a mount for secondary mirror of an astronomical telescope are presented. The mount has five freedom degrees. The control allows to focus with errors of +/- 1 micron and to align with inclinations and displacements with error of +/- 3.48 arcsec and +/- 8.3 micron respectively. The optical tests are presented before and after placing this mount, as well as control electronics and mechanical details.
Major Projects
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Mexican infrared optical new technology telescope (TIM) project
Luis Salas, Elfego Ruiz Schneider, Irene Cruz-Gonzales, et al.
We present the Mexican Infrared-Optical New Technology Telescope Project (TIM). The design and construction of a 7.8 m telescope, which will operate at the Observatorio Astronomico Nacional in San Pedro Martir, B.C. (Mexico), are described. The site has been selected based on seeing and sky condition measurements taken for several years. The f/1.5 primary mirror consists of 19 hexagonal off-axis parabolic Zerodur segments. The telescope structure will be alt-az, lightweight, low cost, and high stiffness. It will be supported by hydrostatic bearings. The single secondary will complement a Ritchey-Chretien f/15 design, delivering to Cassegrain focus instrumentation. The telescope will be infrared optimized to allow observations ranging from 0.3 to 20 microns. The TIM mirror cell provides an independent and full active support system for each segment, in order to achieve both, phasing capability and very high quality imaging (0.25 arcsec).
Poster Papers on Mirror Support, Active Optics, Coatings, and Telescope Enclosures
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Coronagraphic and low-emissivity astronomical reflector (CLEAR): heat trap design
Walter A. Siegmund
The heat trap in a coronagraphic telescope is located at its prime focus and blocks the transmission of radiation from unwanted portions of the solar disk to subsequent optics in the telescope. This reduces light scattered and heat absorbed by these optics. For observations of the corona, the solar disk is completely blocked, whereas for observations of the disk, typically 90% or more of the disk is blocked. The proposed heat trap design is constructed largely of fused silica plates, partially coated with platinum, and cooled with air. It is robust and handles high irradiance, i.e., almost f megawatt/m2 at f/3.75, without degrading the image quality of the telescope or contributing significant stray light to the focal surface.
New Concepts and Approaches
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Optical very large array (OVLA) prototype telescope: status report and perspective for large mosaic mirrors
Julien Dejonghe, Luc Arnold, Olivier Lardiere, et al.
The OVLA will be a kilometric-size interferometric array of N equals 27 or more 1.5 m telescopes. It is expected to provide visible to infra-red snap-shot images, containing in densified pupil mode N2 10-4 arc-second wide resolved elements in yellow light. The prototype telescope is under construction at Observatoire de Haute Provence and will be connected in 2000 to the GI2T, Grand Interferometre a 2 Telescopes, thus upgraded to a GI3T. The prototype telescope has a spherical mount, well suited for multi- aperture interferometric work, and a thin active 1.5 m f/1.7 mirror weighting only 180 kg with the active cell. This meniscus-shaped mirror, made of low-cost ordinary window glass, is only 24 mm thick and supported by 32 actuators. We describe the telescope optical concept with emphasis on opto-mechanical aspects and the test results of the active optics system. We also discuss the application of this mirror concept to large mosaic mirrors of moderate cost.
Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
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SOFIA lightweight primary mirror
Jean Espiard, Michel Tarreau, Joel Bernier, et al.
Thanks to its experience in lightweighting ceramic glass mirrors by machining, R.E.O.S.C. won the contract for designing and manufacturing the primary mirror and its lateral fixations of the 2.7 m. SOFIA telescope which will be installed aboard a 747 SP Boeing aircraft to constitute the Stratospheric Observatory for Infrared Astronomy (SOFIA).
Poster Papers on Mirror Support, Active Optics, Coatings, and Telescope Enclosures
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Protected silver coating for astronomical mirrors
Norman L. Thomas, Jesse D. Wolfe, Joseph C. Farmer
A durable protected silver coating was designed and fabricated for use on flashlamp reflectors in the National Ignition Facility to avoid tarnishing under corrosive conditions and intense visible light. This coating provides a valuable alternative for telescope mirror coatings where high reflectance and durability are important requirements. This paper describes a protected silver coating having high reflectance from 400 mm to 10,000 nm. An alternate coating design extends the high reflectance down to 300 nm while maintaining high reflectance out to 10,000 nm. The specular reflectance is between 95% and 97% in the visible region and 98% or better in the infrared region.
Poster Papers on Telescope Projects, Mirror Fabrication, Polishing,and Handling
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Performances of the LBT after the main structure optimization
Gianpietro Marchiori, Gianfranco Rigato, Alberto Zanon, et al.
This paper reports the structural performances of the LBT telescope after the detail design phase completion. It explains, through some significant examples of construction drawings, the process of optimization of the manufacturing methods, guaranteeing the main characteristics of rigidity and masses of the mechanical structures of the telescope.
Poster Papers on Chopping Secondary Mirror Systems and Next-Generation Concepts
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Single-axis four-mirror system: large spherical primary and small fields
Andre Baranne
A catoptric corrector of modest size can be used for large spherical primaries, easily integrated at the prime focus, this corrector gives back to the system, aspect and properties of 2-mirrors classical telescopes. In the last few years, progress in active and adaptative optics makes possible a lot of things, progress in measuring distances, new ideas on optical coatings, new materials and so on in a near future, all that makes the instrumentalist dreamy It is said that nobody knows today if the size of 3rd millennium telescopes will be limited or not by a theoretical, physical or technical phenomenon, thus let us imagine but with thoughtfulness because our projects will be surely restricted by financial considerations