A turnkey observatory with 6.5-m telescope has been developed for a broad range of science applications. The observatory includes the telescope, mount and enclosure, installed on site and ready for operation. The telescope’s primary mirror is an f/1.25 honeycomb sandwich of borosilicate glass, similar to that of the MMT and Magellan telescopes. The baseline optical design is for a Gregorian Nasmyth focus at f/11. A Gregorian adaptive optics secondary that provides a wide-field focus corrected for ground layer turbulence (0.25 arcsecond images over a 4 arcminute field) as well as a narrow-field diffraction-limited focus is optional. Another option is a corrected f/5 focus with a 1° field. The observatory, built by partners from academia and industry with extensive experience, can be delivered within five years at a fixed price.
Segment production for the Giant Magellan Telescope is well underway, with the off-axis Segment 1 completed, off-axis
Segments 2 and 3 already cast, and mold construction in progress for the casting of Segment 4, the center segment. All
equipment and techniques required for segment fabrication and testing have been demonstrated in the manufacture of
Segment 1. The equipment includes a 28 m test tower that incorporates four independent measurements of the segment's
figure and geometry. The interferometric test uses a large asymmetric null corrector with three elements including a 3.75
m spherical mirror and a computer-generated hologram. For independent verification of the large-scale segment shape,
we use a scanning pentaprism test that exploits the natural geometry of the telescope to focus collimated light to a point.
The Software Configurable Optical Test System, loosely based on the Hartmann test, measures slope errors to submicroradian
accuracy at high resolution over the full aperture. An enhanced laser tracker system guides the figuring
through grinding and initial polishing. All measurements agree within the expected uncertainties, including three
independent measurements of radius of curvature that agree within 0.3 mm. Segment 1 was polished using a 1.2 m
stressed lap for smoothing and large-scale figuring, and a set of smaller passive rigid-conformal laps on an orbital
polisher for deterministic small-scale figuring. For the remaining segments, the Mirror Lab is building a smaller, orbital
stressed lap to combine the smoothing capability with deterministic figuring.
The University of Arizona has developed a new dish-based High Concentration Photovoltaic (HCPV) system which is in
the process of being commercialized by REhnu, Inc. The basic unit uses a paraboloidal glass reflector 3.1 m x 3.1 m
square to bring sunlight to a high power point focus at a concentration of ~20,000x. A unique optical system at the focus reformats the concentrated sunlight so as to uniformly illuminate 36 triple junction cells at 1200x geometric
concentration1. The relay optics and cells are integrated with an active cooling system in a self-contained Power Conversion Unit (PCU) suspended above the dish reflector. Only electrical connections are made to the PCU as the active cooling system within is completely sealed. Eight of these reflector/PCU units can be mounted on a single two axis tracking structure2. Our 1st generation prototype reflector/PCU unit consistently generated 2.2 kW of power normalized to 1kW/m2 DNI in over 200 hours of on-sun testing in 20113. Here, we present on-sun performance results for our 2nd generation prototype reflector/PCU unit, which has been in operation since June 2012. This improved system consistently generates 2.7 kW of power normalized to 1kW/m2 DNI and has logged over 100 hours of on-sun testing. This system is currently operating at28% DC net system efficiency with an operating cell temperature of only 20°C above ambient. Having proven this system concept, work on our 3rd generation prototype is underway with a focus on manufacturability, lower cost, and DC efficiency target of 32% or better.
KEYWORDS: Telescopes, Space telescopes, Reliability, Error analysis, Optical instrument design, Observatories, Camera shutters, Current controlled current source, Spherical lenses, Interfaces
After several years of operation the enclosure rotation system of the LBT is exhibiting wear and other performance
issues that may impact operations. This paper reviews the system design and assumptions used, describes the current
performance and observed symptoms, and discusses recent improvements made to improve performance and reliability.
The rotating enclosure of the LBT is a 2200 ton structure riding on four bogies with a total of 20 wheels. Identified
deficiencies include wheel bearing capacities, bogie misalignment, and rail loading. These are partially due to excess
enclosure weight, which was supposed to be 1600 tons, but also due to design errors.
The most serious problem was the failure of several wheel bearings. The bearings were not designed for field
serviceability, so a crash program began to determine how to replace them. This got us back on sky quickly, but a
review of the engineering calculations identified an error which led to the use of undersized bearings. A method of
installing a larger bearing was found, and these have been installed.
One set of bogie wheels are misaligned so severely the structure makes loud popping and banging noises when the
direction of building rotation changes. The bogie needs to be rotated about its vertical axis, but there was no provision in
the design for this.
The circular rail the bogies roll on is wearing faster than expected. The rails are extremely difficult to replace, so the
short term plan is to study the problem.
The paper describes a new system architecture optimized for utility-scale generation with concentrating photovoltaic
cells (CPV) at fossil fuel price. We report on-sun tests of the architecture and development at the University of Arizona
of the manufacturing processes adapted for high volume production. The new system takes advantage of triple-junction
cells to convert concentrated sunlight into electricity. These commercially available cells have twice the conversion
efficiency of silicon panels (40%) and one-tenth the cost per watt, when used at 1000x concentration. Telescope
technology is adapted to deliver concentrated light to the cells at minimum cost. The architecture combines three novel
elements: large (3.1 m x 3.1 m square) dish reflectors made as back-silvered glass monoliths; 2.5 kW receivers at each
dish focus, each one incorporating a spherical field lens to deliver uniform illumination to multiple cells; and a
lightweight steel spaceframe structure to hold multiple dish/receiver units in coalignment and oriented to the sun.
Development of the process for replicating single-piece reflector dishes is well advanced at the Steward Observatory
Mirror Lab. End-to-end system tests have been completed with single cells. A lightweight steel spaceframe to hold and
track eight dish/receiver units to generate 20 kW has been completed. A single 2.5 kW receiver is presently under
construction, and is expected to be operated in an end-to-end on-sun test with a monolithic dish before the end of 2010.
The University of Arizona has granted an exclusive license to REhnu, LLC to commercialize this technology.
The Giant Magellan Telescope has a 25 meter f/0.7 near-parabolic primary mirror constructed from seven 8.4 meter
diameter segments. Several aspects of the interferometric optical test used to guide polishing of the six off-axis
segments go beyond the demonstrated state of the art in optical testing. The null corrector is created from two obliquelyilluminated
spherical mirrors combined with a computer-generated hologram (the measurement hologram). The larger
mirror is 3.75 m in diameter and is supported at the top of a test tower, 23.5 m above the GMT segment. Its size rules out
a direct validation of the wavefront produced by the null corrector. We can, however, use a reference hologram placed at
an intermediate focus between the two spherical mirrors to measure the wavefront produced by the measurement
hologram and the first mirror. This reference hologram is aligned to match the wavefront and thereby becomes the
alignment reference for the rest of the system. The position and orientation of the reference hologram, the 3.75 m mirror
and the GMT segment are measured with a dedicated laser tracker, leading to an alignment accuracy of about 100
microns over the 24 m dimensions of the test. In addition to the interferometer that measures the GMT segment, a
separate interferometer at the center of curvature of the 3.75 m sphere monitors its figure simultaneously with the GMT
measurement, allowing active correction and compensation for residual errors. We describe the details of the design,
alignment, and use of this unique off-axis optical test.
The Large Binocular Telescope (LBT) is built around two lightweight borosilicate honeycomb mirrors which, at
8.4 meters in diameter, are the largest operational examples of this technology. Since the mirrors are relatively
stiff, the LBT mirror support system relies on passive position control and active force control. Passive position
control is performed by six extendable hardpoints organized as a truncated hexapod, which may be positioned
as required by the active optics control loop. The hardpoints rely on their axial stiffness to maintain the mirror
position against residual external disturbances. The active force control system minimizes the force exerted by
the hardpoints on the glass. Additionally, the axial component of the nominally uniform active support forces
can be perturbed to distort the mirror as required by the active optics control loop. Because of the relatively
large CTE of borosilicate glass, the differential temperature of the mirror is critical. Thus, the force control
system must support a 16 metric ton mirror using less than 100 Watts of electrical power. The authors present
a description of the primary mirror support system as implemented at the LBT. Initial stability problems made
the mirrors nearly unusable in freezing temperatures. The authors explain the reason for this instability and
describe the solutions implemented. Data demonstrating the current performance of the primary mirror support
system are also presented.
The Giant Magellan Telescope achieves 25 meter aperture and modest length using an f/0.7 primary mirror made from
8.4 meter diameter segments. The systems that will be used for measuring the aspheric optical surfaces of these mirrors
are in the final phase of development. This paper discusses the overall metrology plan and shows details for the
development of the principal test system - a system that uses mirrors and holograms to provide a null interferometric test
of the surface. This system provides a full aperture interferometric measurement of the off-axis segments by
compensating the 14.5 mm aspheric departure with a tilted 3.8-m diameter powered mirror, a 77 cm tilted mirror, and a
computer generated hologram. The interferometric measurements are corroborated with a scanning slope measurement
from a scanning pentaprism system and a direct measurement system based on a laser tracker.
Surface measurements represent a significant part of the cost for manufacturing large aspheric optics. Both polished and
rough ground surfaces must be measured with high precision and spatial resolution. We have developed a system that
couples a commercial laser tracker with an advanced calibration technique and a system of external references. This
system was built to measure the off-axis primary mirror segments for the Giant Magellan Telescope where it will guide
loose abrasive grinding and initial polishing. The system is further expected to corroborate the optical interferometric
tests of the completed mirrors, in several low-order aberrations. The design, analysis, calibration, and measured
performance of this system will be presented.
The first of the 8.4 m off-axis segments for the primary mirror of the Giant Magellan Telescope is being manufactured at
the Steward Observatory Mirror Lab. In addition to the manufacture of the segment, this project includes the
development of a complete facility to make and measure all seven segments. We have installed a new 28 m test tower
and designed a set of measurements to guide the fabrication and qualify the finished segments. The first test, a laser-tracker
measurement of the ground surface, is operational. The principal optical test is a full-aperture interferometric test
with a null corrector that includes a 3.75 m spherical mirror, a smaller sphere, and a computer-generated hologram. We
have also designed a scanning pentaprism test to validate the measurement of low-order aberrations. The first segment
has been cast and generated, and is in the process of loose-abrasive grinding.
The Large Synoptic Survey Telescope uses a unique optomechanical design that places the primary and tertiary mirrors
on a single glass substrate. The honeycomb sandwich mirror blank was formed in March 2008 by spin-casting. The
surface is currently a paraboloid with a 9.9 m focal length matching the primary. The deeper curve of the tertiary mirror
will be produced when the surfaces are generated. Both mirrors will be lapped and polished using stressed laps and other
tools on an 8.4 m polishing machine. The highly aspheric primary mirror will be measured through a refractive null lens,
and a computer-generated hologram will be used to validate the null lens. The tertiary mirror will be measured through a
diffractive null corrector, also validated with a separate hologram. The holograms for the two tests provide alignment
references that will be used to make the axes of the two surfaces coincide.
The design, manufacture and support of the primary mirror segments for the GMT build on the successful primary mirror systems of the MMT, Magellan and Large Binocular telescopes. The mirror segment and its support system are based on a proven design, and the experience gained in the existing telescopes has led to significant refinements that will provide even better performance in the GMT. The first 8.4 m segment has been cast at the Steward Observatory Mirror Lab, and optical processing is underway. Measurement of the off-axis surface is the greatest challenge in the manufacture of the segments. A set of tests that meets the requirements has been defined and the concepts have been developed in some detail. The most critical parts of the tests have been demonstrated in the measurement of a 1.7 m off-axis prototype. The principal optical test is a full-aperture, high-resolution null test in which a hybrid reflective-diffractive null corrector compensates for the 14 mm aspheric departure of the off-axis segment. The mirror support uses the same synthetic floatation principle as the MMT, Magellan, and LBT mirrors. Refinements for GMT include 3-axis actuators to accommodate the varying orientations of segments in the telescope.
The Large Synoptic Survey Telescope (LSST) is an 8-meter class telescope with a proposed field of view between 3.0 and 3.5 degrees. The scientific goals of the survey establish a cadence that sets the telescope performance. The proposed cadence of the LSST telescope will typically require movements and settling of the telescope of approximately 3 degrees in 5 seconds. This dictates a high bandwidth to the telescope servo and thus a high locked rotor resonant frequency. In this study, the structure must accommodate three optical surfaces, the 8.4-meter primary, the 3-meter class secondary, and a 5-meter class tertiary in a long-tube configuration. The instrument must be accommodated in a "Trapped Focus" in the middle of the telescope. This imposes very stringent requirements on the structure and drives. This structure will require performance beyond the existing class of 8-meter telescopes. This can be achieved with the C-ring and azimuth platform concept demonstrated with the Large Binocular Telescope. The structure requires a low rotational inertia and a very high locked rotor resonant frequency at all angles of the sky. This is a challenging problem that can be overcome with this innovative solution.
A concept design has been developed for the Giant Magellan Telescope (GMT). The project is a collaboration by a group of U.S. universities and research institutions to build a 21.5-meter equivalent aperture optical-infrared telescope in Chile. The segmented primary mirror consists of seven 8.4-meter diameter borosilicate honeycomb mirrors that will be cast by the Steward Observatory Mirror Laboratory. The fast primary optics allow the use of unusually compact telescope and enclosure structures. A wide range of secondary trusses has been considered for the alt-az mount. The chosen truss employs carbon fiber and steel and, due to its unique geometry, achieves high stiffness with minimal wind area and primary obscuration. The mount incorporates hydrostatic supports and a C-ring elevation structure similar in concept to those implemented on the Magellan 6.5-m and LBT dual 8.4-m telescopes. Extensive finite element analysis has been used to optimize the telescope structure, achieving a lowest telescope resonant frequency of ~5 Hz. The design allows for removal and replacement of any of the 7 subcells for off-telescope mirror coating with no risk to the other
mirrors. A wide range of instruments can be used which mount to the top or underside of a large instrument platform below the primary mirror cells. Large instruments are interchanged during the day while small and medium-sized instruments can be enabled quickly during the night. The large Gregorian instruments will incorporate astatic supports to minimize flexure and hysteresis.
The Large Binocular Telescope has two 8.4 meter mirrors, one of which is now in the telescope. Handling and moving the 8.4-meter honeycomb mirrors calls for moving 16 metric ton mirrors while maintaining very low stresses. We have now handled the first LBT mirror off the furnace, turned on edge, cleaned out, turned upside down, on the grinder, turned again, put on a polishing cell, moved under the polishing machine, lifted with a vacuum lifting fixture, moved to the telescope cell, to a transportation box, down the highway, onto a multi-axle trailer on edge, up Mount Graham, into the telescope building, back into the telescope cell and up through a hatch onto the telescope itself. The second LBT mirror is in the polishing stage. We have designed and manufactured many pieces of specialized equipment to handle the task. This equipment must be able to handle the mirrors without exceeding 0.7 MPa (100 psi) stress in the glass.
The 20 20 Telescope is a 30 meter class telescope comprised of two 21.2m collector telescopes on a 100m circular track. Each collector telescope has a focal ratio of F: 0.7 and is comprised of seven 8.4 m segments. There is an instrument bridge that carries the combining instrument. The proposal for 20 20 is to have discrete combiner stations for 30,60,and 100 meter baselines. Additional focal stations are implemented for Nasmyth and bent Cassegrain. The Track has the same segmented construction and tracking motion on hydrostatic bearings as LBT. The collector telescope buildings will co-track and co-rotate on separate tracks.
The 30m design has the same basic shape as a single 21 meter Collector but many aspects are different. The 30 meter telescope is a single hexagonal aperture with a primary at F: 0.5. There are 13 that are 8.74m hexagons and 6 half hexagons. The 30m telescope has primarily Nasmyth platforms behind the primary mirror. Both telescopes have a 30 meter equivalent circular aperture. Both telescopes have high structural performance, at 6.5 Hz and 5.3 Hz respectively. Both are balanced, and use similar designed components. Comparison of their characteristics and design differences can show the strengths and weaknesses of each.
Any future giant ground-based telescope must, at a minimum, provide foci for seeing-limited imaging over a wide field and for diffraction-limited imaging over ~1 arcminute fields corrected by adaptive optics (AO). While this is possible with a number of design concepts, our choices are constrained if we anticipate wanting to later add a second telescope for imaging with still higher resolution, and very high contrast imaging for exoplanet studies. This paper explores designs that allow for such future development. Higher resolution imaging by interferometric combination of the AO-corrected fields of two telescopes is possible without loss of point-source sensitivity or field of view, as long as the baseline can be held perpendicular to the source and can be varied in length. This requirement is made practical even for very large telescopes, provided both can move continuously on a circular track. The 20/20 telescope illustrates this concept. Telescopes so mounted can additionally be operated as Bracewell nulling interferometers with low thermal background, making possible the thermal detection of planets that would have been unresolvable by a single 20 m aperture. In practice, limits set by funding and engineering experience will likely require a single 20 or 30 m telescope be built first. This would be on a conventional alt-az mount, but it should be at a site with enough room for later addition of a companion and track. In anticipation of future motion it should be compact and stiff, with a fast primary focal ratio. We envisage the use of large, highly aspheric, off-axis segments, manufactured using the figuring methods for strong aspherics already proven for 8 m class primaries. A compact giant telescope built under these guidelines should be able to perform well on its own for a broad range of astronomical observations, with good resistance to wind buffeting and simple alignment and control of its few, large segments. We compare here configurations with adjacent hexagonal segments and close-packed circular segments. For given segment parent size and number, the largest effective aperture is achieved if the segments are left as circles, when also the sensitivity and resolution for diffraction-limited operation with AO is higher. Large round segments can also be individually apodized for high-contrast imaging of exoplanets with the entire telescope-for example 8.4 m segments will yield 10-6 suppression 0.05 arcsec from a star at 1 μm wavelength, and at 0.25 arcsec at 5 μm.
This paper presents designs of compact 21 and 30 m aperture telescopes with primary focal of f/0.7 and f/0.56. The 20 20 telescope moves on three axes; the elevation axis (which is below the primary vertex), the azimuth axis, and a tracking axis at the center of 100 m diameter tracks. The 30 m telescope has an elevation and azimuth axis. All of the axes move on hydrostatic bearings. A primary requirement for such large telescopes is stiffness against deformation by wind gusts. The mass and stiffness needed for the structure is substantially independent of the primary mirror mass, which can therefore be set by thermal and diffraction issues. For the 21 m design, whose primary has seven 8.4 m glass segments weighing 128 tons, the total moving mass is 905 tons, and the lowest resonant frequency 6.5 Hz. For the 30 m design, whose primary has, 13 whole and 6 half, glass segments 8.7 m, across the points, weighing 256 tons, the total moving mass is 3,460 tons, and the lowest resonant frequency 5.3 Hz. These practical designs offer two versatile telescopes with high performance.
The Large-aperture Synoptic Survey Telescope, LSST, will have an effective aperture of ~6.5 m and a 3 degree field of view. Its 3-mirror optical system with 8.4 m primary, 3.5 m secondary, 4.2 meter tertiary mirrors and a trapped focus offer unique telescope design challenges. The operation of this telescope will require quick slewing, accurate tracking and alignment maintained actively for 0.25 arcsec images in the presence of wind and gravity perturbations. We describe our current design for which finite element models show a lowest frequency resonance above 7 Hertz. Further refinement promises an even stiffer structure. The design has been optimized for low mass (230 tons), minimal inertia (2.4×106 kg-m2 in elevation, 3.2×106 kg-m2 in azimuth) for fast response. It takes advantage of several concepts proven in the Large Binocular Telescope mount, which has shown high performance at low cost. These include elevation motion on C rings placed under the primary mirror, a primary mirror cell built as an integral part of the structure, and the elevation axis placed behind and off to the side of the primary vertex, to achieve balance with minimum mass.
We report on the design of the two tertiary mirrors of the Large Binocular Telescope. The tertiary mirrors are flat octagonal shaped 540 X 640 mm Hextex honeycombs made of Schott borosilicate. Each mirror cell is mounted on three linear actuators for the active control of the mirror pointing and for the adjustment of the telescope optical path length. Each tertiary mirror unit embeds a rotator stage to point at four different instrument stations on the telescope. Particular effort is developed to the optimization of the honeycomb mirror support system to minimize the optical surface RMS deformation at the different mirror attitudes.
The use of a team approach by contractors, engineers and management to build the Large Binocular Telescope (LBT) has been successful in maintaining quality construction at a reasonable price. No matter how efficient the team, the building of a 16-story building, with a totally unique design, and on just 1.2 acres of land at an elevation of 3191 meters does present formidable problems. This paper will present the current status of the LBT construction on Mt. Graham and how the team approach has continued to be successful in providing quality solutions on a timely basis while keeping the costs of construction to a minimum. The paper will discuss many issues that project managers must plan for when undertaking new and unique designs and what steps managers can take to avoid costly delays.
We will present a system to perform closed-loop optical tests of the 64 cm diameter, 336 actuator adaptive secondary made at the Steward Observatory Mirror Laboratory. Testing will include Shack-Hartmann wavefront sensing and modal correction of static and dynamic aberrated wavefronts. The test optical system is designed so that experiments can be made with both the focal plane instrument and secondary installed in their normal configuration at the MMT, or with the same 9 m spacing in a laboratory test tower. The convex secondary will be illuminated at normal incidence through two 70 cm diameter lenses mounted just below. The artificial, aberrated star is projected from near the wavefront sensor in the Cassegrain focus assembly. Computer generated holograms correct for spherical aberration in the really optics at the test wavelengths of 0.594 and 1.5 micrometers . Atmospheric turbulence is reproduced by two spinning transmission plates imprinted with Kolmogorov turbulence. The Shimmulator will give us the opportunity to test fully the adaptive optics system before installation at the new MMT, hence saving much precious telescope time.
We present the results of a compete set of static and dynamic runs of the FEA model of the MMT adaptive secondary. The thin mirror is the most delicate component of the MMT adaptive secondary unit, as it provides the deformable optical surface able to correct the incoming wavefront. The static performances are evaluated as a function of the various load cases arising form gravitational loads and from the forces deriving from the magnetic interactions between actuators. In addition, computations were performed to assess the dynamic response to the high bandwidth, adaptive correcting force.s In both cases, the performances of the adaptive mirror design are able to accommodate the severe specifications.
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.
Hubert Martin, R. Allen, James Roger Angel, James Burge, Warren Davison, Scott DeRigne, Lee Dettmann, Dean Ketelsen, W. Kittrell, Stephen Miller, Peter Strittmatter, Steve West
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.
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.
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...
The main final results in terms of stresses and optical performances are reported for the large binocular telescope (LBT) primary mirrors. The two borosilicate LBT primary mirrors f/1.14 have 8.4 diameter and are produced at the Steward Observatory Mirror Lab (SOML). They are honeycomb shaped in order to achieve light weight, short thermal constant and high stiffness. The back plate is flat and the upper is paraboloid shaped. Each elementary cell has, in the lower plate, one circular hole permitting the ventilation of cell itself. The material used is the borosilicate Ohara E6. Different supporting systems have been analyzed from the mirror casting to the operative conditions, i.e.: supporting system during the cooling of the casting phase; supporting system for the handling after the casting phase and before the optical surface grinding and polishing; supporting system for the handling after the optical surface polishing and for maintenance; passive support system in non-operative condition; supporting system in operative condition. The stress checks carried out show that the values of the maximum principal tensile stresses are below 0.7 MPa for long times and/or stresses affecting large volumes, and are below 1.05 MPa for short times and small volumes. Optical performances in operative condition respect the specification.
Operated by the Multiple Mirror Telescope Observatory (MMTO), the multiple mirror telescope (MMT) is funded jointly by the Smithsonian Institution (SAO) and the University of Arizona (UA). The two organizations equally share observing time on the telescope. The MMT was dedicated in May 1979, and is located on the summit of Mt. Hopkins (at an altitude of 2.6 km), 64 km south of Tucson, Arizona, at the Smithsonian Institution's Fred Lawrence Whipple Observatory (FLWO). As a result of advances in the technology at the Steward Observatory Mirror Laboratory for the casting of large and fast borosilicate honeycomb astronomical primary mirrors, in 1987 it was decided to convert the MMT from its six 1.8 m mirror array (effective aperture of 4.5 m) to a single 6.5 m diameter primary mirror telescope. This conversion will more than double the light gathering capacity, and will by design, increase the angular field of view by a factor of 15. Because the site is already developed and the existing building and mount will be used with some modification, the conversion will be accomplished for only about $20 million. During 1995, several major technical milestones were reached: (1) the existing building was modified, (2) the major steel telescope structures were fabricated, and (3) the mirror blank was diamond wheel ground (generated). All major mechanical hardware required to affect the conversion is now nearly in hand. Once the primary mirror is polished and lab-tested on its support system, the six-mirror MMT will be taken out of service and the conversion process begun. We anticipate that a 6 - 12 month period will be required to rebuild the telescope, install its optics and achieve f/9 first light, now projected to occur in early 1998. The f/5.4 and f/15 implementation will then follow. We provide a qualitative and brief update of project progress.
We present the final design of the alt/az structure of the large binocular telescope. As a final report of the structural performances of the telescope, this paper describes how the azimuth platform and the primary mirror cells have been modeled. Furthermore, a definition of the simulation of the various structural interfaces is given. Finally, the static and dynamic responses at various zenith angles are reported.
KEYWORDS: Mirrors, Actuators, Polishing, Finite element methods, Surface finishing, Telescopes, Error analysis, Active optics, Monochromatic aberrations, Control systems
We describe the active support system and optimization of support forces for a 3.5-m honeycomb sandwich mirror. The optimization was based on interferometric figure measurements made in a vibration-isolated test tower. We obtained actuator influence functions by measurement and by finite-element analysis. The two sets of influence functions are similar in shape, but the computed figure changes are 25% smaller in magnitude than the measured figure changes. We achieved a figure accuracy of 25 nm rms surface error with the computed influence functions and only slightly worse with the measured influence functions, but were unable to reproduce the 21-nm rms surface error obtained on the passive polishing support. This implies that subtle differences between the polishing support and operational support caused small, uncorrectable figure changes. The optimization was performed by singular-value decomposition of the influence functions into normal modes. The best results were obtained using 20 - 30 out of a possible 37 modes.
We describe the solutions adopted for the most important mechanical subsystems of the Large Binocular Telescope (LBT, formerly Columbus Project), which is now in the phase of detailed design. We report in particular the design and the results of static and dynamic finite element analysis of the open telescope elevation structure, of the azimuth structure, of the cantilever swing arms supporting the auxiliary optics, and of the primary mirror cells.
This paper will describe and discuss the methods which are being developed to support the large borosilicate honeycomb mirrors from the Steward Observatory Mirror Lab which are being used in the MMT 6.5 m conversion and the Large Binocular Telescope. The technique is similar to previous work carried out for the 3.5 m Phillips Lab mirror support.
The Large Optical Generator (LOG) was originally installed as a precision generator at the University of Arizona. It has since been relocated to the Steward Observatory Mirror Laboratory, where, in addition to its tasks as generator, it can be reconfigured as a polishing machine. As such, utilizing the Mirror Lab's stressed-lap techniques, LOG has recently finished a series of three 3.5 meter mirrors to high accuracy. It is currently configured as a generator for work on the 6.5 meter MMT upgrade. LOG's operating parameters and level of performance both as generator and polisher will be discussed, along with some of the unique safety features that have been built into its operation.
David Anderson, James Roger Angel, James Burge, Warren Davison, Scott DeRigne, B. Hille, Dean Ketelsen, W. Kittrell, Hubert Martin, Robert Nagel, Thomas Trebisky, Steve West, R. Young
The stressed-lap polishing technique has been developed to meet the challenge of polishing 8- m-class mirrors with highly aspheric figures to an accuracy consistent with the best ground- based telescope sites. The method is currently being demonstrated in the polishing of two primary mirrors, a 1.8-m f/1.0 ellipsoid and a 3.5-m f/1.5 paraboloid. The figure accuracies achieved at the time of writing are 43 nm rms surface error for the 1.8-m mirror, and 190 nm rms surface error for the 3.5-m mirror. Polishing is proceedings on both mirrors. In this paper we describe the process used for the 3.5-m mirror and the progress through the early stages of fabrication. We also summarize progress on the 1.8-m mirror.
A technique has been developed for casting glass in the form of a honeycomb structure possessing good stiffness despite its low weight, and facilitating thermal control via forced ventilation of the honeycomb cells. To date, 3.5 m diameter mirrors of this type have been successfully cast; fully 8 m diameter mirrors are expected to be cast by 1992. No mirror of diameter as large as 8 m has ever been polished, however, and the difficulties which will be encountered shall be compounded by the shorter focal lengths required for advanced telescopic optics. A novel method, designated 'stressed lap polishing', has been developed to address these problems.
The reasons are presented for the high performance of the Columbus Project Telescope whose design is based on two short focal mirrors, large drive and support radii, and a short load path to the ground. The radius squared is argued to be the most important tool for the inprovement of performance in the large optical telescopes, since the stiffness of the mechanics is proportional to the radius at which they act squared. The six Columbus telescope finite element models show that optimization of a structure depends more on the initial conditions (radius squared) than the truss shape or elements. It is concluded that future exploitation of radius squared could lead to higher performance for the very large telescope.
The design of aluminization systems for the MMT Conversion 6.5 m mirror and the Columbus
Project 8 m mirror has led us to reconsider many of the design issues and tradeoffs for such systems.
Coating of the large honeycomb mirrors will be done in situ on the telescope with a portable bell jar
forming the front half of a two-stage vacuum system. The mirror cell forms a "dirty" vacuum behind
the mirror to eliminate excess force on the glass. A multi-ring source geometry has been proposed to
allow a 1.0 m spacing between the mirror surface and the sources thereby minimizing the size of the
vacuum chamber. Evaporation source models have been developed to optimize the number of
sources, ring spacing, and high incidence angle emission to achieve better than 5% rms deviation in
coating thickness over the diameter. Code results are compared to empirical thickness profiles
measured at the University of Arizona's (UA) Sunnyside 2.0 m coating facility. Cryoadsorption
pumps are considered for reasons of economy, quality of vacuum, pumping speed, and reliability.
The interaction of the cryopumps and getter pumping with the pumping/cleaning/deposition cycle is
studied. Glow discharge cleaning is discussed and the results of deposition tests in 10' Torr residual
argon are given. Electrical requirements are estimated and a novel transformer design may decrease
the current entering the chamber from 12,000 A to less than 600 A.
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