We present an overview of the preliminary design of the Telescope Structure System (STR) of Thirty Meter Telescope (TMT). NAOJ was given responsibility for the TMT STR in early 2012 and engaged Mitsubishi Electric Corporation (MELCO) to take over the preliminary design work. MELCO performed a comprehensive preliminary design study in 2012 and 2013 and the design successfully passed its Preliminary Design Review (PDR) in November 2013 and April 2014. Design optimizations were pursued to better meet the design requirements and improvements were made in the designs of many of the telescope subsystems as follows: 1. 6-legged Top End configuration to support secondary mirror (M2) in order to reduce deformation of the Top End and to keep the same 4% blockage of the full aperture as the previous STR design. 2. “Double Lower Tube” of the elevation (EL) structure to reduce the required stroke of the primary mirror (M1) actuators to compensate the primary mirror cell (M1 Cell) deformation caused during the EL angle change in accordance with the requirements. 3. M1 Segment Handling System (SHS) to be able to make removing and installing 10 Mirror Segment Assemblies per day safely and with ease over M1 area where access of personnel is extremely difficult. This requires semi-automatic sequence operation and a robotic Segment Lifting Fixture (SLF) designed based on the Compliance Control System, developed for controlling industrial robots, with a mechanism to enable precise control within the six degrees of freedom of position control. 4. CO2 snow cleaning system to clean M1 every few weeks that is similar to the mechanical system that has been used at Subaru Telescope. 5. Seismic isolation and restraint systems with respect to safety; the maximum acceleration allowed for M1, M2, tertiary mirror (M3), LGSF, and science instruments in 1,000 year return period earthquakes are defined in the requirements. The Seismic requirements apply to any EL angle, regardless of the operational status of Hydro Static Bearing (HSB) system and stow lock pins. In order to find a practical solution, design optimization study for seismic risk mitigation was carried out extensively, including the performing of dynamic response analyses of the STR system under the time dependent acceleration profile of seven major earthquakes. The work is now moving to the final design phase from April 2014 for two years.
The Thirty Meter Telescope (TMT) is a public-private-international partnership to build an extremely large optical-infrared telescope on the summit of Mauna Kea on the island of Hawai'i. This paper summarizes the design and predicted performance of TMT, and provides updates on the status of the development and prototype testing activities. TMT is currently in its preconstruction phase. The roles of the partner institutions for developing and delivering observatory subsystems are now well defined, the design work is maturing and plans for construction are in place. Current plans are for start of construction in April 2014, with first light including all 492 segments installed by the end of 2021.
This paper presents refinements to the design of the TMT primary mirror segment passive-support system that are
effective in reducing gravity print-through and thermal distortion effects. First, a novel analytical method is presented
for tuning the axial and lateral support systems in a manner that results in improved optical performance when subject to
varying gravity fields. The method utilizes counterweights attached to the whiffletrees to cancel astigmatic and comatic
errors normally resulting when the lateral support system resists transverse loads induced by gravity. Secondly, several
central diaphragm designs are presented and analyzed to assess lateral-gravity and thermal distortion performance: 1) a
simple flat diaphragm, 2) a stress-relieving diaphragm having a slotted outer rim and a circumferential convolution near
the outside diameter, and 3) a flat diaphragm having a slotted outer rim. The latter design is chosen based on results from
analytical studies which show it to have better overall optical performance in the presence of gravity and thermal
The Thirty Meter Telescope (TMT) project, a partnership between ACURA, Caltech, and the University of California, is
currently developing a 30-meter diameter optical telescope. The primary mirror will be composed of 492 low expansion
glass segments. Each segment is hexagonal, nominally measuring 1.44m across the corners. Because the TMT primary
mirror is curved (i.e. not flat) and segmented with uniform 2.5mm nominal gaps, the resulting hexagonal segment
outlines cannot all be identical. All segmentation approaches studied result in some combination of shape and size
variations. These variations range from fractions of a millimeter to several millimeters. Segmentation schemes for the
TMT primary mirror are described in some detail. Various segmentation approaches are considered, with the goal being
to minimize various measures of shape variation between segments, thereby reducing overall design complexity and
cost. Two radial scaling formulations are evaluated for their effectiveness at achieving these goals. Optimal tuning of
these formulations and detailed statistics of the resulting segment shapes are provided. Finally, we present the rationale
used for selecting the preferred segmentation approach for TMT.
The Thirty Meter Telescope (TMT) project has revised the reference optical configuration from an Aplanatic Gregorian
to a Ritchey-Chrétien design. This paper describes the revised telescope structural design and outlines the design
methodology for achieving the dynamic performance requirements derived from the image jitter error budget. The usage
of transfer function tools which incorporate the telescope structure system dynamic characteristics and the control
system properties is described along with the optimization process for the integrated system. Progress on the structural
design for seismic considerations is presented. Moreover, mechanical design progress on the mount control system
hardware such as the hydrostatic bearings and drive motors, cable wraps and safety system hardware such as brakes and
absorbers are also presented.
This paper documents the methods used for the seismic design and analysis of the Thirty Meter Telescope (TMT)2. The
seismic analysis includes response spectrum and nonlinear time history methods. Several seismic restraint design options
are considered, both linear and nonlinear, and the seismic performance is presented for these options. The paper
addresses several issues specific to large optical telescope seismic design and analysis: generation of appropriate
response spectra and time histories; use of operational and survival level earthquakes; selection of damping coefficients;
use of reduced degree of freedom models and their calibration with more detailed models; and local response spectra for
This paper describes the studies performed to establish a baseline conceptual design of the Segment Support Assembly
(SSA) for the Thirty Meter Telescope (TMT) primary mirror. The SSA uses a combination of mechanical whiffletrees
for axial support, a central diaphragm for lateral support, and a whiffletree-based remote-controlled warping harness for
surface figure corrections. Axial support whiffletrees are numerically optimized to minimize the resulting gravityinduced
deformation. Although a classical central diaphragm solution was eventually adopted, several lateral support
concepts are considered. Warping harness systems are analyzed and optimized for their effectiveness at correcting
second and third order optical aberrations. Thermal deformations of the optical surface are systematically analyzed
using finite element analysis. Worst-case performance of the complete system as a result of gravity loading and
temperature variations is analyzed as a function of zenith angle using an integrated finite element model.
The Thirty Meter Telescope (TMT) project is a partnership between ACURA, AURA, Caltech, and the University of California. The design calls for a 3.6 m diameter secondary mirror and an elliptical tertiary mirror measuring more than 4 m along its major axis. Each mirror will weigh more than two metric tons and must be articulated to compensate for deformation of the telescope structure. The support and control of these "smaller optics" pose significant challenges for
the designers. We present conceptual designs for active and passive figure control and articulation of these optics.
The Thirty Meter Telescope (TMT) project has chosen a reference configuration with the telescope elevation axis above the primary mirror. The TMT telescope design has a segmented primary mirror, with 738 segments, nominally 1.2 m across corners, and it uses an articulated tertiary mirror to feed science light to predefined instrument positions on two large Nasmyth platforms. This paper outlines the development of the telescope structural design to meet the motion requirements related to the image quality error budget. The usage of opto-structural performance evaluation tools such as Merit Function Routine are described in addition with the optimization techniques used during the telescope structure design development.
The U.S. National Observatories have responded to the call of the astronomy decadal survey committee to develop a Giant Segmented Mirror Telescope by forming the AURA New Initiatives Office. Drawing on the engineering and scientific staffs of the National Optical Astronomy Observatory and the Gemini Observatory, NIO has for the past 30 months carried out studies aimed at: understanding the key science drivers for a thirty-meter telescope; developing a feasible point design that is responsive to the science goals; and identifying key technical issues that must be solved in order to successfully build such a telescope. In parallel, NIO has followed the charge of the decadal survey to identify potential private and international partners to fulfill the committee vision of a public-private partnership to build and operate this facility. NIO has now joined with two other groups -- the CELT Development Corporation (a partnership between the University of California and the California Institute of Technology) and the Association of Canadian Unviersities for Research In Astronomy (ACURA) -- to initiate the next step, the design & development (D & D) phase of a joint project that is being called the Thirty-Meter Telescope (TMT) Project. This paper reviews the plans for the TMT D & D phase, including the organizational structure, science requirements, and plans for conceptual design studies, technology development, and site selection.
Wind loading is a critical issue for large telescopes and will be even more problematic for proposed future giant telescopes. As the current-generation telescopes were being designed in the 1980s and 1990s, numerous studies were made to understand airflow through the enclosures and wind loading of the telescopes. Now that these telescopes are in operation, it is important to consider what can be learned from them to: (1) verify the assumptions and predictions of previous studies; (2) establish procedures to optimize telescope operation; and (3) guide the design of future extremely large telescopes. With these goals in mind, Gemini Observatory conducted a campaign during the integration of the southern Gemini telescope to simultaneously measure wind velocities inside and outside the enclosure and pressure variations on the (dummy) primary mirror. The data collected in this campaign have been analyzed and results are presented that address these three goals. This paper points out several results that are different from the assumptions of previous studies. It presents a rule of thumb for allowable wind speed around the Gemini primary mirror. Results are shown indicating that the average pressure pattern on the Gemini mirror is primarily produced by airflow around the telescope structure, but that much of the dynamic pressure variation at the mirror comes from turbulence generated by the enclosure. Also described is a strategy for developing realistic wind loading input for simulating the performance of an extremely large telescope, including the spatial and temporal variation of pressure on the primary mirror.
We describe a 'point design' for a 30m Giant Segmented Mirror Telescope (GSMT) aimed at meeting a set of initial science goals developed over a period of two years by working groups comprised of more than 60 astronomers. The paper summarizes these goals briefly, captures the top-level performance requirements that follow from them, and describes a plausible, first-cut technical solution developed as part of an overall systems-level analysis. The key features of the point design are: (1) a fast (f/1) primary; (2) an adaptive secondary that serves both to compensate for the effects of wind buffeting and as the first stage of three adaptive optics systems: (i) multi-conjugate AO; (ii) high-performance on-axis AO; (iii) ground-level seeing compensation; (3) a radio telescope structure; (4) multiple instrument ports (prime focus; Nasmyth foci; direct Cass); (5) an hierarchical control system comprising multiple active and adaptive elements.
For future giant telescopes, control of construction and operation costs will be the key factor in their success. The best way to accomplish this cost control, while maximizing the performance of the telescope, will be through design-to-cost methods that use value engineering techniques to develop the most cost-effective design in terms of performance per dollar. This will require quantifiable measures of performance and cost, including: (1) a way of quantifying science value with scientific merit functions; (2) a way of predicting telescope performance in the presence of real-world disturbances by means of integrated modeling; and (3) a way of predicting the cost of multiple design configurations.
Design-to-cost methods should be applied as early as possible in the project, since the majority of the life-cycle costs for the observatory will be locked in by choices made during the conceptual design phase. However, there is a dilemma: how can costs be accurately estimated for systems that have not yet been designed? This paper discusses cost estimating methods and describes their application to estimating the cost of ELTs, showing that the best method to use during the conceptual design phase is parametric cost estimating. Examples of parametric estimating techniques are described, based on experience gained from instrument development programs at NOAO.
We then describe efforts underway to collect historical cost information and develop cost estimating relationships in preparation for the conceptual design phase of the Giant Segmented Mirror Telescope.
Ground-based telescopes operate in a turbulent atmosphere that affects the optical path across the aperture by changing both the mirror positions (wind induced vibrations) and the air refraction index. Although the characteristics of the atmosphere are well understood in the inertial range, the validity of the homogeneous, isotropic field assumption is questionable inside the enclosure and in the close vicinity of the structure. To understand the effect of wind on an actual telescope, we conducted extensive wind measurements at the Gemini South Telescope. Simultaneous measurements were made of pressures at multiple points on the mirror surface, as well as wind velocity and direction at several locations inside and outside the dome. During the test we varied the dome position relative to the wind, the telescope elevation angle, the position of windscreens in the observing slit, and the size of the openings in the ventilation gates. The data sets have been processed to provide the temporal and spatial characteristics of the pressure variations on the primary mirror in comparison to the theory of atmospheric turbulence. Our investigation is part of an effort leading to the development of a scalable wind model for large telescope simulations, which describes the forces due to air turbulence on the primary mirror and telescope structure reasonably well even inside an enclosure.
One of the critical design factors for large telescopes is control of primary mirror distortion caused by wind pressure variations. To quantify telescope wind loading effects, the Gemini Observatory has conducted a series of wind tests under actual mountaintop conditions. During commissioning of the southern Gemini Telescope, simultaneous measurements were made of pressures at multiple points on the mirror surface, as well as wind velocity and direction at several locations inside and outside the dome. During the test we varied the dome position relative to the wind, the telescope elevation angle, the position of windscreens in the observing slit, and the size of the openings in the ventilation gates. Five-minute data records were made for 116 different test conditions, with a data-sampling rate of ten per second. These data sets have been processed to provide pressure maps over the surface of the mirror at each time instant. From these pressure maps, the optical surface distortions of the primary mirror have been calculated using finite-element analysis. Data reduction programs have been developed to enhance visualization of the test data and mirror surface distortions. The test results have implications for the design of future large telescopes.
The Gemini primary mirror support incorporates a system of hydraulic whiffletrees to carry the mirror weight and define its position. The six orthogonal kinematic degrees of freedom are controlled by six hydraulic zones--three axial, two lateral, plus a transverse lateral. By varying the fluid volumes in these hydraulic zones the mirror position can be adjusted in all six degrees of freedom. Because of the finite lengths of the linkages that connect the mirror to the lateral supports, any shift in mirror position changes the amplitudes and directions of the applied forces with a resulting effect on the static balance and mirror figure. These effects have been calculated for mirror translations and rotations in all six degrees of freedom, resulting in predictions of the changes in the axial and lateral support forces and in the mirror figure. This paper describes the modeling as well as experimental verification of the results.
There are many excellent science teachers, each of whom has a number of well-tested, successful demonstrations or ideas on how to teach some aspect of the field of optics. This reservoir of ideas is a valuable resource for integrating optics teaching into other science disciplines and into the general curriculum. How do we tap this resource? This paper outlines the usefulness of collecting ideas on optics teaching with the intention of publishing the collection in book form, while giving credit to each contributor. The authors are currently involved in collecting a wide range of ideas on teaching optics in order to crate a book that is useful in a variety of teaching situations. We hope that one result of this will be to encourage the teaching of optics in a large number of classrooms and at all grade levels.
This paper describes work done on the design of the thermal management system for the primary mirrors of the Gemini telescopes. The concept developed has a set of radiating plates behind the mirror, which can be used to heat or cool the mirror. In addition, there is a provision for heating the front surface of the mirror by passing a current through the reflective coating. It is shown that the heating and cooling together can be used to raise or lower the temperature of the surface of the mirror by about 1 degree per hour. Experiments and calculations are reported which show that the system can meet the target temperature range up to 90% of the time. The temperature gradients induced in the mirror have little effect on the optical performance. Experiments have shown that no degradation to the surface is caused by the current passing process. This approach potentially will allow thick mirrors of low thermal expansivity to follow rapid ambient air temperature changes, thereby avoiding mirror seeing.
The primary mirror selected for the Gemini 8-m Telescopes is a thin meniscus made of Corning ULETM glass. The conceptual design of the Gemini support system has evolved in response to the properties of the meniscus mirror and the functional requirements of the Gemini Telescopes. This paper describes the design requirements, the design features, and predicted performance of this system.
This paper describes optical testing procedures used at the National Optical Astronomy Observatories (NOAO) for testing large optics. It begins with a discussion of the philosophy behind the testing approach and then describes a number of different testing methods used at NOAO, including the wire test, full-aperture and sub-aperture Hartmann testing, and scatterplate interferometry. Specific innovations that enhance the testing capabilities are mentioned. NOAO data reduction software is described. Examples are given of specific output formats that are useful to the optician, using illustrations taken from recent testing of a 3.5- meter, f/1.75 borosilicate honeycomb mirror. Finally, we discuss some of the optical testing challenges posed by the large optics for the Gemini 8-meter Telescopes Project
The National Optical Astronomy Observatories has been working to extend existing fabrication techniques necessary to polish and figure large, steeply curved, structured borosilicate glass mirrors to exceedingly close tolerances. This paper describes the generation, grinding, and polishing techniques used to transform a 3.5 m diameter, f/1.75, glass casting into a precision spherical surface. The accuracy of the finished sphere was 0.52 (lambda) peak- to-valley and 0.066 (lambda) (42 nm) rms
An active optics system for a 3.5-meter f/1.75 borosilicate honeycomb mirror has been designed and built. The system hardware and software are described, and preliminary test results are presented that demonstrate the structured mirror responds well to the active optics control. Plans for extensive further testing are described. The results of the testing will guide a redesign of the system, before installation of the second-generation system in the WIYN Telescope, to be built on Kitt Peak in Arizona.
The National Optical Astronomy Observatories (NOAO) have been working for several years to develop the technology for 8-m telescopes using structured borosilicate glass primary mirrors. In March 1989 the final stage in this technology development program began with the delivery of a 3.5-m mirror blank, cast under NOAO contract at the University of Arizona Mirror Lab. The project will have four phases: (1) initial fabrication, (2) testing of support and thermal systems, (3) aspherizing the mirror and rework of the support and thermal systems, and (4) final acceptance test. At the conclusion of this effort there will be a finished 3.5-m f/1.75 mirror, which will then become the heart of the new WIN telescope on Kitt Peak.
The Astrophysical Research Consortium, the Magellan Project, and the National Optical Astronomy Observatories are collaborating in an experiment to study temperature control of a honeycomb mirror in an observatory environment. A full-scale mockup of a 60-deg sector of a 3.5-m borosilicate honeycomb mirror was constructed of glass and installed in the Apache Point Observatory (APO) 3.5-m telescope mount. The response of this mockup (the wedge) to diurnal temperature variations was monitored by measuring the glass temperature at about 300 locations distributed throughout its volume. The temperature of the wedge can be controlled via air circulated through its interior.