Thomas A. Sebring

Cornell Caltech Atacama Telescope (CCAT): a 25 m aperture telescope above 5000 m altitude

Cornell, California Institute of Technology (Caltech), and Jet Propulsion Lab (JPL) have joined together to study development of a 25 meter sub-millimeter telescope (CCAT) on a high peak in the Atacama region of northern Chile, where the atmosphere is so dry as to permit observation at wavelengths as short as 200 μm. The telescope is designed to deliver high efficiency images at that wavelength with a total one-half wavefront error of about 10 μm. With a 20 arc min field of view, CCAT will be able to accommodate large format bolometer arrays and will excel at carrying out surveys as well as resolving structures to the 2 arc sec resolution level. The telescope will be an ideal complement to ALMA. Initial instrumentation will include both a wide field bolometer camera and a medium resolution spectrograph. Studies of the major telescope subsystems have been performed as part of an initial Feasibility Concept Study. Novel aspects of the telescope design include kinematic mounting and active positioning of primary mirror segments, high bandwidth secondary mirror segment motion control for chopping, a Calotte style dome of 50 meter diameter, a mount capable of efficient scanning modes of operation, and some new approaches to panel manufacture. Analysis of telescope performance and of key subsystems will be presented to illustrate the technical feasibility and pragmatic cost of CCAT. Project plans include an Engineering Concept Design phase followed by detailed design and development. First Light is planned for early 2012.

The Cornell Caltech Atacama Telescope: progress and plans 2010

The CCAT Project is an effort to construct a 25 meter aperture telescope above 5600 meters altitude operating down to wavelengths as short as 200 μm. CCAT has developed some new and innovative approaches to telescope and optics design, added new partners to the project, and has plans for substantially increased activities over the next two years. Begun by Cornell University and the California Institute of Technology, CCAT currently has six national and university partners. Funding has been increased and significant technical activities are underway to investigate the key enabling technologies. Areas of development include telescope optical design, mount design, application of CFRP materials to the telescope, sensing and control of primary mirror segments, and control system architecture. Schedules and budgets for the Project have been updated and an overall approach leading to first light in 2016-2017 has been developed. CCAT promises to have a significant scientific impact on submillimeter astronomy and the prospects for success has never looked better.

Development of lightweight, stiff, stable, replicated glass mirrors for the Cornell Caltech Atacama Telescope (CCAT)

The 25 m aperture Cornell Caltech Atacama Telescope (CCAT) will be the first segmented telescope of its size and precision. A new technology was required to be able to economically manufacture the segments for the primary mirror. This technology had to be a low cost, low risk, volume manufacturing process in addition to meeting all of the optical and mechanical requirements. The segments had to be lightweight (10-15 kg/m2), have high specific stiffness and be thermally stable. The segments had to have sufficient robustness for practical transport and use and be compatible with high-reflectivity coatings. ITT has designed a replicated, lightweight glass mirror solution to these manufacturing problems. This technology can be used to fabricate segments for CCAT. It can be used to fabricate segments for visible wavelength segmented telescopes or any other application requiring lightweight optics in large quantities. This technology enables the fabrication of large, lightweight mirror segments in a few weeks to a couple of months, depending on the figure requirements. This paper discusses the design of these mirrors and presents demonstrated results to date, including a 0.5 m diameter, 8 kg/m2 borosilicate mirror blank and 0.2 m diameter replicated borosilicate mirrors.

Final design of the LSST hexapods and rotator

The Large Synoptic Survey Telescope (LSST) is a large (8.4 meter) wide-field (3.5 degree) survey telescope, which will be located on the Cerro Pachón summit in Chile. Both the Secondary Mirror (M2) Cell Assembly and Camera utilize hexapods to facilitate optical positioning relative to the Primary/Tertiary (M1M3) Mirror. A rotator resides between the Camera and its hexapod to facilitate tracking. The final design of the hexapods and rotator has been completed by Moog CSA, who are also providing the fabrication and integration and testing. Geometric considerations preclude the use of a conventional hexapod arrangement for the M2 Hexapod. To produce a more structurally efficient configuration the camera hexapod and camera rotator will be produced as a single unit. The requirements of the M2 Hexapod and Camera Hexapod are very similar; consequently to facilitate maintainability both hexapods will utilize identical actuators. The open loop operation of the optical system imposes strict requirements on allowable hysteresis. This requires that the hexapod actuators use flexures rather than more traditional end joints. Operation of the LSST requires high natural frequencies, consequently, to reduce the mass relative to the stiffness, a unique THK rail and carriage system is utilized rather than the more traditional slew bearing. This system utilizes two concentric tracks and 18 carriages.

CFRP truss for the CCAT 25 m diameter submillimeter-wave telescope

CCAT will be a 25 m diameter submillimeter-wave telescope that will operate inside a dome located on Cerro Chajnantor in the Atacama Desert. The telescope must have high aperture efficiency at a wavelength of 350 microns and good performance out to a wavelength of 200 microns. A conceptual design for a carbon fiber reinforced plastic (CFRP) truss and primary reflector support truss has been developed. This design yields a telescope with a net ½ wave front error of <10 microns using a lookup table to adjust the segment actuators to compensate for gravitational deflections. Minor corrections may be required to compensate for the expected 20 C temperature excursions. These can be handled using a coarse lookup table.

The Cornell Caltech Atacama Telescope status and technical progress

Five partners have currently joined a Consortium to develop the Cornell Caltech Atacama Telescope (CCAT.) Included are Cornell University, the California Institute of Technology (Caltech), the University of Colorado at Boulder, the United Kingdom as represented by the Astronomy Technology Centre (ATC), and Canada as represented by the Universities of British Columbia and Waterloo. This consortium has continued work toward the design of the telescope and instrumentation, pursued fund raising, and further developed the science case for CCAT. An Engineering Design Phase is being planned for 2009-2011 with construction planned to begin shortly thereafter. CCAT continues as a wide field (20 arc min) FOV telescope operating from a shortest wavelength of 200µ. Testing has continued near the summit of Cerro Chajnantor in the Atacama Region of Chile above 5600 meters altitude and data indicates significantly lower water vapor in the seeing column than measured at the ALMA site on the plateau below. Work over the past two years has included research on manufacturing methods for optical segments, extensive study of mirror alignment sensing and control techniques, additional concepts for major structures, and further development of instrumentation.

The Cornell Caltech atacama telescope (CCAT)

The CCAT will be a 25 m telescope for submillimeter astronomy located at 5600 m altitude in northern Chile. An international consortium has formed to carry out the project. CCAT will combine high sensitivity, a wide field of view (20acute ), and a broad wavelength range (2 mm-200 mum) to provide an unprecedented capability for deep, large area multi-color submillimeter surveys that complement narrow field, high resolution studies with ALMA. Science objectives include galaxy formation and evolution, star formation, protoplanetary and debris disks, and Kuiper belt objects. Instrumentation will include bolometer cameras, direct detection spectrometers, and heterodyne receiver arrays.

Cornell Caltech Atacama Telescope primary mirror surface sensing and controllability

To meet the 10 µm RMS half wavefront error requirement for the 25 m diameter Cornell Caltech Atacama Telescope (CCAT), active control of the approximately 200 primary mirror panels is required. The CCAT baseline design includes carbon fiber aluminum honeycomb sandwich mirror panels. Distortions of the panels due to thermal gradients, gravity and the mounting scheme need to be taken into consideration in the control system design. We have modeled the primary mirror surface as both flat and curved surfaces and have investigated mirror controllability with a variety of sensor types and positions. To study different mirror segmentation schemes and find acceptable sensor configurations, we have created a software package that supports multiple segment shapes and reconfigurable panel sizing and orientation. It includes extensible sensor types and flexible positioning. Inclusion of panel and truss deformations allows modeling the effects of thermal and gravity distortions on mirror controllability. Flat mirrors and curved mirrors with the correct prescription give similar results for controlled modes, but show significant differences in the unsensed flat mirror modes. Both flat and curved mirror models show that sensing schemes that work well with rigid, thermally stable panels will not control a mirror with deformable panels. Sensors external to the mirror surface such as absolute distance measurement systems or Shack-Hartmann type sensors are required to deal with panel deformations. Using a combination of segment based sensors and external sensors we have created a promising prototype control system for the CCAT telescope.

LSST secondary mirror system final design

The Large Synoptic Survey Telescope (LSST) has a 10 degrees square field of view which is achieved through a 3 mirror optical system comprised of an 8.4 meter primary, 3.5 meter secondary (M2) and a 5 meter tertiary mirror. The M2 is a 100mm thick meniscus convex asphere. The mirror surface is actively controlled by 72 axial electromechanical actuators (axial actuators). Transverse support is provided by 6 active tangential electromechanical actuators (tangent links). The final design has been completed by Harris Corporation. They are also providing the fabrication, integration and testing of the mirror cell assembly, as well as the figuring of the mirror. The final optical surface will be produced by ion figuring. All the actuators will experience 1 year of simulated life testing to ensure that they can withstand the rigorous demands produced by the LSST survey mission. Harris Corporation is providing optical surface metrology to demonstrate both the quality of the optical surface and the correctablility produced by the axial actuators.

Design concepts for primary mirror structures of large optical telescopes for optical and submillimeter astronomy

Technologies of modern optical telescopes with large primary mirror are based on adaptive optics. These telescopes operate with many small mirror segments, so that all the segments work as a large piece of a reflective curved plate, i.e. a paraboloid. Each mirror segment is independently attached to a support structure via adjustable warping harnesses. A support structure is required to be extremely rigid in order to maintain the reflective surface. This paper describes the conceptual approach for the design of such support structures. A system proven to fulfill these requirements with efficient structural material use is a node-and-bar system, so-called space frame. The rules for geometry of space frame structures are based on the system of the five platonic solids: The edges of the conceptually assembled solids can be replaced by the bar members of a space frame to achieve maximum stiffness. This conceptual approach is demonstrated with examples in the paper, by illustrating the determination of the geometry and examining the deformation due to the telescope rotations during operation. This paper also demonstrates design solutions for other issues relevant to space frame geometry, such as effects of gradient thermal load and redundancy of the structures.