The Haystack radio telescope is being upgraded to support imaging radar applications at 96 GHz. The Cassegrain antenna includes a 37 m diameter primary reflector comprising 432 reflector panels and a 2.84 m diameter hexapod mounted subreflector. Top-level antenna performance is based on meeting diffraction-limited performance over an elevation range of 10 - 40° resulting in a maximum RF half pathlength error requirement of 100 μm RMS. RF-mechanical performance analyses were conducted that allocated subsystem
requirements for fabrication, alignment, and environmental effects. Key contributors to system level performance are discussed. The environmental allocations include the effects of gravity, thermal gradients, and diurnal thermal variations which are the dominant error source. Finite element methods and integrated optomechanical models were employed to estimate the environmental performance of the antenna and provide insight into thermal management strategies and subreflector compensation. Fabrication and alignment errors include the manufacturing of the reflector surface panels and assembly of overall reflector surface.
This paper will discuss analysis and design of large ground based telescopes for seismic hazard. Seismic hazard is an
important issue for both the observatory and the telescope structure. Properly defined seismic specifications are vital.
These specifications should include performance objective that matches performance levels and probabilistic based
hazard levels for operational and survival conditions. The paper will discuss specific tools that utilize results of existing
seismic hazard assessment programs and can be used for initial seismic assessment during site selection. In the final
stage of site selection, site specific probabilistic seismic-hazard studies that account for local geological settings and
active faults should be used. The results of these site specific studies usually include response spectra and time history
records in horizontal and vertical directions for operational and survival conditions. Different methods to analyze the
telescope structure for seismic loadings, such as, equivalent static analysis, response spectrum analysis, linear and
nonlinear time history analysis, are discussed. Devices that mitigate seismic forces and/or deformations are also
Many antennas, such as the 100-m Green Bank Telescope, use a wheel-on-track systems in which the track segments
consist of wear plates mounted on base plates. The wear plates are typically 2 to 3 inches thick and are case hardened or
through hardened. The base plates are usually 3 to 4 times thicker than the wear plates and are not hardened. The wear
plates are typically connected to the base plates using bolts. The base plates are supported on grout and anchored to the
underlying concrete foundation. For some antennas, slip has been observed between the wear plate and base plate, and
between the base plate and the grout, with the migration in the wheel rolling direction. In addition, there has been wear
at the wear plate/base plate interface. This paper is an update on the evaluation of GBT track retrofit. The paper
describes the use of three-dimensional non-linear finite element analyses to understand and evaluate the behavior of (1)
the existing GBT wheel-on-track system with mitered joints, and (2) the various proposed modifications. The
modifications include welding of the base plate joints, staggering of the wear plate joints from the base plate joints,
changing thickness of the wear plate, and increasing bolt diameter and length. Parameters included in the evaluation
were contact pressure, relative slip, wear at the wear plate/base plate interface, and bolt shears and moments.
Optimization programs are used routinely in the design of precision structures such as radio telescopes, but design optimization is merely the beginning, rather than the end, of the process. After selecting appropriate member sizes for the optimized solution, careful attention must be paid to the design of the connections to ensure that the effective stiffness of a given member matches that of the optimized design. Given that members rarely span directly to their working points, the effective stiffness of a member is actually a combination of the member cross-section stiffness and the stiffness of the connections at each end. This paper describes the procedure used for the design of a large radio telescope with tubular members and bayonet style connections. Initially, a parametric study was performed to establish gusset plate thicknesses and widths for various tube cross-sections so that the resulting tube/gusset subassembly would match the stiffness of the tubing; the results of this study provided a starting point for designing the connections. Following optimization and member selection for the overall structure, detailed finite element models were constructed
for selected connections to assess the effective stiffness of each member framing into these connections. The overall goal was to hold the effective stiffnesses to within a given tolerance. This was accomplished by adjusting plate thicknesses, plate widths, tube-to-plate engagement lengths, and, in a few cases, actually changing the member crosssection to compensate for excessive stiffness or softness at a given connection.
Haystack, MIT's 37-m radio telescope, was built in the early 1960s. At the time considered to be a high-performance antenna, Haystack produced a number of outstanding scientific results. The antenna, originally designed to operate at 8-10 GHz, was upgraded at various times, notably in 1993 with the addition of a deformable subreflector to allow operations at 115 GHz. Planning is now underway for a major upgrade with the replacement of the entire elevation structure that is supported on the existing yoke and tower. The new antenna should be capable of operating at up to 325 GHz. In this paper, we will describe the limitations of the original design, the solutions used in the previous upgrades, and how the lessons learned led to the approach used in the planned upgrade. The major issues limiting the further upgrade of the existing telescope were in the elevation structure; these included fabrication tolerances and gravity sag of the reflector panels, thermal lag of a ring plate supporting the reflector panels, non-repeatable behavior of the sliding joint at the elevation bearing and shear pins, and the interaction of the steel yoke and the aluminum backstructure.
Many antennas use wheel-on-track systems in which track segments consist of wear plates mounted on base plates. The hardened wear plates are typically connected to the base plates using bolts, and the base plates are supported on grout and anchored to the underlying concrete foundation. For some antennas, slip has been observed between the wear plate and base plate, and between the base plate and the grout, with migration in the wheel rolling direction. In addition, there has been wear at the wear plate/base plate interface. This paper describes the use of finite element models (FEM's) of the wheel, track, and foundation to understand the behavior of the wheel-on-track system, and to evaluate possible retrofit concepts. The FEM’s are capable of representing friction and slip, and the opening and closing of gaps at the interfaces between the wheel, wear plate, base plate, and grout. The FEM’s can capture the behavior of the components as the wheel rolls forward. The paper also describes a method to estimate the amount of wear at the wear plate/base plate interface based on the relative slip and contact pressure between the wear plate and base plate.
The Giant Segmented Mirror Telescope (GSMT) is a 30-m fully-steerable ground-based optical/infra-red telescope with actively controlled segmented mirrors. This paper presents the initial point design for the telescope structure which was used to evaluate the expected structural behavior needed in developing other aspects of the telescope design, in particular, the adaptive optics system. The primary mirror consists of 618 hexagonal 1.152-m mirror segments supported on 91 rafts. A typical raft supports seven mirror segments. There are two level of actuation; each mirror segment is supported by actuators mounted on the raft structure and each raft is supported by another level of actuators mounted on the elevation structure. A radio telescope-type design is used for its structural advantages as well as for accommodation of large instruments. In particular, the location of the elevation bearings allows for an efficient support of the primary and a smaller azimuth structure. The elevation structure consists of (1) a space truss backstructure that supports the rafts, (2) a braced tripod structure that supports the prime focus instrument or secondary mirror, and (3) the transition structure and elevation wheel that connect the backstructure to the elevation bearings and provide support for the drive arc and the counterweight. The azimuth structure accommodates a large Nasmyth platform. Issues covered in this paper include: natural frequencies, response to wind buffeting, deflection limits to accommodate actuator strokes, span between elevation bearings, braced tripod to minimize blockage, and configuration to maximize space for instruments.
A novel track joint was developed for the azimuth track of the 50-m diameter Large Millimeter Telescope (LMT) now under construction in Mexico at an elevation of 4,600 m. The track, which is 430 mm wide by 230 mm deep, must be flat to within ± 0.3 mm, and the material hardness at least 290 Brinell. This design uses a partial penetration narrow gap groove weld on the top surface of the track and a splice plate welded to the underside of the track. Pre-camber of the joint compensates for weld shrinkage which is small because of the use of the narrow gap groove weld. The residual deviations from flatness are reduced to the required tolerance by adjusting anchor bolts using an optimization procedure. The feasibility of the design with respect to fabrication, strength, fatigue, and alignment was demonstrated by detailed finite element analyses, trial welding and alignment of full scale joints, and testing of the mechanical properties of the joint and adjacent metal.
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.
The LMT is a 50-meter-diameter fully-steerable radio telescope to be used for astronomy at millimeter wavelengths. The specifications call for a surface accuracy of 70 micrometer RMS and a pointing accuracy of 0.7 arc sec RMS. This paper addresses two critical issues of the design development. To achieve the required accuracy it was proposed that the primary reflector of the LMT utilize an actively controlled segmented surface with a real time closed loop control system. In the approach developed for the preliminary design systematic deformations of the segments result in the magnification and propagation of surface errors; this problem is examined and possible solutions presented. Limits on the applicability of homology to the design of millimeter wave radio telescopes are discussed in the context of the need for an active surface. A proposed approach for computing in real time the pointing errors due to thermal deformations is evaluated. The results show the limits of applicability of this approach.
The Multiple Mirror Telescope (MMD on Mt. Hopkins AZ, which uses an array of six telescopes on a single mount to achieve an effective aperture of 4.5-rn, is to be converted to a 6.5-rn telescope with a single prirnary. This paper describes the objectives, constraints, and solutions for a conceptual design of the Optics Support Structure (OSS) for the 6.5-rn telescope.