Joseph Antebi

Upgrading the Haystack radio telescope for operation at 115 GHz

The Haystack 37-m radio telescope has been upgraded for operation at frequencies up to 115 GHz. A unique deformable subreflector with active actuator control has been developed to correct for gravitational distortion effects including astigmatism and deflections associated with the particular reflector surface. Active thermal compensation of the surface has also been implemented to conduct for both thermal lag effects and circularly symmetric gravitational deviations. Holographic mapping of the antenna surface deviations was achieved using 12-GHz geostationary satellite transmissions, which required the use of special techniques to correct for diffraction and multiple reflection effects involving the space-frame radome that covers the antenna. Realignment of the antenna surface utilized a finite-element structural model to translate surface deviations to motions of the unusual adjustment mechanisms on the antenna. The currently measured rms surface deviation (Dec. 23, 1992) is 210 /spl mu/m. The telescope has been equipped with a two-channel cryogenically cooled 3-mm SIS receiver, covering the range from 84 to 115 GHz. A new flexible digital spectrometer has been constructed for spectral line astronomy. Configurations can range from a widest bandwidth coverage of 160 MHz at 512 lags to 0.66 MHz at 4096 lags. Examples are given of surface holographic maps and radio measurements of aperture efficiency, pointing, and other performance parameters. >

Optimal surface adjustment of Haystack antenna

The primary reflector panels of the 37-m (120-ft) diameter Haystack antenna are prestressed to form an integrated parabolic shell of revolution. The adjustment mechanisms of the reflector surface are highly interacting, and the region of influence of each adjustment mechanism is large and intersects in a major way the influence regions of other adjustment mechanisms. The influence surface for each adjustment is computed using a detailed finite-element model of the antenna and the reflector structures. The optimal adjustments, i.e. the adjustments that minimize surface RMS, are obtained using the computed influence surfaces by solving a quadratic programming problem. The resolution of holography introduces errors in the holography map, but the resulting error in the computed adjustments are eliminated by using, in lieu of the actual influence surfaces, the transformed influence surfaces obtained by the convolution of the actual influence surfaces with the holography resolution function. The procedure, which was used to reduce surface RMS of the Haystack from 639 micron (25.1 mil) to 194 micron (7.6 mil), is applicable to other antennas. >

A deformable subreflector for the Haystack radio telescope

A deformable subreflector was designed and implemented to compensate for part of the gravity deformations of the primary reflector of Haystack, a 37-m-(120-ft-)diameter Cassegrain radio telescope. This was done to allow it to operate at 100+ GHz, as compared to the 1-to-10 GHz range for which it was originally designed. The design, analysis, construction, testing, and the results of preliminary measurements of performance are presented. The deformable subreflector consists of a fiberglass shell, supported on an aluminum back structure. The homologous components of deformations are compensated for by optimal positioning of the subreflector, which can be displaced axially and laterally, and tilted. The deformation modes of the subreflector compensate for astigmatic deformations of the back structure of the primary, and for part of the symmetric and anti-symmetric components of gravity sag of the panels of the primary reflector. Analyses show that, due to the deformable subreflector, the surface RMS due to gravity has been reduced from 494 mm (19.4 mil) down to 146 mm (5.7 mil), as the antenna travels over its operating range of 15 to 70 degrees elevation. Combining the reduced gravity effects with surface adjustment and thermal errors results in a predicted combined surface error of 250 mm (9.8 mil), at the extremes of the operating range.<<ETX>>

Precision continuous high-strength Azimuth track for large telescopes

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.

RF-mechanical performance for the Haystack radio telescope

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.

Conversion of the MMT to a 6.5-m telescope: the optics support structure and the enclosure

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.

Seismic hazard: analysis and design of large ground-based telescopes

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 presented.

Update on slip and wear in multi-layer azimuth track systems

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.

Slip and wear in multilayer azimuth track systems

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 (FEMs) 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.

Replacement of elevation structure to upgrade Haystack 37-m radio telescope

Haystack, MITs 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.