Philippe Raffin

Improvement of the IRAM 30-m telescope from temperature measurements and finite-element calculations

Millimeter wavelength radio telescopes built in a conventional way from steel and aluminum require elaborate thermal control to guarantee small structural deformations and good observational performance. We describe the temperature monitoring system of the Institut de Radioastronomie Millime/spl acute/trique 30-m telescope and the use of temperature measurements in finite-element calculations of structural deformations. These calculations reproduce with good precision the measured thermal deformations of the telescope and allow the investigation and localization of thermally important elements in the telescope structure. The data are used for calculation of temperature induced main reflector surface deformations and of the associated actual beam pattern, and for prediction and real-time correction of the focus. The pointing cannot be fully predicted since the available finite-element model does not include the Nasmyth focus cabin (and the concrete pedestal). The long-term investigation of the telescopes thermal behavior led to an improvement of the thermal control system and to a better performance of the telescope.

The AMiBA Hexapod Telescope Mount

The Array for Microwave Background Anisotropy (AMiBA) is the largest hexapod astronomical telescope in current operation. We present a description of this novel hexapod mount with its main mechanical components—the support cone, universal joints, jack screws, and platform—and outline the control system with the pointing model and the operating modes that are supported. The AMiBA hexapod mount performance is verified based on optical pointing tests and platform photogrammetry measurements. The photogrammetry results show that the deformations in the inner part of the platform are less than 120 μm rms. This is negligible for optical pointing corrections, radio alignment, and radio phase errors for the currently operational seven-element compact configuration. The optical pointing error in azimuth and elevation is successively reduced by a series of corrections to about 0 4 rms which meets our goal for the seven-element target specifications.

Greenland telescope project: Direct confirmation of black hole with sub‐millimeter VLBI

A 12 m diameter radio telescope will be deployed to the Summit Station in Greenland to provide direct confirmation of a Super Massive Black Hole (SMBH) by observing its shadow image in the active galaxy M87. The telescope (Greenland Telescope: GLT) is to become one of the Very Long Baseline Interferometry (VLBI) stations at sub-millimeter (submm) regime, providing the longest baseline >9000 km to achieve an exceptional angular resolution of 20 µas at 350 GHz, which will enable us to resolve the shadow size of ~40 µas. The triangle with the longest baselines formed by the GLT, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, and the Submillimeter Array (SMA) in Hawaii will play a key role for the M87 observations. We have been working on the image simulations based on realistic conditions for a better understanding of the possible observed images. In parallel, retrofitting of the telescope and the site developments are in progress. Based on 3 years of opacity monitoring at 225 GHz, our measurements indicate that the site is excellent for submm observations, comparable to the ALMA site. The GLT is also expected to make single-dish observations up to 1.5 THz.

CFRP platform and hexapod mount for the Array of MIcrowave Background Anisotropy (AMiBA)

AMiBA consists of a 90 GHz interferometric array telescope with dishes ranging in size from 0.3 to 2.4 meter in diameter, mounted on a 6-meter fully steerable platform. The dishes are attached to the receivers, which are mounted on a platform controlled by a six degree of freedom hexapod mount. The hexapod mount is a parallel connection manipulator also called Stewart Platform. The basic reference for this mechanism is a paper by Stewart. The Stewart Platform is a unique kinematically constrained work platform. It can be manipulated through the six degrees of freedom. The hexapod also provides better accuracy, rigidity, load to weight ratio and load distribution than a serial manipulator or traditional manipulator. The advantages of the hexapod shows that it is a great choice for the AMiBA project. Vertex Antennentechnik GmbH fabricates the hexapod. Testing has started in Germany. The telescope will be delivered in the summer of 2004. The 6m in diameter hexagonal platform is made of carbon fiber reinforced plastics (CFRP) and consists of seven pieces of three different unique types. The platform can be disassembled and fits in a container for transportation. The mounting plane flatness is an important issue for the platform assembly. The deflection angle of the mounting plane relative to any other mounting position must be less than 20 arcsec. Meanwhile, the platform must endure a loading of 3 tons. The platform has been built by Composite Mirror Applications, Inc. (CMA) in Tucson, and mounted on the Hexapod in Germany. This report describes the design and testing of platform and mount for the AMiBA telescope.

Progress of the array of microwave background anisotropy (AMiBA)

The Academia Sinica, Institute for Astronomy and Astrophysics (ASIAA) is installing the AMiBA interferometric array telescope at the Mauna Loa Observatory, Hawaii. The 6-meter carbon fiber fully steerable platform is mounted on the Hexapod Mount. After integration and equipment with dummy weights, the platform has been measured by photogrammetry to verify its behavior predicted by Finite Element Analysis. The Hexapod servo control is now operational and equipment of the platform with the initial 7 60-cm dishes, the correlator and electronics is underway. Pointing has started with the aid of the optical telescope. We present the status of the telescope after the servo and initial pointing tests have been carried out. We also present the results of platform measurements by photogrammetry.

THE AMIBA PROJECT

The Array for Microwave Background Anisotropy is a 7-element interferometer to be sited on Mauna Loa, Hawaii. The seven 1.2m telescopes are mounted on a 6-meter platform, and operates at 3mm wavelength. At the time of this meeting, the telescope is under construction at the Vertex factory in Germany. It is due to be delivered in the middle of 2004. A 2-element prototype instrument has already been deployed to Mauna Loa where initial tests are underway.

1.2 m Shielded Cassegrain Antenna for Close-Packed Radio Interferometer

Interferometric millimeter observations of the cosmic microwave background and clusters of galaxies with arcminute resolutions require antenna arrays with short spacings. Having all antennas co-mounted on a single steerable platform sets limits to the overall weight. A 25 kg lightweight novel carbon-fiber design for a 1.2 m diameter Cassegrain antenna is presented. The finite element analysis predicts excellent structural behavior under gravity, wind, and thermal load. The primary- and secondary-mirror surfaces are aluminum-coated with a thin TiO2 top layer for protection. A low beam sidelobe level is achieved with a Gaussian feed-illumination pattern with edge taper, designed based on feed-horn antenna simulations and verified in a far-field beam-pattern measurement. A shielding baffle reduces interantenna coupling to below ~-135 dB. The overall antenna efficiency, including a series of efficiency factors, is estimated to be around 60%, with major losses coming from the feed spillover and secondary blocking. With this new antenna, a detection rate of about 50 clusters yr-1 is anticipated in a 13-element array operation.

Photogrammetry measurement of the AMiBA 6-meter platform

This paper describes the photogrammetry method as a mean to measure the deformation of the 6-meter carbon fiber reinforced plastic (CFRP) Platform of the AMiBA interferometric array telescope installed at the Mauna Loa Observatory, Hawaii. The Platform was surveyed at a series of elevation, azimuth and polarization angles. Photogrammetry demonstrates that the deformation of the Platform is not only gravity-induced but also due to the Hexapod mount actuator. The measurement results verify the predictions of the Finite Element Analysis (FEA).

Platform deformation refined pointing and phase correction for the AMiBA hexapod telescope

The Array for Microwave Background Anisotropy (AMiBA) is a radio interferometer for research in cosmology, currently operating 7 0.6m diameter antennas co-mounted on a 6m diameter platform driven by a hexapod mount. AMiBA is currently the largest hexapod telescope. We briefly summarize the hexapod operation with the current pointing error model. We then focus on the upcoming 13-element expansion with its potential difficulties and solutions. Photogrammetry measurements of the platform reveal deformations at a level which can affect the optical pointing and the receiver radio phase. In order to prepare for the 13-element upgrade, two optical telescopes are installed on the platform to correlate optical pointing tests. Being mounted on different locations, the residuals of the two sets of pointing errors show a characteristic phase and amplitude difference as a function of the platform deformation pattern. These results depend on the telescopes azimuth, elevation and polarization position. An analytical model for the deformation is derived in order to separate the local deformation induced error from the real hexapod pointing error. Similarly, we demonstrate that the deformation induced radio phase error can be reliably modeled and calibrated, which allows us to recover the ideal synthesized beam in amplitude and shape of up to 90% or more. The resulting array efficiency and its limits are discussed based on the derived errors.

The Greenland Telescope: antenna retrofit status and future plans

Since the ALMA North America Prototype Antenna was awarded to the Smithsonian Astrophysical Observatory (SAO), SAO and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) are working jointly to relocate the antenna to Greenland. This paper shows the status of the antenna retrofit and the work carried out after the recommissioning and subsequent disassembly of the antenna at the VLA has taken place. The next coming months will see the start of the antenna reassembly at Thule Air Base. These activities are expected to last until the fall of 2017 when commissioning should take place. In parallel, design, fabrication and testing of the last components are taking place in Taiwan.