Ming-Tang Chen

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.

Absorption Properties of Supercooled Liquid Water between 31 and 225 GHz: Evaluation of Absorption Models Using Ground-Based Observations

Microwave radiometers (MWR) are commonly used to quantify the amount of supercooled liquid water (SLW) in clouds; however, the accuracy of the SLW retrievals is limited by the poor knowledge of the SLW dielectric properties at microwave frequencies. Six liquid water permittivity models were compared with ground-based MWR observations between 31 and 225GHz from sites in Greenland, the German Alps, and a low-mountain site; average cloud temperatures of observed thin cloud layers range from 08 to 2338C. A recently published method to derive ratios of liquid water opacity from different frequencies was employed in this analysis. These ratios are independent of liquid water path and equal to the ratio of aL at those frequencies that can be directly compared with the permittivity model predictions. The observed opacity ratios from all sites show highly consistent results that are generally within the range of model predictions; however, none of the models are able to approximate the observations over the entire frequency and temperature range. Findings in earlier published studies were used to select one specific model as a reference model for aL at 90GHz; together with the observed opacity ratios, the temperature dependence of aL at 31.4, 52.28, 150, and 225GHz was derived. The results reveal that two models fit the opacity ratio data better than the other four models, with one of the two models fitting the data better for frequencies below 90GHz and the other for higher frequencies. These findings are relevant for SLW retrievals and radiative transfer in the 31–225-GHz frequency region.

A wideband analog correlator system for AMiBA

A wideband correlator system with a bandwidth of 16 GHz or more is required for Array for Microwave Background Anisotropy (AMiBA) to achieve the sensitivity of 10μK in one hour of observation. Double-balanced diode mixers were used as multipliers in 4-lag correlator modules. Several wideband modules were developed for IF signal distribution between receivers and correlators. Correlator outputs were amplified, and digitized by voltage-to-frequency converters. Data acquisition circuits were designed using field programmable gate arrays (FPGA). Subsequent data transfer and control software were based on the configuration for Australia Telescope Compact Array. Transform matrix method will be adopted during calibration to take into account the phase and amplitude variations of analog devices across the passband.

AMiBA WIDEBAND ANALOG CORRELATOR

A wideband analog correlator has been constructed for the Yuan-Tseh Lee Array for Microwave Background Anisotropy. Lag correlators using analog multipliers provide large bandwidth and moderate frequency resolution. Broadband intermediate frequency distribution, back-end signal processing, and control are described. Operating conditions for optimum sensitivity and linearity are discussed. From observations, a large effective bandwidth of around 10 GHz has been shown to provide sufficient sensitivity for detecting cosmic microwave background variations.

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.

Receiver-beam characterization for the SMA

This paper describes the measurement and subsequent data analysis of the signal beam of the receiver system of the Submillimeter Array (SMA). To measure the receiver beam patterns, a 2-axis planar scanning stage is mounted on top of the receiver assembly. Scanning in a plane orthogonal to the optical path, the near-field measuring system maps out both the amplitude and phase at frequencies of 242 and 265 GHz. By analyzing the measured patterns we can inspect the alignment of an individual receiver, and the optical assembly common to all frequency bands. The data also allows us to determine how the receiver couples to the beam waveguide that feeds the SMA antennas. The measured beam patterns of two different receivers, covering frequency bands of 176 - 256 and 250 - 350 GHz, are presented, as well as the analysis of these data. We believe that this is the first time such a rigorous full vectorial radio alignment technique has been applied to millimeter or submillimeter receiving systems for radio astronomy.

W-band dual-polarization receiver for array of microwave background anisotropy (AMiBA)

This is to report on our development for a dual-polarization receiver to detect the cosmic microwave background (CMB) in 85 to 105 GHz band. The receiver is based on a MMIC, HEMT-based LNA developed in the Jet Propulsion Laboratory. A W-band, orthomode transducer (OMT) is used for polarization separation. Most of the RF front-end is located in cryogenics environment at 20K. We have developed a MMIC sub-harmonically pumped diode mixer, operating at 42 GHz, for signal down-conversion. The entire base-band, 2 to 18 GHz, is correlated in a lag-correlator system. The receiver design details and the lab test results will be described in this report.