Chih-Chiang Han

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.

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.

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.

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.

The Greenland Telescope (GLT): antenna status and future plans

The ALMA North America Prototype Antenna was awarded to the Smithsonian Astrophysical Observatory (SAO) in 2011. SAO and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), SAO’s main partner for this project, are working jointly to relocate the antenna to Greenland to carry out millimeter and submillimeter VLBI observations. This paper presents the work carried out on upgrading the antenna to enable operation in the Arctic climate by the GLT Team to make this challenging project possible, with an emphasis on the unexpected telescope components that had to be either redesigned or changed. Five-years of inactivity, with the antenna laying idle in the desert of New Mexico, coupled with the extreme weather conditions of the selected site in Greenland have it necessary to significantly refurbish the antenna. We found that many components did need to be replaced, such as the antenna support cone, the azimuth bearing, the carbon fiber quadrupod, the hexapod, the HVAC, the tiltmeters, the antenna electronic enclosures housing servo and other drive components, and the cables. We selected Vertex, the original antenna manufacturer, for the main design work, which is in progress. The next coming months will see the major antenna components and subsystems shipped to a site of the US East Coast for test-fitting the major antenna components, which have been retrofitted. The following step will be to ship the components to Greenland to carry out VLBI

AMiBA: Cluster Sunyaev-Zel'dovich Effect Observations with the Expanded 13-Element Array

The Yuan-Tseh Lee Array for Microwave Background Anisotropy (AMiBA) is a co-planar interferometer array operating at a wavelength of 3 mm to measure the Sunyaev-Zel’dovich effect (SZE) of galaxy clusters at arcminute scales. The first phase of operation ‐ with a compact 7-element array with 0.6 m antennas (AMiBA7) ‐ observed six clusters at angular scales from 5 0 to 23 0 . Here, we describe the expansion of AMiBA to a 13-element array with 1.2 m antennas (AMiBA-13), its subsequent commissioning, and cluster SZE observing program. The most noticeable changes compared to AMiBA-7 are (1) array re-configuration with baselines ranging from 1.4 m to 4.8 m, allowing us to sample structures between 2 0 and 10 0 , (2) thirteen new lightweight carbon-fiber-reinforced plastic (CFRP) 1.2 m reflectors, and (3) additional correlators and six new receivers. Since the reflectors are co-mounted on and distributed over the entire six-meter CFRP platform, a refined hexapod pointing error model and phase error correction scheme have been developed for AMiBA-13. These effects ‐ entirely negligible for the earlier central close-packed AMiBA-7 configuration ‐ can lead to additional geometrical delays during observations. Our correction scheme recovers at least 80 5% of point source fluxes. We, therefore, apply an upward correcting factor of 1.25 to our visibilities to correct for phase decoherence, and a 5% systematic uncertainty is added in quadrature with our statistical errors. We demonstrate the absence of further systematics with a noise level consistent with zero in stackeduv-visibilities. From the AMiBA-13 SZE observing program, we present here maps of a subset of twelve clusters with signal-to-noise ratios above five. We demonstrate combining AMiBA-7 with AMiBA-13 observations on Abell 1689, by jointly fitting their data to a generalized Navarro‐Frenk‐White (gNFW) model. Our cylindrically-integrated Compton-y values for five radii are consistent with results from the Berkeley-Illinois-Maryland Array (BIMA), Owens Valley Radio Observatory (OVRO), Sunyaev-Zel’dovich Array (SZA), and the Planck Observatory. We also report the first targeted SZE detection towards the optically selected cluster RCS J1447+0828, and we demonstrate the ability of AMiBA SZE data to serve as a proxy for the total cluster mass. Finally, we show that our AMiBA-SZE derived cluster masses are consistent with recent lensing mass measurements in the literature. Subject headings: cosmology: cosmic background radiation — galaxies: clusters: general — instrumentation: interferometers

Platform Deformation Phase Correction for the AMiBA-13 Coplanar Interferometer

We present a new way to solve the platform deformation problem of coplanar interferometers. The platform of a coplanar interferometer can be deformed due to driving forces and gravity. A deformed platform will induce extra components into the geometric delay of each baseline and change the phases of observed visibilities. The reconstructed images will also be diluted due to the errors of the phases. The platform deformations of The Yuan-Tseh Lee Array for Microwave Background Anisotropy (AMiBA) were modeled based on photogrammetry data with about 20 mount pointing positions. We then used the differential optical pointing error between two optical telescopes to fit the model parameters in the entire horizontal coordinate space. With the platform deformation model, we can predict the errors of the geometric phase delays due to platform deformation with a given azimuth and elevation of the targets and calibrators. After correcting the phases of the radio point sources in the AMiBA interferometric data, we recover 50%-70% flux loss due to phase errors. This allows us to restore more than 90% of a source flux. The method outlined in this work is not only applicable to the correction of deformation for other coplanar telescopes but also to single-dish telescopes with deformation problems. This work also forms the basis of the upcoming science results of AMiBA-13.

A wideband 240 GHz receiver for the submillimeter array

We report on the design of a 240 GHz double-side-band receiver for the Submillimeter Array (SMA). The heart of this receiver is a 3-junction series connected SIS mixer, which allows it to provide intermediate frequency (IF) output up to more than 12 GHz. We have custom built a low noise Amplifier-Multiplier Chain for use as the receiver’s Local Oscillator module, which is tunable from 210 to 270 GHz. The receiver has demonstrated low noise performance in laboratory. 7 out of the 8 SMA antennas are now equipped with this receiver. The receiver has already participated in Event Horizon Telescope observations in April 2016, working with the SMA-200 receiver to provide dual polarization coverage for the EHT Hawaii Station. This receiver has enabled the SMA to provide 32 Gbit per second data stream to the EHT observations. We are currently trying to improve the on-sky beam co-alignment of this receiver with respect to other SMA receivers.