We present the first high-angular resolution (up to 0.7, ~5000 AU) polarization and thermal dust continuum images toward the massive star-forming region W51 North. The observations were carried out with the Submillimeter Array (SMA) in both the subco
mpact (SMA-SubC) and extended (SMA-Ext) configurations at a wavelength of 870 micron. W51 North is resolved into four cores (SMA1 to SMA4) in the 870 micron continuum image. The associated dust polarization exhibits more complex structures than seen at lower angular resolution. We analyze the inferred morphologies of the plane-of-sky magnetic field (B_bot) in the SMA1 to SMA4 cores and in the envelope using the SMA-Ext and SMA-SubC data. These results are compared with the B_bot archive images obtained from the CSO and JCMT. A correlation between dust intensity gradient position angles (phi_{nabla I}) and magnetic field position angles (phi_B) is found in the CSO, JCMT and both SMA data sets. This correlation is further analyzed quantitatively. A systematically tighter correlation between phi_{nabla I} and phi_B is found in the cores, whereas the correlation decreases in outside-core regions. Magnetic field-to-gravity force ratio (Sigma_B) maps are derived using the newly developed polarization - intensity gradient method by Koch, Tang & Ho 2012. We find that the force ratios tend to be small (Sigma_B <= 0.5) in the cores in all 4 data sets. In regions outside of the cores, the ratios increase or the field is even dominating gravity (Sigma_B > 1). This possibly provides a physical explanation of the tightening correlation between phi_{nabla I} and phi_B in the cores: the more the B field lines are dragged and aligned by gravity, the tighter the correlation is. Finally, we propose a schematic scenario for the magnetic field in W51 North to interpret the four polarization observations at different physical scales.
Interferometric millimeter observations of the cosmic microwave background and clusters of galaxies with arcmin resolutions require antenna arrays with short spacings. Having all antennas co-mounted on a single steerable platform sets limits to the o
verall 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 TiO$_2$ top layer for protection. A low beam sidelobe level is achieved with a Gaussian feed illumination pattern with edge taper, designed based on feedhorn antenna simulations and verified in a far field beam pattern measurement. A shielding baffle reduces inter-antenna coupling to below $sim$ -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 per year is anticipated in a 13-element array operation.
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 hex
apod 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.
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 con
trol 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 micron rms. This is negligible for optical pointing corrections, radio alignment and radio phase errors for the currently operational 7-element compact configuration. The optical pointing error in azimuth and elevation is successively reduced by a series of corrections to about 0.4 arcmin rms which meets our goal for the 7-element target specifications.
