Edward Tong

Evidence for dynamically important magnetic fields in molecular clouds

Recent observational evidence that magnetic fields are dynamically important in molecular clouds, compared to self-gravity and turbulence, is reviewed and illustrated with data from the NGC 2024 region. One piece of evidence, turbulence anisotropy, was found in the diffuse envelope of a cloud (A v ≈ 1; Heyer et al. 2008); our data further suggest turbulence anisotropy in the cloud (A v > 7) and even near the cloud core (A v ∼ 100). The data also show that magnetic fields can channel gravitational contraction even for a region with supercritical N(H 2 )/2B los ratio (the ratio between the observed column density and two times the line-of-sight observed field strength), a parameter which has been widely used by observers to estimate core mass-to-flux ratios. Although the mass-to-flux ratio is constant under the flux-freezing condition, we show that N(H 2 )/2B los grows with time if gravitational contraction is anisotropic due to magnetic fields.

Large-Signal Frequency Response of an HEB Mixer: From 300 MHz to Terahertz

We present a study of the large signal frequency response of an HEB mixer over a wide frequency range. In our experiments, we have subjected the HEB mixer to incident electromagnetic radiation from 0.3 GHz to 1 THz. The mixer element is an NbN film deposited on crystalline quartz. The mixer chip is mounted in a waveguide cavity, coupled to free space with a diagonal horn. At microwave frequencies, electromagnetic radiation is applied through the coaxial bias port of the mixer block. At higher frequencies the input signal passes via the diagonal horn feed. At each frequency, the incident power is varied and a family of I-V curves is recorded. From the curves we identify 3 distinct regimes of operation of the mixer separated by the phonon relaxation frequency and the superconducting energy gap frequency observed at about 3 GHz and 660 GHz respectively. In this paper, we will present observed curves and discuss the results of our experiment.

Microwave stabilization of HEB mixer by a microchip controller

The stability of a Hot Electron Bolometer (HEB) mixer can be improved by the use of microwave injection. In this article we report a refinement of this approach. We introduce a microchip controller to facilitate the implementation of the stabilization scheme, and demonstrate that the feedback loop effectively suppresses drifts in the HEB bias current, leading to an improvement in the receiver stability. The measured Allan time of the mixers IF output power is increased to > 10 s.

Suppression of astronomical sources using the McMath-Pierce Solar Telescope and starshades with flight-like optics

The external starshade is a method for the direct detection and spectral characterization of terrestrial planets around other stars, a key goal identified in ASTRO2010. Tests of starshades have been and continue to be conducted in the lab and in the field using non-collimated light sources. We extend the current approach to performing night-time observations of astronomical objects using small-scale (10-30cm) starshades and the McMath-Pierce Solar Telescope at Kitt Peak National Observatory. This configuration allows us to make measurements of stars with a Fresnel number close to those expected in proposed full-scale space configurations. We present the results of our engineering runs conducted in 2015.

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

A field-deployed 810 GHz receiver incorporating a superconducting mixer developed for herschel space telescope and a SiGe low noise amplifier

We have constructed an 810 GHz receiver system incorporating a HIFI Band-3 superconductor-insulator-superconductor (SIS) mixer developed for Herschel space observatory and a wide-band SiGe low noise amplifier (LNA) designed at Caltech. The instrument is currently installed at the RLT telescope (elevation 5500 m) in northern Chile. Hot/cold (280K/72K) load measurements performed at the telescope yield noise temperatures of 225 K (Y-factor = 1.7) including receiver optics. First-light observations indicate that the receiver is highly sensitive and functions stably. We present details of the receiver system, its performance at the telescope, and first-light observations with a Herschel mixer.

The LUVOIR science and technology definition team (STDT): overview and status

This presentation gives an overview of the Large UV/Optical/IR Surveyor mission study progress, presented on behalf of the (community + NASA) study team.

Vector near-field beam scanner for the SMA

Here we describe the principles behind the design, construction, and implementation of a vector near-field beam scanner for the antennas of the Submillimeter Array. The Submillimeter Array (SMA) is a radio interferometer array operating at frequencies ranging from 200 { 700 GHz at the summit of Maunakea in Hawaii. A set of 4 receivers cover the key atmospheric windows over which the SMA operates. Each receiver insert is equipped with an ambient optical insert, which is pre-aligned in the lab prior to installation at the summit. However, as a result of receiver upgrades and problems, some receiver inserts may no longer be matched to the original optics inserts. Since the SMA is used extensively in dual-receiver observations, such beam mis-alignments lead to a relative pointing error between a pair of receivers during the observation. In order to address this issue, we have designed a near-field beam scanner which can be used to map out the receiver beam of each antenna. The setup employs the existing radio references available in each antenna for the vector beam measurement. We have successfully used this scanner to improve the on-sky co-alignment of receiver beams. In this presentation, we will describe the system and operational aspect of this in-situ radio frequency alignment technique.