D. M. Luong-Van

Exceptional terahertz transparency and stability above Dome A, Antarctica

We present the first direct measurements of the terahertz atmospheric transmission above Dome A, the highest point on the Antarctic plateau at an elevation of 4.1 km. The best-quartile atmospheric transmission during the Austral winter is 80% at a frequency of 661 GHz (453 μm), corresponding to a precipitable water vapor column of 0.1 mm. Daily averages as low as 0.025 mm were observed. The Antarctic atmosphere is very stable, and excellent observing conditions generally persist for many days at a time. The exceptional conditions over the high Antarctic plateau open new far-infrared spectral windows to ground-based observation. These windows contain important spectral-line diagnostics of star formation and the interstellar medium which would otherwise only be accessible to airborne or space telescopes. Online material: color figures

The PLATO Dome A Site-Testing Observatory : instrumentation and first results

The PLATeau Observatory (PLATO) is an automated self-powered astrophysical observatory that was deployed to Dome A, the highest point on the Antarctic plateau, in 2008 January. PLATO consists of a suite of site-testing instruments designed to quantify the benefits of the Dome A site for astronomy, and science instruments designed to take advantage of the unique observing conditions. Instruments include CSTAR, an array of optical telescopes for transient astronomy; Gattini, an instrument to measure the optical sky brightness and cloud cover statistics; DASLE, an experiment to measure the statistics of the meteorological conditions within the near-surface layer; Pre-HEAT, a submillimeter tipping radiometer measuring the atmospheric transmission and water vapor content and performing spectral line imaging of the Galactic plane; and Snodar, an acoustic radar designed to measure turbulence within the near-surface layer. PLATO has run completely unattended and collected data throughout the winter 2008 season. Here we present a detailed description of the PLATO instrument suite and preliminary results obtained from the first season of operation.

The PLATO Dome A site-testing observatory: Power generation and control systems

The atmospheric conditions above Dome A, a currently unmanned location at the highest point on the Antarctic plateau, are uniquely suited to astronomy. For certain types of astronomy Dome A is likely to be the best location on the planet, and this has motivated the development of the Plateau Observatory (PLATO). PLATO was deployed to Dome A in early 2008. It houses a suite of purpose-built site-testing instruments designed to quantify the benefits of Dome A site for astronomy, and science instruments designed to take advantage of the observing conditions. The PLATO power generation and control system is designed to provide continuous power and heat, and a high-reliability command and communications platform for these instruments. PLATO has run and collected data throughout the winter 2008 season completely unattended. Here we present a detailed description of the power generation, power control, thermal management, instrument interface, and communications systems for PLATO, and an overview of the system performance for 2008.

Covariance profiling for an adaptive Kalman filter to suppress sensor quantization effects

This paper presents a generic approach to model the noise covariance associated with discrete sensors such as incremental encoders and low resolution analog to digital converters. The covariance is then used in an adaptive Kalman filter that selectively and appropriately carries out measurement updates. The temporal as well as system state measurements are used to predict the quantization error of the measurement signal. The effectiveness of the method is demonstrated by applying the technique to incremental encoders of varying resolutions. Simulation of an example system with varying encoder resolutions is presented to show the performance of the new filter. Results show that the new adaptive filter produces more accurate results while requiring a lower resolution encoder than a similarly designed conventional Kalman filter, especially at low velocities.

PHOTOMETRY OF VARIABLE STARS FROM DOME A, ANTARCTICA

Dome A on the Antarctic plateau is likely one of the best observing sites on Earth thanks to the excellent atmospheric conditions present at the site during the long polar winter night. We present high-cadence time-series aperture photometry of 10,000 stars with i < 14.5 mag located in a 23 deg(2) region centered on the south celestial pole. The photometry was obtained with one of the CSTAR telescopes during 128 days of the 2008 Antarctic winter. We used this photometric data set to derive site statistics for Dome A and to search for variable stars. Thanks to the nearly uninterrupted synoptic coverage, we found six times as many variables as previous surveys with similar magnitude limits. We detected 157 variable stars, of which 55% were unclassified, 27% were likely binaries, and 17% were likely pulsating stars. The latter category includes delta Scuti, gamma Doradus, and RR Lyrae variables. One variable may be a transiting exoplanet.

The PLATO Antarctic site testing observatory

Over a decade of site testing in Antarctica has shown that both South Pole and Dome C are exceptional sites for astronomy, with certain atmospheric conditions superior to those at existing mid-latitude sites. However, the highest point on the Antarctic plateau, Dome A, is expected to experience colder atmospheric temperatures, lower wind speeds, and a turbulent boundary layer that is confined closer to the ground. The Polar Research Institute of China, who were the first to visit the Dome A site in January 2005, plan to establish a permanently manned station there within the next decade. As part of this process they conducted a second expedition to Dome A, arriving via overland traverse in January 2008. This traverse involved the delivery and installation of the PLATeau Observatory (PLATO). PLATO is an automated self-powered astrophysical site testing observatory, developed by the University of New South Wales. A number of international institutions have contributed site testing instruments measuring turbulence, optical sky background, and sub-millimetre transparency. In addition, a set of science instruments are providing wide-field high time resolution optical photometry and terahertz imaging of the Galaxy. We present here an overview of the PLATO system design and instrumentation suite.

The First Release of the CSTAR Point Source Catalog from Dome A, Antarctica

In 2008 January the twenty-fourth Chinese expedition team successfully deployed the Chinese Small Telescope ARray (CSTAR) to Dome A, the highest point on the Antarctic plateau. CSTAR consists of four 14.5 cm optical telescopes, each with a different filter (g, r, i, and open) and has a 4.5° × 4.5° field of view (FOV). It operates robotically as part of the Plateau Observatory, PLATO, with each telescope taking an image every ~30 s throughout the year whenever it is dark. During 2008, CSTAR 1 performed almost flawlessly, acquiring more than 0.3 million i-band images for a total integration time of 1728 hr during 158 days of observations. For each image taken under good sky conditions, more than 10,000 sources down to ~16th magnitude could be detected. We performed aperture photometry on all the sources in the field to create the catalog described herein. Since CSTAR has a fixed pointing centered on the south celestial pole (decl. = -90°), all the sources within the FOV of CSTAR were monitored continuously for several months. The photometric catalog can be used for studying any variability in these sources, and for the discovery of transient sources such as supernovae, gamma-ray bursts, and minor planets.

Sky Brightness and Transparency in the i-band at Dome A, Antarctica

The i-band observing conditions at Dome A on the Antarctic plateau have been investigated using data acquired during 2008 with the Chinese Small Telescope Array. The sky brightness, variations in atmospheric transparency, cloud cover, and the presence of aurorae are obtained from these images. The median sky brightness of moonless clear nights is 20.5 mag arcsec(-2) in the SDSS i band at the south celestial pole (which includes a contribution of about 0.06 mag from diffuse Galactic light). The median over all Moon phases in the Antarctic winter is about 19.8 mag arcsec(-2). There were no thick clouds in 2008. We model contributions of the Sun and the Moon to the sky background to obtain the relationship between the sky brightness and transparency. Aurorae are identified by comparing the observed sky brightness to the sky brightness expected from this model. About 2% of the images are affected by relatively strong aurorae.

Thickness of the Atmospheric Boundary Layer Above Dome A, Antarctica, during 2009

The domes, or local elevation maxima, on the Antarctic plateau provide a unique opportunity for ground-based astronomy in that the turbulent boundary layer is so thin that a telescope on a small tower can be in the free atmosphere, i.e., the portion of the atmosphere in which the turbulence is decoupled from the effect of the Earths surface. There, it can enjoy a free atmosphere which itself appears to offer superior conditions to that of temperate sites. This breaks the problem of characterizing the turbulence at Antarctic plateau sites into two separate tasks: determining the variability, distribution and thickness of the boundary layer, and characterizing the free atmosphere. In this article we tackle the first of these tasks using a high-resolution, low minimum sample height sonic radar (SODAR) called Snodar that has been specifically designed to characterize the Antarctic bound- ary thickness and structure. Snodar delivers a vertical resolution of 0.9 m, with a minimum sampling height of 8 m. Snodar sampled the first 180 m of the atmosphere with 0.9 m resolution every 10 s at Dome A, Antarctica between 2009 February 4 and 2009 August 18. The median thickness of the boundary layer over this period was 13.9 m, with the 25th and 75th percentiles at 9.7 m and 19.7 m, respectively. The data collected from Dome A also show that, while the boundary layer can be stable for several hundred hours at a time, it can also be highly variable and must be sampled on the time scale of minutes to properly characterize its thickness.

Site testing for submillimetre astronomy at Dome C, Antarctica

Aims. Over the past few years a major effort has been put into the exploration of potential sites for the deployment of submillimetre astronomical facilities. Amongst the most important sites are Dome C and Dome A on the Antarctic Plateau, and the Chajnantor area in Chile. In this context, we report on measurements of the sky opacity at 200 μm over a period of three years at the French-Italian station, Concordia, at Dome C, Antarctica. We also present some solutions to the challenges of operating in the harsh polar environment. Methods. The 200-μm atmospheric opacity was measured with a tipper. The forward atmospheric model MOLIERE (Microwave Observation LIne Estimation and REtrieval) was used to calculate the atmospheric transmission and to evaluate the precipitable water vapour content (PWV) from the observed sky opacity. These results have been compared with satellite measurements from the Infrared Atmospheric Sounding Interferometer (IASI) on Metop-A, with balloon humidity sondes and with results obtained by a ground-based microwave radiometer (HAMSTRAD). In addition, a series of experiments has been designed to study frost formation on surfaces, and the temporal and spatial evolution of thermal gradients in the low atmosphere. Results. Dome C offers exceptional conditions in terms of absolute atmospheric transmission and stability for submillimetre astronomy. Over the austral winter the PWV exhibits long periods during which it is stable and at a very low level (0.1 to 0.3 mm). Higher values (0.2 to 0.8 mm) of PWV are observed during the short summer period. Based on observations over three years, a transmission of around 50% at 350 μm is achieved for 75% of the time. The 200-μm window opens with a typical transmission of 10% to 15% for 25% of the time. Conclusions. Dome C is one of the best accessible sites on Earth for submillimetre astronomy. Observations at 350 or 450 μm are possible all year round, and the 200-μm window opens long enough and with a sufficient transparency to be useful. Although the polar environment severely constrains hardware design, a permanent observatory with appropriate technical capabilities is feasible. Because of the very good astronomical conditions, high angular resolution and time series (multi-year) observations at Dome C with a medium size single dish telescope would enable unique studies to be conducted, some of which are not otherwise feasible even from space.