D. D. Thornton

The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI

The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.

Imaging the HL Tauri Disk at λ = 2.7 Millimeters with the BIMA Array

We have obtained a subarcsecond image of the disk associated with the T Tauri star HL Tau at a wavelength of 2.7 mm using the new high-resolution capability of the BIMA Array. The disk is elongated with a deconvolved Gaussian source size of 10 ± 02 × 05 ± 02, implying a semimajor axis of 70 ± 15 AU for a distance of 140 pc; the minor axis may be unresolved. The position angle of the major axis (125° ± 10°) is orthogonal to the axis of the optical jet. The disk centroid is coincident with the VLA λ = 3.6 cm source position and nearly coincident with recent measurements of the near-infrared emission peak. The λ = 2.7 mm images, along with previous interferometric measurements at λ = 0.87 mm and flux measurements from 10 μm to 1.3 cm, are well fitted by a simple power-law disk model with a shallow radial dependence to the surface density [Σ(r) ∝ r0 to r-1], an outer radius between 90 and 160 AU, and a dust opacity law proportional to ν1.

THE ALLEN TELESCOPE ARRAY Pi GHz SKY SURVEY. I. SURVEY DESCRIPTION AND STATIC CATALOG RESULTS FOR THE BOÖTES FIELD

The Pi GHz Sky Survey (PiGSS) is a key project of the Allen Telescope Array. PiGSS is a 3.1 GHz survey of radio continuum emission in the extragalactic sky with an emphasis on synoptic observations that measure the static and time-variable properties of the sky. During the 2.5 year campaign, PiGSS will twice observe similar to 250,000 radio sources in the 10,000 deg(2) region of the sky with b > 30 degrees to an rms sensitivity of similar to 1 mJy. Additionally, sub-regions of the sky will be observed multiple times to characterize variability on timescales of days to years. We present here observations of a 10 deg(2) region in the Bootes constellation overlapping the NOAO Deep Wide Field Survey field. The PiGSS image was constructed from 75 daily observations distributed over a 4 month period and has an rms flux density between 200 and 250 mu Jy. This represents a deeper image by a factor of 4-8 than we will achieve over the entire 10,000 deg(2). We provide flux densities, source sizes, and spectral indices for the 425 sources detected in the image. We identify similar to 100 new flat-spectrum radio sources; we project that when completed PiGSS will identify 10(4) flat-spectrum sources. We identify one source that is a possible transient radio source. This survey provides new limits on faint radio transients and variables with characteristic durations of months.

Primary Beam and Dish Surface Characterization at the Allen Telescope Array by Radio Holography

The Allen Telescope Array (ATA) is a cm-wave interferometer in California, comprising 42 antenna elements with 6-m diameter dishes. We characterize the antenna optical accuracy using two-antenna interferometry and radio holography. The distortion of each telescope relative to the average is small, with RMS differences of 1% of beam peak value. Holography provides images of dish illumination, characterizing as-built mirror surfaces. Maximal distortions across ~ 2 meter lengths appear to result from mounting stresses or solar radiation. Experimental RMS errors are 0.7 mm at night and 3 mm under worst-case solar illumination. For frequencies 4, 10, and 15 GHz, the nighttime values indicate sensitivity losses of 1, 10 and 20%, respectively. ATAs wide-bandwidth receiver permits observations over a continuous range 0.5-11.2 GHz. We probe the antenna optical gain and beam pattern stability as a function of focus position and observation frequency, concluding that ATA can produce high fidelity images over a decade of simultaneous observation frequencies. We quantify solar heating effects on antenna sensitivity and pointing accuracy. We find that during the day, observations >;5 GHz will suffer some sensitivity loss and it may be necessary to make antenna pointing corrections on a 1-2 hourly basis.

Venus: New Microwave Measurements Show No Atmospheric Water Vapor

Two sets of passive radio observations of Venus—measurements of the spectrum of the disk temperature near the 1-centimeter wavelength, and interferometric measurements of the planetary limb darkening at the 1.35-centimeter water vapor resonance—show no evidence of water vapor in the lower atmosphere of Venus. The upper limit of 2 x 10-3 for the mixing ratio of water vapor is substantially less than the amounts derived from the Venera space probes (0.5 x 10-2 to 2.5 x 10-2). This amount of water vapor cannot produce dense clouds, and it is doubtful that it may contribute significantly to a greenhouse effect.

The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera

The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in Northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting panoramic surveys of the astrophysical sky. The fundamental scientific program of this new telescope is varied and exciting; we here discuss some of the first astronomical results.

Automated tuning of the Berkeley-Illinois-Maryland array receivers

The Berkeley Illinois Maryland Association (BIMA) array consists of 6 antennas, each 6 meters in diameter, operating at a wavelength of 3 mm. The telescope control is fully automated, allowing round-the-clock observing with little or no supervision. The array can also be controlled from a remote site. One of the major challenges of automated operation is the ability to tune multiple receivers to the desired operating frequency in a reliable manner. The large tuning range required at millimeter wavelengths (85 to 115 GHz), the nonlinear response of the microwave cavities and oscillator phase lock problems have been stumbling blocks for remote receiver tuning. At BIMA, we have developed an automated system capable of tuning all the receivers to any observing frequency within a few minutes. The system uses a Sun workstation in conjunction with dedicated hardware to control the receivers. Large disparities between receiver characteristics are handled in an efficient manner through the use of lookup tables. We describe the system design in detail, and discuss the problems encountered along with appropriate solutions. The generation of lookup tables in the laboratory is also presented.