John Lugten

THE BERKELEY-ILLINOIS-MARYLAND-ASSOCIATION MILLIMETER ARRAY

We describe the characteristics of the BIMA millimeter wave array at Hat Creek, California. The array is an aperture synthesis instrument consisting of 9 6-meter diameter antennas which may be deployed in three differenet configurations, with spacings ranging from 7 meters up to 1.3 km. At an observing frequency of 100 GHz these configurations yield maps with angular resolutions of 5, 2, and 0.4, over a 2 field. Larger fields may be mapped by using multiple pointings. For all but the oldest telescopes, the surface accuracy is ≤ 30 microns rms, and the aperture efficiency is 77% at 100 GHz. Background emission from antenna losses and spillover is very low, about 5 K after subtraction of the cosmic Bv(2.7K). Each antenna contains a single dewar which accommodates up to four separate receivers. SIS mixers are cooled to 3.2 K with novel Gifford-McMahon cycle refrigerators. Both the upper and lower sidebands of the first local oscillator are received and separated, providing two bands extending from 70-900 MHz on each side of the first local oscillator. The correlation spectrometer covers a bandwidth of up to 800 MHz, and provides up to 2048 channels for each antenna pair. There are four independently tunable spectral windows (in each sideband), allowing simultaneous observations of several different spectral lines. The spectral resolution ranges from 6 kHz to 3 MHz. For a single 8 hour track in one configuration, the sensitivity is approximately 1 mJy/beam in the 800 MHz wide continuum. Measurements of atmospheric phase fluctuations as functions of both time and baseline have been made; these indicate that routine imaging at angular resolutions of less than 1 at 100 GHz is possible only if self-calibration or some other means of phase correction can be applied. Examples of a few recent results are included. We note that 30% of the observing time on the array is granted to visitors.

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.

Imaging Simulations of Large‐Scale Flux Recovery at Millimeter Wavelengths

We present multiple-field imaging simulations that explore the issue of large-scale flux recovery using five current and future millimeter-wavelength interferometers. The simulations show that nonlinear deconvolutions routinely applied to interferometric maps interpolate and extrapolate to unsampled spatial frequencies and thereby reconstruct much larger scale structures than an analytical treatment of the flux recovery issue would suggest. We show that the fraction of flux recovered for a given observation is a function of the signal-to-noise ratio (S/N) of the map; however, even for S/Ns as low as 3, a deconvolved map reconstructs more flux than an un-deconvolved map. Both the noise-free and the noise-added simulations demonstrate that in order to make accurate maps of even moderately large (20) sources at millimeter wavelengths, it is generally necessary to include single-dish (total-power) or very short spacing data. We demonstrate that for high-S/N data, how well an individual telescope recovers large-scale structure for a mosaiced observation is more closely related to the minimum distance between its dishes, Smin – D, than to the minimum center-to-center distance, Smin. For a source that is large in one dimension but small in another (e.g., an elliptical Gaussian), the simulations show that the flux recovery is more closely determined by the small dimension than by the long dimension. This may help with the real imaging of spiral galaxies, where the rotation tends to confine the emission in any given channel map to a relatively small region in one dimension. These simulations were motivated by our work with mosaics from the Berkeley-Illinois-Maryland Association Survey of Nearby Galaxies (BIMA SONG), but the results apply generally to millimeter-wavelength interferometric images.

Optical path length fluctuations in the atmosphere.

The effect of atmospheric fluctuations on measurements of distance has become increasingly important as laser interferometers become more widely used. The present study evaluates the precision with which dimensions can be monitored by interferometry involving optical path lengths in the open air under good atmospheric conditions. The size and time scale of optical path length fluctuations over distances of one meter, and correlations between fluctuations in nearby optical paths, are measured and analyzed.

The Allen Telescope Array

The Allen Telescope Array, a joint project between the SETI Institute and the Radio Astronomy Laboratory at the University of California Berkeley, is currently under development and construction at the Hat Creek Radio Observatory in northern California. It will consist of 350 6.1-m offset Gregorian antennas in a fairly densely packed configuration, with minimum baselines of less than 10 m and a maximum baseline of about 900 m. The dual-polarization frequency range spans from about 500 MHz to 11 GHz, both polarizations of which are transported back from each antenna. The first generation processor will provide 32 synthesized beams of 104 MHz bandwidth, eight at each of four tunings, as well as outputs for a full-polarization correlator at two of the tunings at the same bandwidth. This paper provides a general description of the Allen Telescope Array.

Excitation gradient of the molecular gas in the Sgr A circum‐nuclear ring

We present new measurements of CO submillimeter (J = 7 ..-->..6) and far-infrared (J = 14 ..-->.. 13, J = 21 ..-->.. 20) lines in the Galactic center. The new data, together with earlier CO measurements, are used to derive the physical conditions of the dense, warm molecular gas at different positions in the Sgr a circum-nuclear ring. From the inner edge of the ring at Rapprox.1.7 pc to Rapprox.6 pc the gas pressure decreases proportional to R/sup -1.6//sup +- //sup 0.4/. The best fit excitation curves indicate that both density and temperature decrease with distance from the center. There is also an azimuthal excitation gradient in the ring. If the molecular gas is directly heated from the center by UV radiation or mass outflow the decrease of excitation is a natural consequence of the R/sup -2/ dependence of the energy density. In the more likely scenario of shock heating fed by the large turbulence of the gas throughout the ring, the excitation gradient may reflect variations in gas density and heating efficiency.

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.

Mapping of C+ far‐infrared emission in the inner galaxy

We have mapped the [CII] 158 μm emission over a region of about 8’ diameter towards Sgr A. The strongest C+ emission comes from a ring of gas centered on SgrA*. The ring has an inner radius of about 1.7 pc and can be traced in C+ emission out to a least 8 pc from Sgr A*. The ring is inclined about 70° to the line of sight and tilted about −20° with respect to the galactic plane. The velocity field of the C+ is that expected for a tilted, inclined thin ring which is predominantly rotating about the center. There is also substantial ‘‘turbulent’’ velocity dispersion within the ring. The C+ rotation velocity decreases with distance from the center. If the velocity field is dominated by gravitational forces, the mass distribution in the center of the Galaxy is more concentrated than that of an isothermal stellar cluster.

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.