From Reference Frames to Relativistic Experiments: Absolute and Relative Radio Astrometry

Reference systems and frames are crucial for high precision absolute astrometric work, and their foundations must be well-defined. The current frame, the International Celestial Reference Frame, will be discussed: its history, the use of the group delay as the measured quantity, the positional accuracy of 0.3 milliarcsec, and possible future improvements. On the other hand, for the determination of the motion of celestial objects, accuracies approaching 0.01 milliarcsec can be obtained by measuring the differential position between the target object and nearby stationary sources. This astrometric technique uses phase referencing, and the current techniques and limitations are discussed, using the results from four experiments. Brief comments are included on the interpretation of the Jupiter gravity deflection experiment of September 2002.Comment: 25 pages, 7 figures--Presented at JENAM meeting in Budapest, 27-30 August 200

Radio Tests of GR

Since VLBI techniques give microarcsecond position accuracy of celestial objects, tests of GR using radio sources as probes of a gravitational field have been made. We present the results from two recent tests using the VLBA: In 2005, the measurement of the classical solar deflection; and in 2002, the measurement of the retarded gravitational deflection associated with Jupiter. The deflection experiment measured PPN-gamma to an accuracy of 0.0003; the Jupiter experiment measured the retarded term to 20% accuracy. The controversy over the interpretation of the retarded term is summarized.Comment: 4 pages: IAU24

Sub-Milliarcsecond Precision of Pulsar Motions: Using In-Beam Calibrators with the VLBA

We present Very Long Baseline Array phase-referenced measurements of the parallax and proper motion of two pulsars, B0919+06 and B1857-26. Sub-milliarcsecond positional accuracy was obtained by simultaneously observing a weak calibrator source within the 40' field of view of the VLBA at 1.5 GHz. We discuss the merits of using weak close calibrator sources for VLBI observations at low frequencies, and outline a method of observation and data reduction for these type of measurements. For the pulsar B1919+06 we measure a parallax of 0.31 +/- 0.14 mas. The accuracy of the proper motions is approximately 0.5 mas, an order of magnitude improvement over most previous determinations.Comment: 11 pages plus 4 figures. In press, Astronomical Journa

ALMA Temporal Phase Stability and the Effectiveness of Water Vapor Radiometer

Atacama Large Millimeter/submillimeter Array (ALMA) will be the world largest mm/submm interferometer, and currently the Early Science is ongoing, together with the commissioning and science verification (CSV). Here we present a study of the temporal phase stability of the entire ALMA system from antennas to the correlator. We verified the temporal phase stability of ALMA using data, taken during the last two years of CSV activities. The data consist of integrations on strong point sources (i.e., bright quasars) at various frequency bands, and at various baseline lengths (up to 600 m). From the observations of strong quasars for a long time (from a few tens of minutes, up to an hour), we derived the 2-point Allan Standard Deviation after the atmospheric phase correction using the 183 GHz Water Vapor Radiometer (WVR) installed in each 12 m antenna, and confirmed that the phase stability of all the baselines reached the ALMA specification. Since we applied the WVR phase correction to all the data mentioned above, we also studied the effectiveness of the WVR phase correction at various frequencies, baseline lengths, and weather conditions. The phase stability often improves a factor of 2 - 3 after the correction, and sometimes a factor of 7 improvement can be obtained. However, the corrected data still displays an increasing phase fluctuation as a function of baseline length, suggesting that the dry component (e.g., N2 and O2) in the atmosphere also contributes the phase fluctuation in the data, although the imperfection of the WVR phase correction cannot be ruled out at this moment.Comment: Proc. SPIE 8444-125, in press (7 pages, 4 figures, 1 table

A Catalog of the East-West Visibility Functions of Radio Sources at 1425 MHz

As part of a general program for determining the radio structure of discrete sources, 532 sources have been observed using the two 90-ft paraboloids at the Owens Valley Radio Observatory as an east-west interferometer. The frequency of the observations was 1425 MHz and up to nine east-west spacings, ranging from 144 wavelengths to 2626 wavelengths, were used. Because of the large amount of data, the discussion of the observations and results will be given in several papers. This paper, the first in the series, will list the visibility functions of the data. The interpretation of a visibility function in terms of a source structure and the statistical interpretation of the structures will be given in following papers

A Catalog of the North-South Visibility Functions of Radio Sources at 1425 MHz

Observations with the North-South interferometer were made in 1966 using the two 90-foot paraboloids at the Owens Valley Radio Observatory to determine the strip-scan distribution in position angle 0° for a large number of extra-galactic radio sources. The frequency of the observations was 1425 MHz. In conjuction with other data taken with an East-West interferometer (Fomalong 1967; hereafter called Paper I) and off-transit data taken in 1966 and 1967, two-dimensional structures can be deduced for the sources. These results will be published elsewhere (small-diameter sources, Fomalont and Moffett 1971; large-diameter sources, Fomalont 1971)

The Measurement of the Light Deflection from Jupiter: Experimental Results

We have determined the relativistic light deflection of the quasar J0842+1835 as Jupiter passed within 3.7' on 2002 September 8, by measuring the time delay using the VLBA and Effelsberg radio telescopes at 8.4 GHz. At closest approach, General Relativity (GR) predicts a radial (static) deflection of 1190 microarcsec, and tangential (retarded) deflection in the direction of Jupiter's motion of 51 microarcsec. Our experiment achieved an rms position error of <10 microarcsec, and measured this retarded deflection to be 0.98 +/- 0.19 (rms error) times that predicted by GR. The increase positional accuracy for this VLBI phase referencing experiment was achieved by using two calibrator sources. Comments on the interpretation of this experiment are given