Radio Frame Research Articles

Astrometric CCD Positions for Extragalactic Radiosources

view Abstract Citations (8) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Astrometric CCD Positions for Extragalactic Radiosources Assafin, M. ; Martins, R. Vieira ; Andrei, A. H. ; Veiga, C. H. Abstract We present results from CCD observations made at the 1.60m Telescope of the Laboratorio Nacional de Astrofisica, Itajuba, Brazil, for 85 extragalactic radiosources uniformly distributed over -80(o) <= delta <= +20(o) declination, from the IAU list of objects scheduled to define the International Celestial Reference System (ICRS). The positions are referred to the Carlsberg Series (CAMC), PPM and ACRS catalogues. They were derived by using a method which allows obtaining precise positions based on corrected positions from the Guide Star Catalogue. A set of 60 fiducial sources was selected by means of their positional arc length differences, to link and compare the optical results and the VLBI radiosource positions. Relative orientation angles between the optical and radio frames were found within 0".02 precision. Comparison of positions on both frames for all radiosources has shown a dispersion of 0".2, consistent with the errors in the reference catalogues used. Publication: The Astronomical Journal Pub Date: June 1997 DOI: 10.1086/118444 Bibcode: 1997AJ....113.2329A full text sources ADS | data products SIMBAD (60)

Optical astrometry of Benchmark radio sources

Optical positions relative to the International Reference Stars Catalogue (IRS), and therefore in the sys- tem of the FK5, have been obtained for the optical coun- terparts of 40 compact extragalactic radio sources south of +6. Many of these sources are being consid- ered as possible Benchmark objects for the establishment of a quasi inertial unied radio/optical reference frame. Precision levels as good as 0.1 00 in both coordinates were achieved. A comparison with VLBI radio positions avail- able for these sources is presented. We give new evidence for the existence of an oset in the origin of right ascension between the radio and optical reference frames as dened at present.

The Position of Sagittarius A*: Accurate Alignment of the Radio and Infrared Reference Frames at the Galactic Center

We present a novel approach to the long-standing problem of locating the position of the compact nonthermal radio source Sgr A* on infrared images of the Galactic center region. Using the Very Large Array, we have detected SiO and H2O maser emission toward several sources within the central parsec of our Galaxy. These masers arise from the innermost parts of circumstellar envelopes of giant and supergiant stars that are members of the nuclear star cluster and appear as compact infrared sources in a diffraction-limited 2.2 μm infrared image. One of the SiO masers is associated with the M-type supergiant IRS 7, the most prominent 2.2 μm point source in the Galactic center region. The radio data allow measurements of the maser positions relative to the compact nonthermal radio continuum source Sgr A* with milliarcsecond accuracy. Because stellar SiO masers near the Galactic center trace their host stars to within a few milliarcseconds, these relative positions can be used to calibrate the plate scale and rotation of the infrared image. Our method allows registration of the radio relative to the infrared reference frame with an estimated accuracy of 003. Using the improved position accuracy we put a stringent upper limit on the 2.2 μm flux density of Sgr A* that is significantly lower than values predicted by recent theoretical model calculations.

Determination of the extragalactic-planetary frame tie from Earth orientation parameters

PRLSENTLY there are three kinds of realization of conventional celestial reference frames (CCRFs), i. e. the optical frame, the planetary frame and the extragalactic radio frame. 'Though the CCRFs are high in precision, due to the difference in (ⅰ) the constants, parameters, formulae and force models used to define the frames; (ⅱ) the methods and techniques of

Catalogues of Intermediate Stars in the Vicinity of Radio Sources

AbstractThe accurate positions of extragalactic radio sources are in use to link the radio and fundamental reference frames and contribute to determining relative orientation of HIPPARCOS and the radio reference frames as well as HIPPARCOS and FK5. Catalogues of intermediate stars chosen in the vicinity of extragalactic radio sources are an important part of the link and can be used for extension and densification of the HIPPARCOS frame. The description of main catalogues of intermediate stars is given. Results of investigation and comparison beetween CAMC, RRS2, PIRS and CRIS are presented.

A compiled catalogue of optical positions of extragalactic radio sources

Based on 28 individual catalogues, in which the positions of optical counterparts of extragalactic radio sources are listed, we compiled a combined catalogue by means of a weighted least squares adjustment. The catalogue is in the system of FK5/J2000.0. It consists of about five hundred sources, among which there are 56 primary sources with position uncertainties as . Comparisons show that the orientation differences between optical and radio frames are Local relative deformations are not obvious within the precision of ground-based optical observations.

Optical positions of radio emitting stars

In order to contribute to the alignment of the radio and optical reference frames, 50 stars with conrmed radio emission and published radio positions were observed by astrographical means. Additionally, two conrmed radio-stars of fainter magnitude were observed using a CCD direct camera with a long focus, large aperture telescope. The reductions are made relative to four catalogues: the Carlsberg Meridian Catalogue #4, the International Reference Stars Catalogue, the Positions and Proper Motion Catalogue and the Astrographical Catalogue of Reference Stars. The best results were obtained with the CAMC catalogue. The plate error of a radio star position is 0: 00 07 for both right ascension and declination, rising to 0: 10 0: 20 for bright stars. Ten of the program stars are not in the HIPPARCOS Input Catalogue and 12 do not belong to any of the major reference catalogues used. As soon as the HIPPARCOS results become available, its reference stars already measured in the plates will enable us to get positions for those 10 radio stars in the HIPPARCOS system.

Pulsars and solar-system ephemerides

The analysis of pulsar time-of-arrival data is intimately bound up with planetary ephemerides. Highly accurate ephemerides are required for Earth and Moon and, to a lesser degree, for the other planets, in order to make full use of the timing data for millisecond-class pulsars. These data, in turn, present an opportunity for improving planetary ephemerides in a variety of ways. Fitting the Earth and Moon orbital parameters to the timing data is the obvious first step, though it is less valuable in the short term for many applications than using the current accumulation of spacecraft-tracking and lunar laser ranging data. By themselves, the pulsar timing data convey no information on the orientation of Earth's orbit, since each pulsar's position on the sky must be determined from those same data. However, independent pulsar position measurements by VLBI, in combination with the timing-derived positions, can serve to fix the orientation of Earth's orbit with respect to the radio reference frame and thereby link the planetary and radio frames. In the long run, the acquisition of timing data over increasing time spans and with improving precision should prove to be an important factor in determining the shape, as well as the orientation, of Earth's orbit. In addition, pulsar timing over a sufficiently long span can directly measure a planet mass through the reaction of the rest of the solar system. The effect must be observed for a major fraction of the orbital period of the planet in question so that the signature can be separated from that of the ordinary spin-down of each pulsar. Finally, pulsar timing can be used to probe gravitational physics, a field with far-reaching consequences and a basic part of the framework for constructing the ephemerides.

Pulsar Astrometry and Improved Terrestrial Clocks

AbstractRecent improvements in cesium and hydrogen terrestrial frequency standards have brought the frequency precision of International Atomic Time (TAI) to a value of 2.5E-15 s/s over an averaging time of a month. In this paper we illustrate the improvement graphically, and discuss the state of the art for frame ties between the radio, dynamical, and optical frames. In a larger paper, available via the World Wide Web, we illustrate the measured accuracy curves of the frequency standards, show their effect on the ensemble time scales, explain the reasons for the confusing array of available time scales, and discuss the inverse problem of using pulsar data to correct the terrestrial time scale.

Astrometry VLBI in space (AVS)

This paper describes a proposal for a new space radio astronomy mission for astrometry which uses very-long-baseline interferometry (VLBI) called Astrometry VLBI in Space (AVS). The ultimate goals of AVS are to improve the accuracy of radio astrometry measurements to the microarcsecond level in one epoch of measurements and to improve the accuracy of the transformation between the inertial radio and optical coordinate reference frames. The scientific objectives of the mission cover a few categories of astrometry tasks such as astrometry of the solar system, reference frames ties, tests of general relativity and cosmology (for references see Lowe and Treuhaft, 1994, Russell et al., 1992, Eubanks et al., 1994).

Improvements to Reference Systems and Their Relationships

Reference Systems include the reference frames and their relationships, time arguments, ephemerides, and the standard constants and algorithms.The extragalactic, or radio, reference frame will be the basic frame. Achieving milli to microarcsecond accuracies at optical wavelengths will reduce the disparity between optical, radar, and radio reference frame determinations. Thus, the relationships and identifications of common sources should be much more accurate. Another significant change should be the ability to determine distances, and thus space motions on a three-dimensional basis, rather than the current two-dimensional basis of proper motions.Improvements in ephemerides provide the opportunity to investigate the difference between atomic and dynamical time, the relationship between the dynamical and extragalactic reference frame and the values of precession and nutation.Also, the relationships between the bright and faint optical catalogs, the infrared, and extragalactic reference frames should be better determined. Reference frames at other wavelengths will become determinable.

The Dynamical Reference Frame

Planetary and lunar ephemerides continue to improve in accuracy as they continue to be adjusted to newer and more accurate observational data. An additional improvement will be that of the orientation of the ephemerides; in the future, the ephemerides produced at JPL will be based upon the reference frame of the radio source catalogues. Recent planetary observations have been made directly with respect to the radio reference frame, and these observations have shown a satisfying degree of absolute accuracy and internal consistency; they enable the automatic orientation of the ephemerides onto the radio reference system during the ephemeris adjustment process.

Some Problems of Wide-Field Astrometry by a Short-Focus CCD-Astrograph

The worldwide application of CCDs in astronomy is concerned mainly with the problems of photometry and spectrophotometry. Astrometry with CCDs is limited by the small fields of view and now is used to solve some special tasks such as parallax determination of faint stars, observations of QSOs, etc. Very interesting results were obtained using a CCD with a meridian circle in a drift scanning mode to solve the problem of the extension of the reference system to stars of 16th–17th mag and the linkage between radio and optical reference frames (Stone 1994).Based on the preliminary results of our observations, some proposals on astrometry with CCDs are discussed in the paper. The aim is to show the use of short-focus telescopes with CCDs in the mode of traditional astrographs. Numerous tasks may be solved with such instruments more simply and correctly than with meridian instruments in a drift scanning mode.The problems of the application of CCDs with short-focus optics are also discussed. Some effects of CCD properties on the accuracy of astrometric and photometric measurements are also considered.

The Extragalactic Optical-Radio Reference Frame

The new extragalactic reference frame will be based primarily on the positions of a globally selected number of about 500 compact radio sources, which almost all display optical counterparts, mainly quasars or BL Lac objects. Precise radio positions of these objects on the mas level have been determined by VLBI techniques and will be monitored and updated on a regular basis. In parallel an extensive optical observing program is underway to determine precise optical positions of these objects. The present optical reference frame is based on the FK5 fundamental catalog, containing 4652 brighter stars. The transfer of the VLBI radio frame to the optical domain will be achieved now by the Hipparcos stellar net, containing about 120,000 stars with comparable accuracy. The paper will address details and consequences of the implemenation of the new system for astrometry and general astronomical applications.

The Orientation of Future JPL Planetary Ephemerides

AbstractIn the future, the ephemerides produced at JPL will be based upon the reference frame of the radio source catalogues. Recent planetary observations have been made directly with respect to the radio frame, and these observations have shown a satisfying degree of absolute accuracy and internal consistency; they provide an automatic frame-tie.

A radio optical reference frame. 7: Additional source positions from a Southern hemisphere short baseline survey

We report accurate radio positions for 107 VLBI sources using Very Long Baseline Inteferometry (VLBI), as part of a continuing program to establish a global reference frame of about 400 extragalactic radio positions. The observations were made with Australian radio telescopes at Tidbinbilla and Hobart during 1990 using Mark III VLBI recording systems. The positions reported here add 50 new sources to the global reference frame catalog, bringing the total number to 403, of which 208 are in the Northern hemisphere and 195 in the Southern. The new positions have median errors of about 5 milliarseconds, limited by the relatively short baseline of 832 km.

A radio/optical reference frame. 5: Additional source positions in the mid-latitude southern hemisphere

We report new accurate radio position measurements for 30 sources, preliminary positions for two sources, improved radio postions for nine additional sources which had limited previous observations, and optical positions and optical-radio differences for six of the radio sources. The Very Long Baseline Interferometry (VLBI) observations are part of the continuing effort to establish a global radio reference frame of about 400 compact, flat spectrum sources, which are evenly distributed across the sky. The observations were made using Mark III data format in four separate sessions in 1988-89 with radio telescopes at Tidbinbilla, Australia, Kauai, USA, and Kashima, Japan. We observed a total of 54 sources, including ten calibrators and three which were undetected. The 32 new source positions bring the total number in the radio reference frame catalog to 319 (172 northern and 147 southern) and fill in the zone -25 deg greater than delta greater than -45 deg which, prior to this list, had the lowest source density. The VLBI positions have an average formal precision of less than 1 mas, although unknown radio structure effects of about 1-2 mas may be present. The six new optical postion measurements are part of the program to obtain positions of the optical counterparts of the radio reference frame source and to map accurately the optical on to the radio reference frames. The optical measurements were obtained from United States Naval Observatory (USNO) Black Birch astrograph plates and source plates from the AAT, and Kitt Peak National Observatory (KPNO) 4 m, and the European Southern Observatory (ESO) Schmidt. The optical positions have an average precision of 0.07 sec, mostly due to the zero point error when adjusted to the FK5 optical frame using the IRS catalog. To date we have measured optical positions for 46 sources.

An Assessment of the Accuracy of the Deep Space Network Extragalactic Reference Frame

The Deep Space Network (DSN) Radio Reference Frame consists of a catalog of angular positions for 281 extragalactic radio sources based on VLBI measurements made during the period from 1978 to 1992. A realistic assessment of the accuracy of these source position estimates must consider both modeled stochastic errors and systematic model deficiencies. Modeled stochastic errors include thermal noise (signal-to-noise ratio) and fluctuations in tropospheric refractivity due to the changing distribution of water vapor. These modeled errors result in a median formal position uncertainty of ≈0.3 milliarcseconds (mas). In particular, we examine the effect of changing the model for inter-observation correlations of water vapor fluctuations on estimated parameters. Next, a comparison of our radio source positions with independently determined positions is presented as evidence of systematic errors at ≤ 0.5 mas. We discuss several aspects of VLBI model accuracy focussing on tidal effects, antenna thermal expansion, pressure loading, source structure, precession and nutation. Prospects for reducing these errors are also discussed. We conclude by combining these estimates of modeled stochastic errors and systematic errors into an overall assessment of &lt; 1.0 mas for the current accuracy of the DSN extragalactic radio frame.

A radio optical reference frame. IV - Additional radio source positions in the Northern Hemisphere

Seventy-seven extragalactic radio sources north of -2 deg declination have been observed using Mark III VLBI during 13 experiments from 1988 October to 1989 November. Radio positions derived from these observations supplement an ongoing project to define and maintain an all-sky radio/optical reference frame of 400 or more extragalactic sources with milliarcsecond (mas) accurate radio and optical positions. Positions for 11 new sources are presented along with improved positions for 54 sources already in the reference-frame catalog. An additional five sources have been determined to be unsuitable reference-frame objects. Observations of seven calibration sources tie the new positions to the existing catalog. The radio positions of the new sources have formal mean errors of about 0.7 mas in each coordinate. Sources for which improved positions are reported now have formal errors not greater than 1 mas, an improvement in some cases by as much as 85 percent. The complementary optical program is addressed briefly.

Occultation of a compact radio source by Venus

view Abstract Citations (4) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Occultation of a Compact Radio Source by Venus Linfield, R. P. Abstract The position of Venus in the radio reference frame was measured on the basis of an observation of an occultation of the compact radio source P0507 + 17 by Venus on July 19, 1988. By comparing model light curves to the observed light curves for both ingress and egress, the frame-tie offsets Delta(lambda) and Delta(beta) (ecliptic longitude and latitude) were determined. The solution for both Delta(lambda) and Delta(beta) gave results consistent with zero offsets, but with an error of about 0.2 arcsec. The egress light curve alone yielded higher accuracy for a linear combination of Delta(lambda) and Delta(beta): Delta(lambda) + 0.51 Delta(beta) = 0.026 +/-0.04 arcsec. Publication: The Astronomical Journal Pub Date: August 1992 DOI: 10.1086/116282 Bibcode: 1992AJ....104..880L Keywords: Radio Occultation; Radio Sources (Astronomy); Venus (Planet); Very Long Base Interferometry; Computational Astrophysics; Light Curve; Planetary Ionospheres; Solar Wind; Astronomy; OCCULTATIONS; PLANETS AND SATELLITES: VENUS full text sources ADS | data products SIMBAD (2)