Comparison of the Optical and Radio Reference Frames

The Very Large Array (VLA) has made possible the measurement of the precise positions of the radio emission associated with stars. This allows the direct comparison of the optical reference frame (FK4) with the radio reference frame which is defined by the quasi-absolute positions of extragalactic radio sources. This comparison is limited by the small number of bright stars that display detectable radio emission and the lack of knowledge of the precise coincidence of the radio emission with the optical photocenter of the star. Since the VLA is the most sensitive astrometrically capable radio telescope, positions of the largest number of stars north of declination -20 degrees can be measured. The accuracy of the positions on the extragalactic reference frame should approach a milliarcsecond.

Download Full-text

The Possibility to Link Radio and Optical Reference Frames With Artificial Satellites

Symposium - International Astronomical Union ◽

10.1017/s0074180900086794 ◽

1990 ◽

Vol 141 ◽

pp. 194-194

Author(s):

Tong Fu

Keyword(s):

Reference Frame ◽

Reference Frames ◽

Artificial Satellites ◽

Radio Sources ◽

Bl Lac Objects ◽

Extragalactic Radio Sources ◽

Bl Lac ◽

Optical Reference ◽

Radio Stars ◽

Fundamental Reference

Based on extragalactic radio sources, a new high precision extragalactic radio reference frame can be established from radio interferometric measurements. To link the optical fundamental reference frame presently represented by the FK4/5 to the extragalactic radio frame, the optical counterparts of extragalactic radio sources (quasars, BL Lac objects etc.) and radio stars are the most important classes of objects. Besides these two classes of objects, are there any other objects which can be used to link the optical and radio frames? A posible answer is that artificial satellites could be a candidate class of objects contributing to this subject.

Download Full-text

Optical Reference Frame Defined by Carlsberg Meridian Catalogue La Palma Number 4

Symposium - International Astronomical Union ◽

10.1017/s0074180900087143 ◽

1990 ◽

Vol 141 ◽

pp. 391-401

Author(s):

LV Morrison ◽

RW Argyle ◽

L Helmer ◽

C Fabricius ◽

OH Einicke ◽

...

Keyword(s):

Reference Frame ◽

Proper Motion ◽

Reference Frames ◽

Radio Sources ◽

Present Epoch ◽

Extragalactic Radio Sources ◽

La Palma ◽

Average Accuracy ◽

Optical Reference

The contribution of the Carlsberg Meridian Catalogue La Palma Number 4 to the determination of the optical reference frame is discussed. This catalogue of almost 51000 stars provides one of the most accurate optical reference frames at the present epoch, having a density of 1 star per square degree, and an average accuracy of 0.″12 in position and 0.″003 per year in proper motion for stars with V<9. The catalogue also contains positions of reference stars with V>11 in the fields of benchmark extragalactic radio sources which can be used in linking the optical reference frame defined by the FK5 to the extragalactic frame.

Download Full-text

Radio Astrometry

Symposium - International Astronomical Union ◽

10.1017/s007418090007844x ◽

1984 ◽

Vol 110 ◽

pp. 339-346 ◽

Cited By ~ 2

Author(s):

K. J. Johnston

Keyword(s):

Reference Frame ◽

Reference Frames ◽

Radio Sources ◽

Inertial Reference Frame ◽

Extragalactic Radio Sources ◽

Optical Reference

Radio astrometry is now the premier astrometric technique for measuring the positions of celestial objects. The precision with which absolute positions can be determined is approaching a few milliarc-seconds. The progress towards establishing an almost inertial reference frame based upon the positions of extragalactic radio sources is reviewed as of June 1983. The outlook for relating this reference frame to optical reference frames is also reviewed.

Download Full-text

Accuracy Assessment of Hipparcos Optical and VLA Radio Positions of Stars

Symposium - International Astronomical Union ◽

10.1017/s007418090017353x ◽

1993 ◽

Vol 156 ◽

pp. 377-380

Author(s):

H. G. Walter ◽

R. Hering ◽

H. Lenhardt ◽

Chr. deVegt ◽

D.R. Florkowski ◽

...

Keyword(s):

Radio Emission ◽

Reference Frame ◽

Accuracy Assessment ◽

Optical Emission ◽

Relative Orientation ◽

Large Array ◽

Proper Motions ◽

Data Noise ◽

Very Large Array ◽

Radio Stars

Optical positions of some 30 radio stars derived from 12 months of HIPPARCOS measurements are compared with their radio positions obtained with the Very Large Array (VLA). — Once the lengths of arcs between optical and radio positions of pairs of stars are calculated the differences of the arcs are formed. They provide an estimate of the coincidence of the optical and radio emission centres. — From the comparison of optical and radio positions infinitesimal rotation angles of the HIPPARCOS frame with respect to the VLA extragalactic reference frame are determined by rigid rotations. After taking account of the relative orientation of the frames the standard deviations of the remaining residuals are approximately of the order of the VLA observation errors, thus demonstrating the reliability of the HIPPARCOS results. However, they also indicate some data noise very likely caused by the low accuracy of optical proper motions used to bridge the HIPPARCOS-radio epoch differences up to 9 years, and possible discrepancies of radio-optical emission centres of some stars.

Download Full-text

Fundamental radio reference frame from VLA observations of minor planets

Symposium - International Astronomical Union ◽

10.1017/s0074180900119333 ◽

1988 ◽

Vol 128 ◽

pp. 97-97 ◽

Cited By ~ 1

Author(s):

G. H. Kaplan ◽

K. J. Johnston ◽

P. K. Seidelmann ◽

C. M. Wade ◽

T. S. Carroll

Keyword(s):

Reference Frame ◽

Reference Frames ◽

Thermal Emission ◽

Optical Observations ◽

Minor Planets ◽

Very Large Array ◽

Test Particles ◽

Optical Reference ◽

Microwave Regime

The weak thermal emission from the largest minor planets can be detected in the microwave regime by the Very Large Array (VLA). Signal-to-noise ratios are sufficiently high to permit precise measurement of the positions of these objects at all points in their orbits with respect to background extragalactic sources. We are in the process of obtaining observations of astrometric accuracy for minor planets 1 Ceres, 2 Pallas, 4 Vesta, and 10 Hygeia.Minor planets have historically served as “test particles” in the solar system, and optical observations of these objects have been valuable in the determination of fundamental astronomical constants. In particular, optical observations of minor planets have played an important role in the establishment of the fundamental optical reference frame by permitting the determination of the orientation of the Earth's orbit relative to the stars defining the frame.Similarly, radio observations of these bodies can play a corresponding role in the establishment of a fundamental radio reference frame. Our observations will provide a direct link between the dynamical and radio reference frames, and provide important information on the relationship between the radio and optical reference frames.

Download Full-text

Opacity in compact extragalactic radio sources and its effect on radio-optical reference frame alignment

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308019546 ◽

2007 ◽

Vol 3 (S248) ◽

pp. 348-351

Author(s):

Y. Y. Kovalev ◽

A. P. Lobanov ◽

A. B. Pushkarev ◽

J. A. Zensus

Keyword(s):

Reference Frames ◽

Reference Sample ◽

Physical Factors ◽

Radio Sources ◽

Positional Accuracy ◽

Extragalactic Radio Sources ◽

Long Baseline ◽

Spectral Bands ◽

Optical Reference ◽

Primary Reference

AbstractAccurate alignment of the radio and optical celestial reference frames requires detailed understanding of physical factors that may cause offsets between the positions of the same object measured in different spectral bands. Opacity in compact extragalactic jets (due to synchrotron self-absorption and external free-free absorption) is one of the key physical phenomena producing such an offset, and this effect is well-known in radio astronomy (“core shift”). We have measured the core shifts in a sample of 29 bright compact extragalactic radio sources observed by Very Long Baseline Interferometry (VLBI) at 2.3 and 8.6 GHz. We report the results of these measurements and estimate that the average shift between radio and optical positions of distant quasars could be of the order of 0.1--0.2 mas. This shift exceeds the expected positional accuracy of Gaia and SIM. We suggest two possible approaches to carefully investigate and correct for this effect in order to align accurately the radio and optical positions. Both approaches involve determining a Primary Reference Sample of objects to be used for tying the radio and optical reference frames together.

Download Full-text

Dense optical reference frames: UCAC and URAT

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308019431 ◽

2007 ◽

Vol 3 (S248) ◽

pp. 310-315

Author(s):

N. Zacharias

Keyword(s):

Reference Frame ◽

Focal Plane ◽

Reference Frames ◽

Field Of View ◽

Radio Sources ◽

Proper Motions ◽

Naval Observatory ◽

Optical Reference ◽

Celestial Reference Frame ◽

The U.S

AbstractA series of ground-based, dedicated astrometric, observational programs have been performed or are in preparation which provide a dense and accurate optical reference frame. Integral to all these programs are new observations to link the Hipparcos Celestial Reference Frame (HCRF) to the International Celestial Reference Frame (ICRF), based on compact, extragalactic radio sources.The U.S. Naval Observatory CCD Astrograph Catalog (UCAC) 3rd release is in preparation. A pixel re-reduction is in progress to improve astrometric and photometric accuracy as well as completeness of this all-sky reference catalog to 16th magnitude. Optical counterparts of ICRF radio sources have been observed with 0.9-meter telescopes contemporaneously. Scanning of over 5000 early-epoch astrograph plates on StarScan has been completed. These data will improve the proper motions of stars in the 10 to 14 mag range for the UCAC3 release.A 111 million-pixel CCD was successfully fabricated in 2006 and test observations at the USNO astrograph are underway. Four of such detectors will be used for the USNO Robotic Astrometric Telescope (URAT) focal plane assembly. Phase I of URAT will use the astrograph to reach 18th magnitude, while the new 0.85-meter telescope with a 4.5 deg diameter field of view will reach 21st magnitude. The URAT primary mirror has been fabricated.

Download Full-text

Link between Hipparcos and VLBI Celestial Reference Frames

International Astronomical Union Colloquium ◽

10.1017/s0252921100063764 ◽

1991 ◽

Vol 127 ◽

pp. 202-205

Author(s):

E.F. Arias ◽

J.-F. Lestrade

Keyword(s):

Reference Frame ◽

Reference Frames ◽

Inertial Frame ◽

Radio Sources ◽

Proper Motions ◽

Extragalactic Radio Sources

AbstractThe celestial frame of HIPPARCOS astrometric satellite will be materialized by the positions and proper motions of about 120000 stars relative to arbitrary origins. As the HIPPARCOS reference frame is not naturally related to fixed directions in space, it has to be linked to an inertial frame of similar quality. The technique of VLBI determines the coordinates of extragalactic radio sources precise at the milliarcsecond level in an equatorial frame. The precision expected for HIPPARCOS normal mission is 0.002ʺ for the positions, yearly proper motions and parallaxes.

Download Full-text

Optical Astrometry of Extragalactic Radio Sources With the Tautenburg Schmidt Telescope

Symposium - International Astronomical Union ◽

10.1017/s0074180900087271 ◽

1990 ◽

Vol 141 ◽

pp. 453-454

Author(s):

W.R. Dick ◽

I.I. Kumkova

Keyword(s):

Working Group ◽

Reference Frames ◽

Position Error ◽

Radio Sources ◽

Schmidt Telescope ◽

Extragalactic Radio Sources ◽

Optical Reference

Optical positions of objects from the IAU Commission 24 Working Group list of benchmark radio sources have been derived which will contribute to the link of the radio and optical reference frames. Results for 11 objects with an r.m.s. position error of 0.″2 are presented and discussed.

Download Full-text

Improvement of Star Positions by a New Axial Meridian Circle with Negligible Systematic Errors

Symposium - International Astronomical Union ◽

10.1017/s0074180900228489 ◽

1995 ◽

Vol 166 ◽

pp. 365-365

Author(s):

G.I. Pinigin ◽

A.V. Shulga ◽

P.N. Fedorov ◽

A.N. Kovalchuk ◽

A.E. Mazhaev ◽

...

Keyword(s):

Reference Frame ◽

Reference Frames ◽

Systematic Errors ◽

Radio Sources ◽

Meridian Circle ◽

Positional Accuracy ◽

Extragalactic Radio Sources ◽

Fully Automatic

The Mykolayiv axial meridian circle (AMC) consists of a horisontal telescope with a pentagon prism of ‘CITALL’ in front of the objective of D=180mm and F=2480mm in the meridian, and a fixed aligned vacuum collimator of D=180mm and F=12360mm.Investigations of the AMC parameters show that: horizontal flexure is negligible and about −0.037″; collimation is stable and can be described by the formula C = C′ + a · t, where t[C] is the temperature, a = 0.026″ and C′ const. The variation of collimator inclination is of the order 0.09″ per 1°C Test observations of FK5 stars show variations of the AMC intrumental system of not more than 0.1″.A CCD micrometer is being developed. Test observations with a pilot micrometer with a CCD of 288 × 256 pixels have started. It appears that the AMC magnitude limit (with cooling) will be about 14 and the positional accuracy 0.1 arcsec.Finally, the expected capabilities of the fully automatic Mykolayiv AMC permit its efficient use for extention of the stellar reference frame to fainter magnitudes, for linking the optical/radio reference frames. In particular, for future observations a programme of 12–14m reference stars around extragalactic radio sources is being prepared.

Download Full-text