Barycentric celestial reference system

Abstract: In this, the third in a series of three papers concerning the SuperCOSMOS Sky Survey, we describe the astrometric properties of the data base. We describe the algorithms employed in the derivation of the astrometric parameters of the data, and demonstrate their accuracies by comparison with external data sets using the first release of data, the South Galactic Cap survey. We show that the celestial coordinates, which are tied to the International Celestial Reference Frame via the Tycho–2 reference catalogue, are accurate to better than ±0.2 arcsec at J,R∼19,18, rising to ±0.3 arcsec at J,R∼22,21, with positional-dependent systematic effects from bright to faint magnitudes at the ∼0.1-arcsec level. The proper motion measurements are shown to be accurate to typically ±10 mas yr−1 at J,R∼19,18, rising to ±50 mas yr−1 at J,R∼22,21, and are tied to zero using the extragalactic reference frame. We show that the zero-point errors in the proper motions are ≤1 mas yr−1 for R>17, and are no larger than ∼10 mas yr−1 for R<17 mas yr−1.

Abstract: 1. Introduction 2. New observational techniques 3. Basic principles and coordinate systems 4. Treatment of astronomical data 5. Principles of relativity 6. Apparent displacements of celestial objects 7. Extragalactic reference frame 8. Dynamical reference frame 9. Terrestrial coordinate systems 10. Earth orientation 11. Stars 12. Double and multiple star systems 13. Astronomical phenomena 14. Application of theory to observations Appendices.

Abstract: Astrometry is the major astronomical technique to measure distances, masses and motions of stars. Dividing astrometric techniques into five types according to the size of the field in which a single instrument can produce measurements, the present achievements of the Earth-based astrometry are described. The astrometric activities such as measurements of star diameters, double star relative positioning or stellar parallaxes, search for invisible companions, photographic plate reduction, visual and photoelectric meridian and astrolable astrometry are reviewed. Then, the methods used to construct a quasi-inertial celestial reference frame and to materialize it by a fundamental catalogue are presented and discussed. A much better definition of an absolute reference frame is made possible by VLBI, but the problem of extending it to stellar positions is not yet satisfactorily resolved. The limitations of the ground based astrometry are: the atmospheric turbulence and refraction, Earth's motions and the impossibility to view the entire sky with a single instrument. These limitations are discussed and it is shown how astrometry from space can overcome them. A priori, a gain of two orders of magnitudes in accuracy for all types of astrometry is expected, but at this new level of precision, new effects and limitations will appear, as already shown in the studies of the approved programs. Then, the ESA astrometric satellite HIPPARCOS presently under development is presented. The satellite and the payload are described as well as the observing procedures. Several limitations, specific to space borne instrumentation and to the milliarc second accuracy expected have been identified. However the main limitation in precision remains the photon noise. The data reduction methods are sketched. The data downlinked at a rate of 20 kilobits per second have to be used with an equal weight all over the 21/2 years of observation. They are expected to yield a mean accuracy of 2 milliarc seconds in position and parallax and 2 m.a.s. per year in proper motion for most of the 100000 stars of the program (M b < 9). Stars to be observed by HIPPARCOS have to be carefully selected. The main fields in which the results of HIPPARCOS will be used are listed from the proposals made by the scientific community. The task of constructing the ‘HIPPARCOS input catalogue’ from these proposals is presented. Another feature of the ESA astrometric satellite is the use of the HIPPARCOS star-mapper as a photometric and position survey of the sky. This experiment, called TYCHO, should give at least 400000 star positions with accuracies of the order of 0″.03 to 0″.15 depending upon the magnitudes. Two colour instantaneous magnitudes should also be obtained to 0.1–0.4 mag. precision. Several Space-Telescope on-board instruments are also capable to make small field astrometric observations. Accurate imaging is possible with the Wide Field and the Faint Object cameras. Lunar occultations will be performed with the High Speed photometer. But the main astrometric mode of the Space Telescope will be the use of the Fine Guidance Sensors to measure the relative positions of stars to ±0″.002. It is described together with its main scientific applications. The establishment of an absolute reference frame is subsequently discussed. Plans using simultaneously VLBI, HIPPARCOS, and Space Telescope observations are described. They consist in linking the HIPPARCOS stellar system to quasars via radio-stars or stars in the vicinity of optical quasars. Finally, several space astrometry proposals are described: long focus space astrometry and two versions of space interferometry.

Abstract: An automatic introduction apparatus for automatically introducing a target celestial object by controlling a rotation of an astronomical telescope around at least two axes comprises: an image-capturing means capable of capturing an image of a celestial object at a plurality of focal distances; a celestial object database; an image processing section for extracting a set of information of each celestial object from the image of celestial object captured by the image-capturing means; and a celestial object identification means for identifying the celestial object whose image has been captured, by comparing the information of each celestial object extracted by the image processing sections with the celestial object information stored in the celestial object database. The alignment process is executed by defining a coordinate transformation information of a coordinate system in the astronomical telescope relative to a celestial coordinate system based on the position information of the identified celestial object. In the automatic introduction, after the introduction of the target celestial object, an image of celestial object is captured, the celestial object in the captured image of celestial object is identified, and the astronomical telescope is controlled by rotating it around two axes so that the target celestial object can be introduced into the center of field based on the position information for the identified celestial object. The alignment precision and the automatic introduction precision can be improved by shifting the focal distance of the image-capturing means in a step-by-step manner toward the telescopic field side.