MERLIN positions of 12 radio stars are used to link the provisional Hipparcos reference frame to the International Celestial Reference Frame. The accuracy of the link using these radio stars is 2.3 milliarcseconds. Further observations are planned to check the accuracy of the link in the future.

The final Tycho Catalogue (ESA 1997b) has been derived from 37 months of observations with the star mapper of the astrometric satellite Hipparcos. The Hipparcos Catalogue (ESA 1997a) with about 120,000 stars is the result of the main Hipparcos mission and has, e.g., been described by Kovalevsky et al. (1995). Both catalogues will be published in 1997.

The Hipparcos mission has come to a close with the end of the data processing on August 1996, only three years after the last observations were performed. About 118,000 stellar objects were observed and subsequently solved for their position, parallax and proper motion together with repeated determinations of their magnitude in the Hipparcos band. This paper outlines the main properties of the solution and explains how and when this information will be made available to the astronomical community.

The most reliable method of cross-wavelength identification is the coincidence of accurately determined positions, both referred to the same reference frame. But objects move with time and the more accurate their positions, the more important their proper motions are.

The first astronomical photographic survey, the Carte du Ciél was initiated in 1887 by a group of French astronomers. The observational campaign was started in 1891, while the last of more than 22,000 total plates were photographed in 1950; most of observations (more than 90%) were performed prior to 1920. Detailed description of the Carte du Ciél development can be found elsewhere (Kolchinsky 1989, Eichhorn 1974, Debarbat et al. 1987).

“Salvaging an astrometric treasure” is an european effort to retrieve the astrometric and photometric information contained in the early Carte du Ciel plates. Five institutes from three european countries are involved. The project was supported by the European Community.

The Version 1.0 of the U.S. Naval Observatory Twin Astrographic Catalog (TAC) contains positions of 705,679 stars (Zacharias et al. 1996). The TAC 1.0 covers over 90% of the sky from +90° to −18° declination; 260 out of 5180 plates are not yet measured. Plates were taken in both the yellow (508-578 nm) and in the blue (410-486 nm) bandpass between 1978 and 1986 at the U.S. Naval Observatory (USNO) in Washington, DC.

A collaboration between STScI and ARI has produced a new astrometric reduction of the Guide Star Catalog (GSC, Lasker et al. 1990). This new version, GSC 1.2, has dramatically reduced the systematic errors present in GSC 1.1. The positions in GSC 1.1 are affected by plate-based systematic distortions which are largest at the plate edges (1.0″, north; 1.2″, south) (Taff et al. 1990a). These positions also suffer from systematic errors which are a function of magnitude and radial distance from the plate center (Morrison et al. 1996). This effect is small for radii under 2.7° from the plate center, then rapidly increases producing an average offset of the faint stars (15m) versus the reference stars (10m) of 0.9″ at the plate edges.

The USNO-A1.0 catalog was generated from the U. S. Naval Observatory's digitization of the Palomar Observatory Sky Survey I O and E survey plates for fields with central declination δ ≥ −30°, and from the European Southern Observatory R and Science Research Council J survey plates for fields with central δ ≤ –35° using the Precision Measuring Machine (PMM) located at the Flagstaff Station. It lists positions (α and δ in J2000) and brightnesses (red and blue magnitudes on the parent plate system) for all objects. A flag indicates if the entry also exists in GSC1.1.

The Second Palomar Sky Survey (POSS-II) is now nearing completion. It will cover the entire northern sky with 894 fields (6.5° square) at 5° spacings, with no gaps in the coverage. Plates are taken in three bands: IIIa-J + GG395, λeff ∼ 480 nm; IIIa-F + RG610, λeff ∼ 650 nm; and IV-N + RG9, λeff ∼ 850 nm. Typical limiting magnitudes reached are BJ ∼ 22.5, RF ∼ 20.8, and IN ∼ 19.5, i.e., ∼ 1m −1.5m deeper than the POSS-I. The image quality is improved relative to the POSS-I, and is comparable to the southern photographic sky surveys. For more details, see Reid et al. (1987), and Reid & Djorgovski (1993).

Although the HST GSC–I (Paper-I: Lasker et al. 1990, Paper-II: Russell et al. 1990, Paper-III: Jenkner et al. 1990) has been used with great success operationally, it was always known that it was possible to improve the scientific and operational usefulness by an increase in scope to include multi-color and multi-epoch data. Once the GSC-II concept was established, it was evident that, even beyond the original motivations in HST operations, it would address a number of other astronomical needs such as increasing demands for fainter catalogues to support remote or queue scheduling capabilities and adaptive optics on the next generation of large-aperture, new-technology telescopes. In addition, the all sky nature of the GSC–II makes it a natural data source for research in galactic structure.

In order to ensure the quality of the source catalogue derived from the SASS processing an automatic as well as a visual screening procedure was applied to 1378 survey fields. Most (94%) of the 18,811 sources were confirmed by this screening process. The rest is flagged for various reasons. Broad band images are available for a subset of the flagged sources. Details of the screening process can be found at www.rosat.mpe-garching.mpg.de/survey/rass-bsc/doc.html.

We would like to start this discussion by attempting to make a useful, working distinction between catalogs and databases of astronomical objects. It seems to us that such a distinction could be made based upon the mode of access to the information: A catalog may be considered as a list of objects, almost invariably ordered by coordinates; a database, on the other hand, may be distinguished by its ability to extract a set of objects based on user-given criteria, such as all objects within a certain sky region with magnitudes brighter than m in the blue. Catalogs form the basis of the database, which adds the means of multi-criteria access to the information. In concrete terms, one usually thinks of SIMBAD, NED and LEDA as databases, while an ftp site containing electronic lists of objects may be thought of as a catalog storage warehouse.

The ROSAT All Sky Survey (RASS), performed between July 1990 and February 1991, provided about 60,000 X-ray sources in a soft X-ray band (0.1–2.4keV) with a flux limit of approximately 5 × 10–13 ergs cm–2 s–1 for exposure times of 400 sec (a typical value in low ecliptic latitudes). A wealth of information can be extracted from the RASS source content and many objects deserve (or have already deserved) extensive follow-up studies. However, this possibility is limited by the fact that the nature of most of the RASS sources is unknown i.e., they are unclassified. A correlation with the SIMBAD data base yields only identifications for about one third of the RASS sources.

There are basically two different approaches to study the physical conditions and the energy transfer processes operating in astronomical objects:

1. – the detailed observations of a few prominent objects over a wide wavelength range.

2. – the study of the broad band properties of suitably choosen samples

We present results of the optical identification of a spatially complete, flux limited sample of about 700 ROSAT All-Sky X-ray sources contained in 6 study areas north of δ = −9° with |bII |> 20° (including one region near the North Galactic pole (NGP), another one near the North Ecliptic pole (NEP)). Countrate limits are 0.01 cts s–1 near the NEP and 0.03 cts s–1 for the other areas. The optical observations were performed at the 2.15-m telescope of the Guillermo Haro Observatory, Mexico, using the Landessterwarte Faint Object Spectrograph Camera which allows to carry out direct CCD imaging and multi-object spectroscopy. The limiting magnitude is about 19m for spectroscopy and about 23m for B and R direct imaging. Our analysis shows a dependency of the ratio of ‘extragalactic’ (e.g., AGN, cluster of galaxies) to ‘stellar’ (e.g., coronal emitters, active binaries) counterparts on N H. In the area near the NGP (low N H) ‘extragalactic’ counterparts dominate, while in the area with the highest N H ‘stellar’ counterparts dominate.

We use a new, robust, method to estimate the identification probabilities of optical matches from digitized plate catalogs (COSMOS/UKST Catalog of the Southern Sky Version 2, Yentis et al. 1992; The Guide Star Catalog Version 1.1, Lasker et al. 1990; The Tycho Input Catalog, Egret et al. 1992; The APM Northern Sky Catalogue, Irwin, Maddox and McMahon 1994) to sources in the IRAS Faint Source Survey (FSS; Moshir et al. 1992), including both the Catalog (FSC) and the Reject File (FSR), utilizing a new random matching procedure with the advantages that it: (1) eliminates systematic uncertainties due to many problems, such as uncertainty in N(m); variations across the optical plate of magnitudes, plate limits or N(m); misclassification of stars and galaxies; the assumption of Gaussian error ellipses, etc. and (2) properly calibrates the identification probabilities. We find that at high SNRs and high galactic latitudes essentially all IR star-colored sources have an optical identification with Pid > 99%. At high SNRs and high galactic latitudes, ∼ 90% of all IR galaxy-colored sources have an optical identification with Pid > 90%.

Astronomy is about to undergo a major paradigm shift, with data sets becoming larger, and more homogeneous, for the first time designed in the top-down fashion. In a few years it may be much easier to “dial-up” a part of the sky, when we need a rapid observation than wait for several months to access a (sometimes quite small) telescope. With several projects in multiple wavelengths under way, like the SDSS, 2MASS, GSC-2, POSS2, ROSAT, FIRST and DENIS projects, each surveying a large fraction of the sky, the concept of having a “Digital Sky,” with multiple, TB size databases interoperating in a seamless fashion is no longer an outlandish idea. More and more catalogs will be added and linked to the existing ones, query engines will become more sophisticated, and astronomers will have to be just as familiar with mining data as with observing on telescopes.

Since the first applications of wide-field photography in astronomy nearly 2 million plates and films have been obtained and stored in archives all over the world. The Wide-Field Plate Database (WFPDB) provides astronomers with detailed information about the wide-field photographic observations. Its preparation started in 1991 as one of the main projects initiated by the Working Group on Wide Field Imaging at the IAU Commission 9.

The SkyView virtual observatory provides a single, simple interface where users can retrieve images of the sky at all wavelengths from the radio through gamma rays. Below we discuss how SkyView works and how it represents a new paradigm in astronomical data archives. Users get to SkyView over the WorldWideWeb at http://skyview.gsfc.nasa.gov. Within a few moments they can have high-quality images from ground- or satellite-based surveys.