Overview of Astrometric and Photometry Results

The results of the lengthy Hipparcos star catalogue construction process were finalised in 1997. The quality of the star positions, proper motions, and distance estimates derived from the early data analyses gave a dramatic indication of the outstanding scientific results that the Hipparcos mission had provided and continues to provide.

The final satellite data were despatched from ESOC to the scientific consortia at the end of 1993. Final iterations of the sphere solutions were carried out during the summer of 1995, and the results from the NDAC and FAST Consortia were finally combined after a careful evaluation of the properties of the two solutions. Details of the progress of the mission have been given in a series of papers in ESA Bulletins 69 and 75, and some three-dimensional celestial representations, using the measured parallaxes, appeared in ESA Bulletin 77.

Full descriptions of the sphere solutions and the merging criteria are given in The Hipparcos and Tycho Catalogues: Volume 3 (Construction of the Hipparcos Catalogue) and Volume 4 (Construction of the Tycho Catalogue).

Final median standard errors for stars brighter than 9 mag, at epoch 1991.25, are as follows:

positions:	0.77/0.64 milliarcsec (two coordinates)
parallaxes:	0.97 milliarcsec
proper motions:	0.88/0.74 milliarcsec/year (two coordinates)

Epoch photometry of the ~120,000 stars of the Hipparcos programme is published in Volume 17 of The Hipparcos and Tycho Catalogues. These provide a dramatic improvement in the quality and quantity of photometric data on these objects. Epoch photometry data can also be accessed using the Epoch Photometry Tool.

Detailed information on double and multiple stars is given in Volume 10 of the Catalogues for more than 20,000 stars; some 3,000 of these systems are newly-discovered by Hipparcos and component solutions have been provided.

It is an impressive by-product of the astrometric mission that star magnitudes or brightnesses can be determined from the modulated stellar signal at the same time as the positional information is extracted. Stars over the entire sky were measured with the same instrumental set-up, and observations were repeated on each star on numerous occasions throughout the mission.

Each star was, of course, only observed when it lay on the path of the scanning motion of the satellite; for any individual object, the scanning pattern typically crossed its field at intervals separated by about 20 minutes (the time separation between the scans of each of the two telescope fields of view), then again two hours later as the scanning 'great circle' swept past it again. Depending on whether the objects lay close to the node or anti-nodes of the circles, the objects may have been viewed repeatedly several times before the scanning motion of the satellite moved on across the sky, returning perhaps a few months later. By very careful calibration of the detector's response using a system of 'standard stars' with magnitudes accurately known from ground-based observations, all such measurements of each star provide an accurate magnitude at a given time (many stars in the pre-planned observing programme were identified as suitable 'magnitude' standards for carrying out the in-flight calibration of the observations). Collecting together the magnitudes relating to every star throughout the course of the mission resulted in an enormous collection of light profiles for every star in the observing programme. A consequence of the Hipparcos mission is that the numbers of stars which are now available as astrometric and photometric standards, for a given area of sky, is considerably larger than the numbers available from existing ground-based knowledge.

Results of the photometric signal processing by the FAST and NDAC Consortia have revealed interesting stretches of light curves for various types of object. Especially for objects close to nodes of certain scanning circles, a relatively large number of measurements were made on each star over an interval of only a few days. Variable stars typically occupy interesting locations within the known evolutionary stellar sequences, and the measurement of their distances with Hipparcos is of great importance. Simultaneous determinations of their light curves, of the discovery of previously unknown, perhaps low-amplitude variability objects, can be expected to enhance greatly the scientific value of the Hipparcos positional measurements. Similar results are obtained from the Tycho data processing, for a very much larger number of stars, and in two spectral bands (B and V).