Transit Data - Hipparcos
The Hipparcos Transit Data are described in Volume 1, Section 2.9 of the Hipparcos and Tycho Catalogues (ESA SP-1200, 1997).
The Hipparcos Transit Data file summarises intermediate astrometric and photometric information at the level of individual crossings of targets across the main grid of the Hipparcos instrument. The data is available for 38535 Hipparcos entries selected mainly on account of their known or suspected multiplicity, or as being parts of observations where the signal may be contaminated by the influence of another target close to the edge of the field of view.
What are the Hipparcos Transit Data?
| The Hipparcos Transit Data (TD for short) are a condensed and calibrated version of the raw data from the main detection system onboard the Hipparcos satellite (modulation grid + image dissector tube). They allow the user to re-interpret the Hipparcos raw data in terms of arbitrary models of the astronomical objects that were observed, without having to worry about complex issues such as the geometric and photometric calibration of the instrument, or how it was pointed as function of time (satellite attitude). In principle, it should be possible to reproduce the astrometric results published in the Hipparcos Catalogue, including its Double and Multiple Systems Annex, by analysing the TD in terms of the same models [single star, double star with fixed components, etc] as was assumed in the Catalogue, although that would be a rather pointless exercise. On the other hand, new information can sometimes be generated by assuming a different model, or by injecting supplementary data (e.g. from speckle interferometry) into the solution. |
Two important restrictions should be noted however:
Why have the Hipparcos Transit Data been made available?
| In the course of the 3.5 year mission, the Hipparcos main detector produced some 13 million elementary observations, such as described above, of some 118,000 different targets. For the vast majority of the targets, the subsequent data reductions succeeded in determining, accurately and without ambiguity, the astrometric and photometric characteristics as published in the Catalogue and its Annexes. For a small fraction of the targets (a few thousand) some doubt remained however whether the correct solution was found. And even if a particular solution is correct, it can sometimes be improved by adding constraints not available at the time of the reductions. It was therefore desirable to retain a possibility to reconsider these solutions in the future. This is only possible by going back to the original (raw) data, or at least to some intermediate stage of the data reductions. The machine-readable version of the Hipparcos Catalogue (on six CD-ROMs) contain three different types of such intermediate data: the Epoch Photometry (see van Leeuwen et al., A&A 323, L61, 1997), the Intermediate Astrometric Data (see van Leeuwen & Evans, A&AS 130, 157, 1998), and the Transit Data. |
The Transit Data were thus made available (1) to allow inspection of the Hipparcos data and their interpretation at a more fundamental level than would otherwise be possible; and (2) to allow improved solutions by re-analysis and/or combination with other (space or ground-based) observations.
How can the Hipparcos Transit Data be used?
|Two possible ways in which the TD can be used are explained in detail in the paper by Quist & Lindegren (1999): aperture synthesis imaging (producing an "image" of a double or multiple star, from which the number, approximate positions and magnitudes of the components can be inferred), and model fitting (non-linear least-squares fitting of arbitrary object models to the data).|
Availability of Data and Software
| The paper by Quist & Lindegren (1999) used the double star HIP 97237 as an example both for aperture synthesis imaging and model fitting. Various input/output files used in that example are available and briefly described below. |
Transit Data (in original form):
td097237.td (text file): this is an exact replica of records 3561383 to 3561635 (inclusive) of the HIP transit data file hip_j.dat on CD-ROM disk 6. It contains 253 records of 127 bytes (including both CR and LF in the last two bytes). The first record is the header record, the second is the pointing record, and then follow 251 transit records, one for each passage of HIP 97237 across the main field of view in the Hipparcos instrument.
Files for aperture synthesis modelling:
td097237.uvf (binary file): UV-FITS file used as input for difmap (produced by td2uv)
Files for model fitting, using GaussFit:
mul2_fix.mod (text file): model file (multiple star with 2 components, fixed relative positions)
td097237.env (text file): GaussFit environment file
td097237.dat (text file): data file - input version (produced by td2gf)
td097237.par (text file): parameter file - input version (starting values)
td097237.res (text file): result file
Note: the data in the result file are not completely identical to those shown in Fig. 9 of the paper by Quist & Lindegren (1999). The differences are of the size expected from running td2gf and GaussFit on different computer platforms.
Known Errors in the Hipparcos Transit Data file
|Six records in the Hipparcos TD file (ESA SP-1200, CD-ROM disk 6, file cats/hip_j.dat) contain errors caused by format overflow in the Fortran program originally generating the file. The corresponding fields thus contain asterisks instead of the intended numerical data. This may cause an error when attempting to read these records. The programmes td2uv.f and td2gf.f mentioned above, which extract data from this file, automatically correct the erroneous items. This file contains a description of the errors.|
| Example of results obtained after the publication of the Hipparcos and Tycho Catalogues, using the TD: |
|Prepared by Lennart Lindegren and Carl Fredrik Quist|