A new reduction of the astrometric data as produced by the Hipparcos mission was published in 2007, claiming accuracies for nearly all stars brighter than magnitude Hp = 8 to be better, by up to a factor 4, than in the original catalogue. This catalogue is known as the Hipparcos-2 Catalogue.
The catalogue is described in "Validation of the new Hipparcos reduction", F. van Leeuwen, A&A, 474, 653-664, (2007).
Details of the data reduction, catalogue construction and verification of the results can be found in Hipparcos, the New Reduction of the Raw Data.
Except for intermediate astrometry data (IAD), the data is available in the Hipparcos and Tycho Legacy Archive via the ESASky Legacy Table Access Protocol (TAP) service (in the Topcat tool, select the option "Table Access Protocol (TAP) Query" in the VO menu and paste the TAP url http://esaskylegacy.esac.esa.int/esasky-legacy-sl-tap/tap in the "Select Service” pane). The data is also available from CDS in VizieR, from ESASky, and from the Gaia Archive.
The astrometric results of the Hipparcos re-reduction exist in three different variants. All three give data which are similar for every star, yet not identical.
Note: one must consistently use the intermediate astrometry data (IAD) together with the catalogue it accompanied. For example, one must not mix catalogue values from Vizier with the IAD from the DVD.
- A Digital Versatile Disc (DVD) is included with the van Leeuwen (2007) book Hipparcos, the New Reduction of the Raw Data. The DVD contains the first version of the re-reduction catalogue (based on iteration 13 of the astrometric solution) as well as the IAD used to construct it. These data are also available from the ESA legacy archive (ESDC TAP server: namespace hipparcos2_dvd).
- The CDS/VizieR catalogue supersedes the DVD data for most applications: it is based on a later iteration (iteration 14) of the astrometric catalogue. For this iteration, double star differential parameters and orbital parameters have in some cases been updated, when additional information became available. The attitude outlier rejection criteria were improved and an improved criterion for VIM solutions reduced the number of stars requiring VIM treatment. Most research focused on Hipparcos-2 stars takes the astrometry from VizieR. However, no intermediate astrometry data is published in VizieR. A copy of the VizieR data is available from the ESA legacy archive (ESDC TAP server: namespace hipparcos2_cds). This copy is additionally also available through the Gaia Archive at ESA (namespace External catalogues: public.hipparcos_newreduction).
- The ESA Cosmos pages include a Java tool to interactively explore the Hipparcos-2 data. It was published first in 2014. The tool comes with a set of binary files containing the astrometric catalogue as well as intermediate astrometry data consistent with it. The astrometric data is based on the same updated reduction criteria as the VizieR dataset, but one additional iteration (iteration 15) of the astrometric solution was done. Therefore, the astrometric results of some sources are different from VizieR, but the difference is almost always within their formal uncertainties.
Van Leeuwen and Michalik (2021) have provided a human readable version of the IAD of the Java tool in a zip file [warning: ~350 MB]. The data is provided as one ASCII file per star. An eleven line header gives auxiliary information together with the astrometric reference parameters. The rest of each file is mostly identical to the IAD format of the DVD: it provides the individual observations of this star expressed as abscissa residuals against the astrometric reference solution. This data is currently not available from the ESA legacy archive.
For most stars in the Hipparcos-2 catalogue, results using the IAD from the DVD are indistinguishable from results using the IAD from the Java tool. One advantage of the Java tool data is that rejected transits are clearly denoted by negative abscissa residual uncertainties (SRES). Rejected observations are not designated in the DVD IAD.
Note: Brandt et al. (2021) provide a summary on their use of Hipparcos and Gaia astrometry. In Section 4 of their paper, these authors address ~6600 Hipparcos-2 stars for which a small number of IAD records seem to have been corrupted. This applies equally to the DVD dataset and the Java tool dataset of the Hipparcos re-reduction. These authors propose how the corrupted records can be ad-hoc corrected and provide a software tool to do so. The ad-hoc correction algorithm requires knowing rejected transits and therefore only works with the IAD from the Java tool.