Data formats
Can I cross-match my own list of positions with a VizieR catalogue?
Yes. Use the Tables management tab on the cross-match web page to upload your own table.
Currently, we support VOTable, FITS table and CSV as the table input format. If your table is in another format, we suggest using TOPCAT to convert it to VOTable.
Here is an example of a simple table you can upload:
RA,DEC
2.514167,+46.39083
4.107500,+53.945278
4.172083,+54.660556
4.626250,+26.139167
7.465000,+38.324167
9.805417,+3.134167
10.665417,+71.366944
233.190417,+68.866667
113.448750,+52.689444
Which format do I retrieve the result in?
The result of a cross-match job consists in a list of rows, one for each association found.
Each row is a concatenation of the following fields:
- distance between the catalogue A and the catalogue B sources
- distance in sigma (if XMatch by positional errors) between the catalogue A and the catalogue B sources
- fields of the catalogue A
- fields of the catalogue B
The result can be retrieved in one of the following formats : CSV, ASCII or VOTable.
Catalogues and columns
My favourite catalogue/table is not available for cross-matching. What should I do?
Please contact us (see section 6 of the XMatch documentation) and give us some details about the table. We might be able to integrate it in our service.Where do data from large catalogues come from?
It comes from special binary files with limited number of columns. Those files have been created from VizieR data.
The selected columns include (at least) VizieR default columns.
Which position columns are used for the cross-match?
Disclaimer: so far VizieR does not make any difference between FK5 Equinox J2000 and ICRS (and converting one system to the other has no effect). Positions labeled as J2000 in VizieR can be either FK5 J2000 for old catalogues, or most likely ICRS for recent catalogues. In practice the FK5 system internal error is of the same order as the FK5/ICRS offset (about 80 mas for the latter).
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For uploaded files: the user has to choose the positions in the Tables management tab on the cross-match web page.
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For VizieR tables which are not in the large tables list (the list appearing when clicking VizieR in the table selection panel): the VizieR computed (
_RAJ2000,_DEJ2000) positions are used. If a table contains proper motions, they are probably taken into account and the position are provided at epoch J2000. In case of doubt, please check the VizieR page of the table with the J2000 and decimal Preferences selected. -
For VizieR large tables: the first positional columns of each table in the XMatch ouput are the ones used for the cross-match. For most large astrometrical catalogues, the positions are probably the epoch J2000 positions (possibly computed by the VizieR team). For photometrical catalogues, the positions are the main positions provided in the original table.
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Specific case: for Gaia DR2, two tables are now provided
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Gaia DR2 (or the vizier identifier I/345/gaia2): the positions computed at epoch J2000.0 (using Gaia DR2 positions at epoch 2015.5 and Gaia DR2 proper motions) are used. This is well suited for cross-matches with galactic objects having positions near the J2000 epoch. Although the positional errors have also been computed at epoch J2000.0, they do not account for the full astrometric covariance martix.
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Gaia DR2 J2015.5: the original positions, at epoch J2015.5. Those positions are the most precise (no propagation of astrometrical errors on a 15.5 year timescale) and particulary suit extra-galactic studies or cross-matches with photometric catalogues providing positions at an epoch closer to J2015.5 than J2000.0.
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Specific case: for Gaia EDR3, two tables are now provided
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Gaia EDR3 (or the vizier identifier I/350/gaiaedr3): the positions computed at epoch J2000.0 (using Gaia EDR3 positions at epoch 2016.0 and Gaia DR3 proper motions) are used. This is well suited for cross-matches with galactic objects having positions near the J2000 epoch. The positional errors have also been computed at epoch J2000.0 taking into account the full astrometric information (all 6 parameters and the associated 6x6 covariance matrix)
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Gaia EDR3 J2016.0: the original positions, at epoch J2016.0. Those positions are the most precise (no propagation of astrometrical errors on a 16 year timescale) and particulary suit extra-galactic studies or cross-matches with photometric catalogues providing positions at an epoch closer to J2016.0 than J2000.0.
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Why VizieR columns are missing in the XMatch output?
Mainly for space (and performance) purposes, only VizieR default columns (which are supposedly the most useful for most users) are available in the XMatch service.
Please contact us if important columns are missing.
XMatch jobs
Do I need to let the Web page open during a cross-match computation?
No, the computation is done remotely, and the whole process is asynchronous. Thus, you can safely close the cross-match Web page after firing up a new job, and come back later to check its status.How long are the results of cross-match jobs kept?
Job results are kept for a duration of 7 days following their submission.Credits and acknowledgements
Who is the team behind the service?
The CDS cross-match service is developed and maintained by François-Xavier Pineau, Thomas Boch and Sébastien Derrière.
We also acknowledge the valuable participation of Brice Gassmann and Grégory Mantelet.