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### A.11.1 CDS Upload X-Match Window

CDS Upload X-Match Window

The CDS Upload X-Match Window allows you to join a local table with any table provided by the VizieR database of astronomical tables or with SIMBAD. You can access the window from the main Control Window using the CDS Upload Match button () in the toolbar, or the Joins or VO menus. This window is an interface to the excellent CDS X-Match service provided by the Centre de Données astronomiques de Strasbourg (CDS). The service is very fast, and in most cases it is the best way to match a local table against a large external table hosted by a service. In particular, it is almost certainly much better than using the Multi-Cone window, though it's less flexible than TAP.

The local table is uploaded to the X-Match service in blocks, and the matches for each block are retrieved in turn and eventually stitched together to form the final result. The tool only uploads sky position and an identifier for each row of the input table, but all columns of the input table are reinstated in the result for reference. These details are mostly transparent when using the service, though it may help to understand how the progress bar moves.

For a better understanding of the details of how this service operates, including exactly what coordinates are matched against the uploaded positions (roughly: integrated to J2000 using proper motions if available) and what columns are included in the output (roughly: a subset of the most commonly used columns), please consult the service documentation. Note that because of the way the service is implemented not all the columns from the VizieR table may be present in the result; if the columns you need are not present, you're out of luck, there's no way to request additional ones.

The window has three panels, described below.

The Remote Table panel allows you to select the CDS table against which to match. The table must be entered by name in the VizieR Table ID/Alias field, and may be in one of the following forms:

- "simbad", to indicate the SIMBAD database of known astronomical objects.
- A VizieR table identifier, for instance "II/246/out". Note these must name the table not just the catalogue (which may contain several tables), so for instance "II/246" will not work. No help is currently provided in this window to search for these IDs, you have to use one of the services or web pages provide by CDS to locate them. A good tip is the table searching facility at the TAPVizieR service, https://tapvizier.cds.unistra.fr/adql/.
- A VizieR table alias, for instance "2mass". A few tens of these are defined for the largest and most-used tables in VizieR, and these can be selected by clicking on the selection box.

When a table name has been entered, additional information (Name, Alias, Description and Row Count) will be downloaded from CDS and displayed in the lower part of the Remote Table panel. If nothing shows up here, then the table name is not legal for the X-Match service, and the Go button will be disabled.

Sky coverage information is also displayed, including the proportion of the sky covered by the table and a graphical indication of the covered regions on the sky.

Service interaction is monitored by the little "lights" to the right of this panel. If things are operating correctly they should be, or quickly turn, green. If they are amber or red, the service may be slow or not responding. Hovering over them with the mouse will give more information (apologies to red-green colourblind users).

The Local Table panel allows you to indicate the local table (the one loaded into TOPCAT) that you want to match. You must select the required table and indicate its Right Ascension and Declination columns.

The Match Parameters panel supplies the other information about how the match will operate:

Radius
  The maximum distance between a local table and remote table position to count as a match. There is a maximum enforced on this value by the CDS X-Match service, currently 2 arcminutes.
Find mode
  Determines in what form the result is generated and used. The following options are available:
  Best
    Load a new table with one row for each local row that matches a remote row, giving the closest match. Unmatched local rows are not included.
  All
    Load a new table with one row for each match between a local row and a remote row. The match is symmetric between local and remote tables.
  Each
    Load a new table with the same number of rows as the local table, in the same order. The best remote match, if any, appears alongside the local row, but if there is no match the remote columns are blank.
  Best Remote
    Load a new table with one row for each remote table row that matches the local table. Note there is currently a bug in this mode; if the match is done in N blocks then a remote row may appear up to N times rather than just one. This may be fixed in a future release, but to avoid it you can make sure that the Block size is greater than the row count.
  Add Subset
    No new table is loaded, but a new Row Subset is added to the local table indicating which rows had at least one match to the remote table.
Rename columns
  If columns from two input tables are combined, it's possible that some may share the same name. Multiple columns in a table with the same name can cause confusion, so this selector allows you to determine whether and how such name clashes are dealt with. Columns from the remote table can be renamed by appending a specified suffix, either always or only in case of duplicate names.
Block size
  The number of rows uploaded to the X-Match service at a time. This should not affect the result, though it may affect performance.

  Large blocksizes tend to be good (up to a point) for reducing the total amount of time a large xmatch operation takes, but they can make it harder to see the job progressing. There is also the danger (for ALL-type find modes) of exceeding the return size limit, which will result in truncation of the returned result. If this happens, a warning will be included in the popup window that reports match completion. At time of writing, the upload limit is 100Mbyte (about 3Mrow), and the maximum return size is 2Mrow.

When all the fields have been filled in, hit the Go button and the match will start; progress will be logged in the progress bar at the bottom of the window. To stop the match before it has completed, hit the Stop button. Note that the Go button is only enabled when a legal table name has been entered at the top.

The remote table in most cases contains only a subset of the the columns in the relevant VizieR table, including the most useful ones. The service currently provides no straightforward way to acquire columns which are not returned by default.

The following item is available in the toolbar and Search menu:

Use Service Coverage
  If this option is selected, then the sky coverage of the service is used to pre-filter local table rows. In this case those rows that fall outside the coverage area of the remote table are not uploaded for matching, since it is known that they will not match. This option should therefore not affect the result, but improve performance, and as such should generally be turned on (the default).

Acknowledgement: CDS note that if the use of the X-Match service is useful to your research, they would appreciate the following acknowledgement:

> "This research made use of the cross-match service provided by CDS, Strasbourg."

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk