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CDS Plugin for Cross-match in the AVO prototype
There is currently one method for cross-match:
Positional cross match
The plugin takes as an input 2 catalogue planes (as VOTable), and
the columns containing RA and DEC in each plane:
- plane A contains the reference objects, i.e. objects for
which we seek counterparts (N_A objects)
- plane B, in which counterparts are to be found (N_B objects)
Object are considered matching if they are separated by a distance
d between two thresholds: d_min <= d <= d_max (i.e. search within rings
centered on sources from list A) (distances are in arcsec).
Setting d_min=0 is useful to find all matches within some radius d_max.
The plugin provides on the output a new VOTable file, containing on each
line:
- all the fields from plane A
- all fields from plane B
- the distance between the 2 sources (in arcsec)
For every candidate source from list A, the output can contain:
[ (the best match) OR (all the matches) ] + [(non-matches)]
In (best match) mode:
- if there are possible matches with d_min <= d <= d_max ,
only that with the smallest d is returned
- if there are no sources, nothing is returned
In (all matches) mode:
- the candidate from list A is repeatedly returned, successively
associated with every source from list B matching d_min <= d <= d_max,
thus giving 0, 1 or more results...
In (non-matches) mode:
- Only sources from A which have NO match with d_min <= d <= d_max
are returned. In that case, distance returned is -1, and fields
corresponding to list B are left blank.
The combination "(best match) + (non-matches)" gives a result containing
exactly N_A objects. This corresponds to the january demo specifications.
By default, the positions used for the plane resulting from the cross-match
are that of the plane A, therefore objects are superposed...
It is strongly recommended to use filters on the plane resulting from
the cross-match to help analyze the result.
Examples
Using the 'ellipse' feature, it is possible to draw around cross-matched
objects circles with radius=d_max, to visually check what candidates were
considered possible/selected matches.
eg:{draw ellipse(2,2,0)}
In order to better visualize this, you can put the cross-match results plane and the filter in a new folder plane
Is is also interesting to study the self-correlation of catalogues, to find
possible dupplicates: select the same plane for lists A and B, and either
- search for 0 <= d <= 2'' and use a filter with condition: dist > 0.001 . eg:
${dist}>0.001 {draw}
- search for 0.001 <= d <= 2''
(we remove matches of sources with themselves that have d=0)
Known limitations
If a catalogue plane holds several tables, only the first one will be taken into account for the cross-match
Some screenshots
1. Once you have loaded one or several catalogue planes, click in the Plugins menu and select Cross-match tool
2. A new windows pops up, select catalogue planes you want to cross-match, as well as correct columns for coordinates (when UCDs are properly set in the catalogue, defaut choice of columns should be the good one)
3. Select also your cross-match parameters : min and max thresholds, and which method you want. Then click on Perform cross-match
4. The cross-match results will appear as a new catalog plane in the stack
Script mode
In addition to the GUI, there is a simple script command, allowing one to cross-match 2 catalogues. The syntax is:
xmatch [plane1] [plane2] [distance in arcsec]
Replace plane1 and plane2 with the corresponding name or index of the plane. The "all-matches" mode is used for the cross-match, looking for all possible associations matching 0 <= d <= distance typed in by the user
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