xMatch Queries (astroquery.xmatch)

Getting started

The xMatch service is a tool to cross-identify sources between very large data sets or between a user-uploaded list and a large catalogue. An example for the latter case can be found below.

First of all, a new CSV file is created which stores a list of coordinates. It has the following content:

ra,dec
267.22029,-20.35869
274.83971,-25.42714
275.92229,-30.36572
283.26621,-8.70756
306.01575,33.86756
322.493,12.16703

Next, the xMatch service will be used to find cross matches between the uploaded file and a VizieR catalogue. The parameters cat1 and cat2 define the catalogues where one of them may point to a local file (in this example, the CSV file is stored in '/tmp/pos_list.csv'). max_distance denotes the maximum distance in arcsec to look for counterparts; it is used here to limit the number of rows in the resulting table for demonstration purposes. Finally, colRa1 and colDec1 are used to denote the column names in the input file.

>>> from astropy import units as u
>>> from astroquery.xmatch import XMatch
>>> table = XMatch.query(cat1=open('/tmp/pos_list.csv'),
...                      cat2='vizier:II/246/out',
...                      max_distance=5 * u.arcsec, colRA1='ra',
...                      colDec1='dec')
>>> type(table)
<class 'astropy.table.table.Table'>
>>> print table
angDist      ra       dec         2MASS       ... Qfl Rfl  X   MeasureJD
-------- --------- --------- ---------------- ... --- --- --- ------------
1.352044 267.22029 -20.35869 17485281-2021323 ... EEU 226   2 2450950.8609
1.578188 267.22029 -20.35869 17485288-2021328 ... UUB 662   2 2450950.8609
3.699368 267.22029 -20.35869 17485264-2021294 ... UUB 662   2 2450950.8609
3.822922 267.22029 -20.35869 17485299-2021279 ... EBA 222   2 2450950.8609
4.576677 267.22029 -20.35869 17485255-2021326 ... CEU 226   2 2450950.8609
0.219609 274.83971 -25.42714 18192154-2525377 ... AAA 211   0 2451407.5033
1.633225 275.92229 -30.36572 18234133-3021582 ... EEE 222   2 2451021.7212
0.536998 283.26621  -8.70756 18530390-0842276 ... AAA 222   0 2451301.7945
1.178542 306.01575  33.86756 20240382+3352021 ... AAA 222   0 2450948.9708
0.853178   322.493  12.16703 21295836+1210007 ... EEA 222   0 2451080.6935
 4.50395   322.493  12.16703 21295861+1210023 ... EEE 222   0 2451080.6935

Reference/API

astroquery.xmatch Package

Classes

XMatchClass()
Conf Configuration parameters for astroquery.xmatch.