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- Astropy v1.1.1 »
- Astronomical Coordinate Systems (`astropy.coordinates`) »
- match_coordinates_sky

# match_coordinates_sky

`astropy.coordinates.``match_coordinates_sky`(matchcoord, catalogcoord, nthneighbor=1, storekdtree=u'_kdtree_sky')

  Finds the nearest on-sky matches of a coordinate or coordinates in a set of catalog coordinates.

  This finds the on-sky closest neighbor, which is only different from the 3-dimensional match if `distance` is set in either `matchcoord` or `catalogcoord`.

  Parameters:

  matchcoord : `BaseCoordinateFrame` or `SkyCoord`

  > The coordinate(s) to match to the catalog.

  catalogcoord : `BaseCoordinateFrame` or `SkyCoord`

  > The base catalog in which to search for matches. Typically this will be a coordinate object that is an array (i.e., `catalogcoord.isscalar == False`)

  nthneighbor : int, optional

  > Which closest neighbor to search for. Typically `1` is desired here, as that is correct for matching one set of coordinates to another. The next likely use case is `2`, for matching a coordinate catalog against itself (`1` is inappropriate because each point will find itself as the closest match).

  storekdtree : bool or str, optional

  > If a string, will store the KD-Tree used for the computation in the `catalogcoord` as an attribute in `catalogcoord` with the provided name. This dramatically speeds up subsequent calls with the same catalog. If False, the KD-Tree is discarded after use.

  Returns:

  idx : integer array

  > Indices into `catalogcoord` to get the matched points for each `matchcoord`. Shape matches `matchcoord`.

  sep2d : `Angle`

  > The on-sky separation between the closest match for each `matchcoord` and the `matchcoord`. Shape matches `matchcoord`.

  dist3d : `Quantity`

  > The 3D distance between the closest match for each `matchcoord` and the `matchcoord`. Shape matches `matchcoord`. If either `matchcoord` or `catalogcoord` don’t have a distance, this is the 3D distance on the unit sphere, rather than a true distance.

  Notes

  This function requires SciPy to be installed or it will fail.