spheregroup

pydl.pydlutils.spheregroup.spheregroup(ra, dec, linklength, chunksize=None)

Perform friends-of-friends grouping given ra/dec coordinates.

Parameters:

ra, dec : numpy.ndarray

Arrays of coordinates to group in decimal degrees.

linklength : float

Linking length for the groups in decimal degrees.

chunksize : float, optional

Break up the sphere into chunks of this size in decimal degrees.

Returns:

tuple()

A tuple containing the group number of each object, the multiplicity of each group, the first member of each group, and the next member of the group for each object.

Raises:

:exc:`PydlutilsException`

If the array of coordinates only contains one point.

Notes

It is important that chunksize >= 4 * linklength. This is enforced.

Warning

Behavior at the poles is not well tested.