def __init__(self, lens_galaxies, image_plane_positions, cosmology=cosmo.Planck15):
image_plane = pl.PlanePositions(redshift=lens_galaxies[<NUM_LIT:0>].redshift, galaxies=lens_galaxies,<EOL>positions=image_plane_positions, compute_deflections=True, cosmology=cosmology)<EOL>source_plane_positions = image_plane.trace_to_next_plane()<EOL>source_plane = pl.PlanePositions(redshift=None, galaxies=None, positions=source_plane_positions,<EOL>compute_deflections=False, cosmology=cosmology)<EOL>super(TracerImageSourcePlanesPositions, self).__init__(planes=[image_plane, source_plane], cosmology=cosmology)<EOL>
Positional ray-tracer for a lens system with two planes, an image-plane and source-plane (source-plane \
        galaxies are not input for the positional ray-tracer, as it is only the proximity that image_plane_positions \
        trace to within one another that needs to be computed).

        By default, this has no associated cosmology, thus all calculations are performed in arc seconds and galaxies \
        do not need input redshifts. If a cosmology is supplied, the plane's angular diameter distances, \
        conversion factors, etc. are used to provide quantities in kpc.

        Parameters
        ----------
        lens_galaxies : [Galaxy]
            The list of lens galaxies in the image-plane.
        image_plane_positions : [[[]]]
            The (y,x) arc-second coordinates of image-plane pixels which (are expected to) mappers to the same \
            location(s) in the source-plane.
        cosmology : astropy.cosmology.Planck15
            The cosmology of the ray-tracing calculation.