def field_points_to_compare(side_length, penumbra_width, rotation,
    num_points_across_penumbra=11, num_points_across_field_edge=51):
    """Points for comparison for field centre finding.

    Creates two sets of points centred around the origin designed
    so that by shifting the points by the centre to be tested and then
    comparing a function looked up at one set, to that at the other set
    a profile centre can be found.
    """
