def pupv_to_vRvz(pu,pv,u,v,delta=1.,oblate=False):
    """
    NAME:

       pupv_to_vRvz

    PURPOSE:

       calculate cylindrical vR and vz from momenta in prolate or oblate confocal u and v coordinates for a given focal length delta

    INPUT:

       pu - u momentum

       pv - v momentum

       u - u coordinate

       v - v coordinate

       delta= focus

       oblate= (False) if True, compute oblate confocal coordinates instead of prolate


    OUTPUT:

       (vR,vz)

    HISTORY:

       2017-12-04 - Written - Bovy (UofT)

    """
    if oblate:
        denom= delta*(sc.sinh(u)**2.+sc.cos(v)**2.)
        vR= (pu*sc.sinh(u)*sc.sin(v)+pv*sc.cosh(u)*sc.cos(v))/denom
        vz= (pu*sc.cosh(u)*sc.cos(v)-pv*sc.sinh(u)*sc.sin(v))/denom
    else:
        denom= delta*(sc.sinh(u)**2.+sc.sin(v)**2.)
        vR= (pu*sc.cosh(u)*sc.sin(v)+pv*sc.sinh(u)*sc.cos(v))/denom
        vz= (pu*sc.sinh(u)*sc.cos(v)-pv*sc.cosh(u)*sc.sin(v))/denom
    return (vR,vz)
NAME:

       pupv_to_vRvz

    PURPOSE:

       calculate cylindrical vR and vz from momenta in prolate or oblate confocal u and v coordinates for a given focal length delta

    INPUT:

       pu - u momentum

       pv - v momentum

       u - u coordinate

       v - v coordinate

       delta= focus

       oblate= (False) if True, compute oblate confocal coordinates instead of prolate


    OUTPUT:

       (vR,vz)

    HISTORY:

       2017-12-04 - Written - Bovy (UofT)