def _mass(self,R,z=0.,t=0.):
        """
        NAME:
           _mass
        PURPOSE:
           calculate the mass out to a given radius
        INPUT:
           R - radius at which to return the enclosed mass
           z - (don't specify this) vertical height
        OUTPUT:
           mass in natural units
        HISTORY:
           2014-01-29 - Written - Bovy (IAS)
        """
        if z is None: r= R
        else: r= numpy.sqrt(R**2.+z**2.)
        return numpy.log(1+r/self.a)-r/self.a/(1.+r/self.a)