code: def function(self, x, y, n_sersic, R_sersic, k_eff, e1, e2, center_x=0, center_y=0):
          
          phi_G, q = param_util.ellipticity2phi_q(e1, e2)
          x_, y_ = self._coord_transf(x, y, q, phi_G, center_x, center_y)
          f_ = self.sersic.function(x_, y_, n_sersic, R_sersic, k_eff)
          return f_
docs: returns Gaussian