code: def potential_from_grid(self, grid):
          
          eta = self.grid_to_elliptical_radii(grid)
          return 2.0 * self.einstein_radius_rescaled * eta
docs: Calculate the potential at a given set of arc-second gridded coordinates.
  
          Parameters
          ----------
          grid : grids.RegularGrid
              The grid of (y,x) arc-second coordinates the deflection angles are computed on.