code: def noise_covariance(fit, dof=2, **kw):
      
      ev = fit.eigenvalues
  
      measurement_noise = ev[-1]/(fit.n-dof)
      return 4*ev*measurement_noise
docs: Covariance taking into account the 'noise covariance' of the data.
      This is technically more realistic for continuously sampled data.
      From Faber, 1993