code: def evaluate(self, x, y, flux, x_0, y_0, sigma):
          
  
          return (flux / 4 *
                  ((self._erf((x - x_0 + 0.5) / (np.sqrt(2) * sigma)) -
                    self._erf((x - x_0 - 0.5) / (np.sqrt(2) * sigma))) *
                   (self._erf((y - y_0 + 0.5) / (np.sqrt(2) * sigma)) -
                    self._erf((y - y_0 - 0.5) / (np.sqrt(2) * sigma)))))
docs: Model function Gaussian PSF model.