def METHOD_NAME(self):
    """
    Covariance matrix of all pixel values in 2d numpy array (only diagonal component)
    The covariance matrix is estimated from the data.
    WARNING: For low count statistics, the noise in the data may lead to biased estimates of the covariance matrix.
    :return: covariance matrix of all pixel values in 2d numpy array (only diagonal component).
    """
    if not hasattr(self, "_C_D"):
        if self._noise_map is not None:
            self._C_D = self._noise_map**2
        else:
            self._C_D = covariance_matrix(
                self._data,
                self.background_rms,
                self.exposure_map,
                self._gradient_boost_factor,
            )
    return self._C_D