def background_noise(self):
        """
        Gaussian sigma of noise level per pixel (in counts per second)

        :return: sqrt(variance) of background noise level
        """
        if self._background_noise is None:
            return data_util.bkg_noise(self.read_noise, self._exposure_time, self.sky_brightness, self.pixel_scale,
                                   num_exposures=self._num_exposures)
        else:
            return self._background_noise