code: def background_noise(self):
          
          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
docs: Gaussian sigma of noise level per pixel (in counts per second)
  
          :return: sqrt(variance) of background noise level