code: def get_background_rms(self):
          
          # TODO: return a proper background RMS ignoring the sources
          # This is an approximate method suggested by PaulH.
          # I have no idea where this magic 1.34896 number comes from...
          if self._rms is None:
              # Get the pixels values without the NaNs
              data = numpy.extract(self.hdu.data > -9999999, self.hdu.data)
              p25 = scipy.stats.scoreatpercentile(data, 25)
              p75 = scipy.stats.scoreatpercentile(data, 75)
              iqr = p75 - p25
              self._rms = iqr / 1.34896
          return self._rms
docs: Calculate the rms of the image. The rms is calculated from the interqurtile range (IQR), to
          reduce bias from source pixels.
  
          Returns
          -------
          rms : float
              The image rms.
  
          Notes
          -----
          The rms value is cached after first calculation.