code: def getReadNoise(self, exten):
          
          rn = self._image[exten]._rdnoise
          if self.proc_unit == 'native':
              rn = self._rdnoise / self.getGain(exten)
          return rn
docs: Method for returning the readnoise of a detector (in counts).
  
          Returns
          -------
          readnoise : float
              The readnoise of the detector in **units of counts/electrons**.