def getReadNoise(self, exten):
        """
        Method for returning the readnoise of a detector (in counts).

        Returns
        -------
        readnoise : float
            The readnoise of the detector in **units of counts/electrons**.

        """
        rn = self._image[exten]._rdnoise
        if self.proc_unit == 'native':
            rn = self._rdnoise / self.getGain(exten)
        return rn