code: def getdarkcurrent(self,exten):
          
          darkrate = 0.005 # electrons / s
          if self.proc_unit == 'native':
              darkrate = darkrate / self.getGain(exten) #count/s
  
          try:
              chip = self._image[0]
              darkcurrent = chip.header['DARKTIME'] * darkrate
  
          except:
              msg =  "#############################################\n"
              msg += "#                                           #\n"
              msg += "# Error:                                    #\n"
              msg += "#   Cannot find the value for 'DARKTIME'    #\n"
              msg += "#   in the image header.  WFPC2 input       #\n"
              msg += "#   images are expected to have this header #\n"
              msg += "#   keyword.                                #\n"
              msg += "#                                           #\n"
              msg += "# Error occured in the WFPC2InputImage class#\n"
              msg += "#                                           #\n"
              msg += "#############################################\n"
              raise ValueError(msg)
  
          return darkcurrent
docs: Return the dark current for the WFPC2 detector.  This value
          will be contained within an instrument specific keyword.
          The value in the image header will be converted to units
          of electrons.
  
          Returns
          -------
          darkcurrent : float
              Dark current for the WFPC3 detector in **units of counts/electrons**.