code: def getdarkcurrent(self,extver):
          
          darkcurrent=0.
          try:
              darkcurrent = self._image[self.scienceExt,extver].header['MEANDARK']
          except:
              str =  "#############################################\n"
              str += "#                                           #\n"
              str += "# Error:                                    #\n"
              str += "#   Cannot find the value for 'MEANDARK'    #\n"
              str += "#   in the image header.  ACS input images  #\n"
              str += "#   are expected to have this header        #\n"
              str += "#   keyword.                                #\n"
              str += "#                                           #\n"
              str += "# Error occured in the ACSInputImage class  #\n"
              str += "#                                           #\n"
              str += "#############################################\n"
              raise ValueError(str)
  
          return darkcurrent
docs: Return the dark current for the ACS 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 value for the ACS detector in **units of electrons**.