code: def _assignSignature(self, chip):
          
          sci_chip = self._image[self.scienceExt,chip]
          ny=sci_chip._naxis1
          nx=sci_chip._naxis2
          detnum = sci_chip.detnum
          instr=self._instrument
  
          sig=(instr+self._detector,(nx,ny),int(detnum)) #signature is a tuple
          sci_chip.signature=sig
docs: Assign a unique signature for the image based
             on the  instrument, detector, chip, and size
             this will be used to uniquely identify the appropriate
             static mask for the image.
  
             This also records the filename for the static mask to the outputNames dictionary.