code: def _subtractSky(image,skyValue,memmap=False):
      
      try:
          np.subtract(image.data,skyValue,image.data)
  
      except IOError:
          print("Unable to perform sky subtraction on data array")
          raise IOError
docs: subtract the given sky value from each the data array
      that has been passed. image is a fits object that
      contains the data and header for one image extension