docs: def calculate_focus(self, reading):
          """
          Determines what the focal point of the downloaded image should be.
  
          Returns:
            focal_point: (x, y)
              The location of the source in the middle observation, in the
              coordinate system of the current source reading.
          """
          middle_index = len(self.source.get_readings()) // 2
          middle_reading = self.source.get_reading(middle_index)
          return self.convert_source_location(middle_reading, reading)