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)
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.