A multipole unit normally consists of a support tube, a threaded ring, a plurality of magnetic rings, a plurality of spacer rings and a spring ring. The reflective layer is characterized in that it attenuates electromagnetic radiation in a 200Hz to 10 Ghz range by more than 10Db, that it is watertight and permeable to water vapor, in addition to being resistant to the elements and designed to produce a potential compensation. When a phase error occurs, an error is compensated using a phase compensation control method of closed loop type through one of the two paths and an image is extracted by capturing an image of object in a state that the image of object is shifted by the set phase value when a phase error is compensated.