PURPOSE: To preclude contamination from outside of an optical apparatus to be mounted on a spacecraft navigation apparatus without additional provision of shutter, etc., and provide the possibility of repeating this contamination preventive processing in a plurality of cycles.

CONSTITUTION: A mirror rotating member 2 rotates a plane mirror 1 for use in normal pointing or scanning in an amount larger than in usual service condition. If any contaminant externally intrudes through a hood 3, it 3 is shut by rotating the plane mirror 1 to the position indicated by the broken line. Thereby the contaminant is shut off by the plane mirror 1 and a baffle 6, and the reflex mirror of the plane mirror 1 and a condensation optical system 5 are prevented from contamination.