A link-mechanism base is prepared within a yoke portion, and a link member is
prepared
in connection with the base within left and right supports. The link member connects
the upper part of the supports or the upper part of the yoke portion, and the link-mechanism
base. The displacement and angle of rotation of the upper part of the yoke portion
are measured by using these link-mechanism base and link member, thereby measuring
a change in the pointing direction of a reflector.
