Abstract: The invention is directed to an optical instrument comprising a housing and at least one optical assembly enclosed by said housing and mounted on a stage plate. The stage plate is supported on the base plate with the aid of supporting means which guarantees on the one hand a precise fixing of the stage plate compared to the base plate and thus the housing, and on the other hand allows the base plate to distort by whatever attacks without a transfer to the stage plate. There are three supporting structures of different kinds which fix three different points in a special way. Upon the disconnection of the third supporting structure the stage plate can be lifted like a lid around an axis which is the middle axis of the first and second supporting structure.