Grob-Feintrieb fuer den Teilkreis eines Winkelmessinstrumentes

摘要

The invention is directed to an angle-measuring instrument such as a theodolite, tacheometer or the like having a coarse-fine drive arrangement for presetting a circle part of a graduated circle to a predetermined read-out value. In a preferred embodiment, a coaxial coarse-fine drive arrangement is actuated by a single control knob which is part of an adjustable drive which can be operated in two positions. In the first position, the adjustable drive functions as a coarse drive and includes a spur gear that meshes with a gear on the carrier of the graduated circle to permit adjusting the circle part to obtain a coarse setting thereof. When the control knob and adjustable drive are moved into the second position, the drive becomes completely disengaged from the carrier and functions as a fine drive that operates on a plane-parallel plate pivotally mounted in the read-out optical transmission path of the instrument to finely adjust the light rays passing therealong. Accordingly, the fine drive does not cause any movement of the circle part so that the coarse and fine drives do not interfere with each other mechanically.