Device for reading graduated circles

The device can be used wherever graduated circles are optoelectronically read, with a large scanning field, particularly for absolute reading systems in theodolites, tachymeters and fine measuring instruments. The invention has the aim of constructing the scanning systems for reading graduated circles to be simpler and having fewer and smaller optical components, and with an oscillator system for rotating the deflection element. The task consists in creating a device for reading graduated circles having a small diameter and a larger scanning field for small, light-weight electronic tachymeters or theodolites, having simple digital angle measuring systems and with graduated-circle reading close to the graduated circle in the telecentric principal optical path of the optical imaging system. This object is achieved by the fact that the deflection element 6 is constructed as a rotatable roof prism, the axis of rotation R-R being arranged in the half of the graduated-circle radius r on the line connecting the graduated-circle centre M with the principal point of the objective of the imaging system and being aligned perpendicularly to the plane of the graduated circle and the angle of rotation phi of the roof prism 6 corresponding to the centring angle phi of the graduated circle 2.