The invention relates to a collimator (1) for defining a beam of energetic rays (2) which is emitted from an essentially punctiform radiation source (3) and is oriented onto an object (1) to be treated. The collimator is especially used for the stereotactic conformation radiotherapy of tumors. The collimator (1) is embodied in such a way that an irregular object (4) can be scanned by rays (2, 2′) which are defined by an opening in the collimator. The invention also relates to a program for controlling the collimator. In order to define the contours (29, 29′, 29″) of the objects (4) to be irradiated in a simple but highly accurate manner, especially with a precise definition of the irradiation fields, the collimator comprises a plurality of different sized openings (5, 5′ 5″). One of the openings (5, 5′, 5″) can be selectively displaced in a polydirectional manner on a strip (6) having a spherical surface, and the central axis (7) thereof is oriented towards the radiation source (3). The other collimator openings (5′, 5″) are shielded form the rays (2). A control system (9) acts on the drives (23, 24, 25) of a drive device (8) in such a way that large openings (5, 5′) are used to scan large irradiation surfaces (26) of the object to be treated (4), and small openings (5′, 5″) are used for precise definition at the edge of the irradiation surfaces (26) of the object to be treated (4), especially in the event of irregular contours (29, 29′, 29″).