Abstract: An X-ray simulator has an X-ray source and collimator connected to the end of a cantilevered horizontal arm mounted on a gantry that rotates about a horizontal axis and extends along a patient supporting table. The horizontal arm has an asymmetrical cross section that causes the arm and therefore the X-ray source and collimator to deflect by differing amounts as the gantry rotates. To compensate for the varying displacement of the X-ray target point and to adjust the position of the target point along the rotation axis of the gantry, the collimator is pivoted in one direction or the other in accordance with the amount of rotation of the gantry axis.