A radiation therapy system includes an X-ray imaging system that is configured with a combined and simplified filter and collimator positioning mechanism. In addition, an X-ray imager of the RT system is only positioned at a few discrete locations within a plane that is a fixed distance from the imaging X-ray source when generating X-ray images. As a result, for each of these discrete imaging positions, the simplified filter and collimator positioning mechanism positions a specific collimator-filter combination in a specific location between the X-ray source and the imager.