Raycast-based calibration techniques are described for determining calibration parameters associated with components of a head mounted display (HMD) of an augmented reality (AR) system having one or more off-axis reflective combiners. In an example, a system comprises an image capture device and a processor executing a calibration engine. The calibration engine is configured to determine correspondences between target points and camera pixels based on images of the target acquired through an optical system, the optical system including optical surfaces and an optical combiner. Each optical surface is defined by a difference of optical index on opposing sides of the surface. At least one calibration parameter for the optical system is determined by mapping rays from each camera pixel to each target point via raytracing through the optical system, the raytracing being based on the index differences, shapes, and positions of the optical surfaces relative to the one or more cameras.