Abstract: Couples an additional optical system to an existing system having viewing optics, including an eyepiece. A pivoted mirror reflects the image scene from the imaging optics of the existing system to the viewing optics. The added system transmits a second image scene through a Pechan prism to the mirror, which when pivoted to a second position, reflects it to the viewing optics. The prism has internal reflecting paths to increase the optical path length of the added system to equal the path length of the existing system so that both image scenes are in focus at the eyepiece. An image intensifier may be employed between the prism and the imaging optics of the added system to brighten the image, and a reticle may be placed at the output of the intensifier to provide for boresighting and alignment of the added system.