A compact SAL seeker for a projectile guidance system comprises an optical sub-assembly to focus incident electro-magnetic radiation (EMR) such that target bearing in object space is mapped to a spatial irradiance in image space and a detector sub-assembly to generate at least one guidance signal in response to the position of the centroid of the focused EMR. The optical sub-assembly includes an integrated filter stack of a primary optical element, a spreader, a filter and secondary optical element in which at least one and typically both of the spreader and filter are immersed within the optical media of the stack. The detector sub-assembly may include a field lens in which the detector is immersed. Immersion reduces the number of "air-to-glass" interfaces, hence improves optical throughput. The detector sub-assembly may be integrally formed with a mounting bracket adapted to mate with mounting features on the optical sub-assembly to provide a self-aligned seeker.