A millimeter-wave optical imaging system including an imaging detector located at a focal plane of the optical imaging system, the imaging detector being responsive to electromagnetic radiation in wavelength range of approximately 5-50 millimeters, an immersion lens directly coupled to the imaging detector and configured to focus the electromagnetic radiation onto the imaging detector, wherein the focal plane is located on a planar surface of the immersion lens and the imaging detector is directly coupled to the planar surface, a positive power primary mirror configured to reflect the electromagnetic radiation towards the immersion lens, and one of a Fresnel lens or a diffraction grating configured to receive and direct the electromagnetic radiation towards the primary mirror.