A guidance system, comprising: a non-imaging optical system (230, 404, 500, 600, 704, 906) to focus incident electro-magnetic radiation, EMR, in a detection band including at least one wavelength in a range of 0.35 microns to 15 microns, comprising: a primary optical element (420, 508, 602, 712) having a first side and a second side, wherein the first side is a first lens element configured to focus incident EMR; a spreader (424, 504, 608, 714) configured to spatially homogenize EMR transmitted through the optical system; a filter (426, 506, 604, 718) configured to reject EMR including wavelengths outside the detection band; and a secondary optical element (428, 510, 606, 720) having a first side and a second side, wherein the second side is a second lens element configured to control the aberration characteristics of the EMR transmitted through the optical system and provide additional focusing; said primary optical element, spreader, filter and secondary optical element optically bonded together in an integrated filter stack having an input aperture, said integrated stack having only a first air-to-glass interface at the first side of the primary optical element (420, 508, 602, 712) and a second air-to-glass interface at the second side of the secondary optical element (428, 510, 606, 720), and said guidance system further comprising an irradiance detector coupled to the optical system and configured to generate at least one guidance signal in response to the spatial displacement of the centroid of the focused incident EMR in the detection band. a non-imaging optical system (230, 404, 500, 600, 704, 906) to focus incident electro-magnetic radiation, EMR, in a detection band including at least one wavelength in a range of 0.35 microns to 15 microns, comprising: a primary optical element (420, 508, 602, 712) having a first side and a second side, wherein the first side is a first lens element configured to focus incident EMR; a spreader (424, 504, 608, 714) configured to spatially homogenize EMR transmitted through the optical system; a filter (426, 506, 604, 718) configured to reject EMR including wavelengths outside the detection band; and a secondary optical element (428, 510, 606, 720) having a first side and a second side, wherein the second side is a second lens element configured to control the aberration characteristics of the EMR transmitted through the optical system and provide additional focusing; said primary optical element, spreader, filter and secondary optical element optically bonded together in an integrated filter stack having an input aperture, said integrated stack having only a first air-to-glass interface at the first side of the primary optical element (420, 508, 602, 712) and a second air-to-glass interface at the second side of the secondary optical element (428, 510, 606, 720), and said guidance system further comprising a primary optical element (420, 508, 602, 712) having a first side and a second side, wherein the first side is a first lens element configured to focus incident EMR; a spreader (424, 504, 608, 714) configured to spatially homogenize EMR transmitted through the optical system; a filter (426, 506, 604, 718) configured to reject EMR including wavelengths outside the detection band; and a secondary optical element (428, 510, 606, 720) having a first side and a second side, wherein the second side is a second lens element configured to control the aberration characteristics of the EMR transmitted through the optical system and provide additional focusing; said primary optical element, spreader, filter and secondary optical element optically bonded together in an integrated filter stack having an input aperture, said integrated stack having only a first air-to-glass interface at the first side of the primary optical element (420, 508, 602, 712) and a second air-to-glass interface at the second side of the secondary optical element (428, 510, 606, 720), and said guidance system further comprising an irradiance detector coupled to the optical system and configured to generate at least one guidance signal in response to the spatial displacement of the centroid of the focused incident EMR in the detection band.