Electroformed off-axis toroidal aspheric three-mirror anastigmat multispectral imaging system

A lightweight, single-aperture, and multispectral imaging system operating from visible to long wavelength infrared has been manufactured and tested based on an innovative three-mirror anastigmat optical design. The complex off-axis aspherical toroidal mirrors of the optics have been obtained by electroforming replication from masters having a shape opposite to the mirror shape and manufactured as stand-alone parts by using five-axis single-point diamond turning. The technology is extendible to full free-form optics without any process modification enabling affordability of complex optics since multiple identical copies of the mirrors can be produced from each master. Moreover, thin (similar to 1 mm) electroformed mirrors keep the imaging system mass less than 3 kg. With an effective focal length of 136 mm, the system is suitable for airborne surveillance applications and provides a full aperture F/ of 1.4, a field of view of 4.3 x 3.1 deg, and a nominal contrast better than 75% in the visible waveband at 25 cycles/mm. A beam splitter can be accommodated to provide two separated channels for two or more spectral wavebands. Stray light and thermostructural design has been performed to comply with airborne applications. Experimental results demonstrate the feasibility of the technology, although process improvements are required to reach the extremely demanding theoretical performance of the optical design. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)