With the enhancement of the spectral resolution and the extension of the spectral coverage of spectral imager, its
prefixing telescopic objective is expected to have a wide passband and large relative aperture. Off-axis three-mirror
optical system has such advantages of achromatic, wide spectral range, no-obstruction excellent aberration correction
and inactivity to its environmental temperature that it is ideal for prefixing telescopic system of hyper-spectral imagers
(HSI). In this paper the aberration characteristic of off-axis three-mirror systems is analyzed. According to the
requirement of HSI and through optimization design, an off-axis three-mirror system with high speed is presented. Its F/
number is 2.47, its focal length 360 mm, and its field of view 2.5 degrees. The designed three-mirror objective is
compact and diffraction-limited. Its uniqueness is that its secondary mirror is convex spherical so that its manufacture
and test difficulty and cost is greatly reduced.
A special kind of ultrathin lenses using annular folded optics is introduced, which has potential for compact imagers.
Their long focal lengths can be contained within a limited track. Light enters the lens through an outer annular aperture
and is focused by a series of concentric zone reflectors to the image plane. It is an approach to implement high sample
resolution imaging, large aperture and ultrathin characteristic at the same time. This optic has the advantages of both the
compactness of miniature cameras and the high imaging quality of large cameras. In this paper, the principle, structure,
and design method of this type of ultrathin lens is introduced. An optimized design and its performance evaluation are
reported. Such lens can be applied to the situation where the bulk and mass are so limited that conventional imaging
systems are unsuitable.