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.
Imaging spectrometers can provide imagery and spectrum information of objects and form so-called three-dimensional
spectral imagery, two spatial and one spectral dimension. Most of imaging spectrometers use conventional spectroscopic
elements or systems, such as reflective diffraction gratings, prisms, filters, spatial modulated interferometers, and so on.
Here a special imaging spectrometer which is based on a novel cemented Prism-Grating-Prism (PGP) is reported. Its
spectroscopic element PGP consists of two prisms and a holographic transmission volume grating, which is cemented
between these prisms. The two prisms mainly function as beam deviation, the grating as a disperser. In addition to the
high light efficiency of the volume gratings that is required for high spectral resolution, the cementing difficulty when
surface relief gratings are used can be avoided due to its voluminal characteristic. The PGP imaging spectrometer has
advantages of direct vision, dispersion uniform, compactness, low cost, and facility to be used. The principle, structure,
and optimized design of the PGP imaging spectrometer are given in detail. Its front collimation optics and rear focusing
lenses are same so as to reduce its cost further. The spectral coverage, resolution, and track length of the designed system
are respectively visible light from 400nm to 800nm, 1.6nm/pixel, and 85mm. From its performance evaluation, it is
shown that the PGP imaging spectrometer has the potentiality to be used in microscopic hyperspectral imagers and
hyperspectral imaging remote sensors.