This paper presents a new idea that designing a high speed telescope system with freeform surface. The system is
designed based on the relation that between the coefficient of Zernike polynomial and Seidel aberration, so that we can
control the aberrations of every field of view and alternation optimize with a specific aim. The primary mirror is
freeform surface, and others are conicoid. It is telecentric in the image space. Its F-number is 2.0, its focal length
360mm, and its field of view 4 degrees. This system is compact and diffraction-limited. It's unique in that the chief ray
on the 0 field of view traces along the line between every vertex of elements, so that its assembly difficulty is greatly
reduced, the working distance is long and good to place the color filters.
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.
The optimized design, alignment and experimental results of a compact convex grating hyper-spectral imager with high
fidelity are reported and evaluated. The imager works in visible wavelength range from 0.4 to 0.78 μm. The numerical
aperture of system is 0.2, and the entrance slit images with -1 magnification and linearly dispersed into 7.5mm in width.
In addition, the spectral sample resolution is 0.76nm/pixel with negligible distortion. In order to get good performance
and facilitate alignment, the optical system is both imagery and objective telecentric. The efficiency of the convex
grating can up to about 40 percents by ion-beam etching. The size is 190mm×180mm×90mm and the mass is less than
1kg. The light weighted compact system is portable, and it is feasible in remote sensing.