Recent advances in large format detector arrays and holographic diffraction gratings have made possible the development of imaging spectrographs with high sensitivity and resolution, ideally suited for space-based remote sensing of earth resources. An optical system composed of dual spectrographs and a common fore-optic has been designed for the visible-near infrared (VNIR) and shortwave bands with 10-nm spectral resolution, providing 30-meter ground resolution from an altitude of 605 km. The spectrograph designs are based on a modified Offner 1-X relay with spherical mirrors and a convex spherical holographic grating for the secondary mirror. The fore-optic is a three-mirror anastigmatic telescope with a 360-mm focal length to match the pixel pitch of the respective 1024x1024 visible silicon CCD and SWIR HgCdTe FPAS. The primary advantages of this design are the relatively low f-number (173), large fiat field (18mm), and low distortion. Preliminary performance results ofa VNIR testbed grating and spectrograph are presented and compared to the design predictions.
For many scanning applications, the scan line straightness, linearity, and influence of mechanical wobble and eccentricity on a scanner's performance are important. Some different 2-D scanners are compared on a common specification of a total resolution of 6.25 million pixels. According to our analyses, the polygon-mirror system offers the best straightness but is the most sensitive to mechanical wobble and eccentricity.1 Different holographic 2-D scanners are also investigated, and their performances depends strongly on design parameters and on the configurations. The scan line straightness of the grating scanner’s system is comparable with that of a polygon mirror.2 The scan line linearity of the scanner with an auxiliary reflector3 is the best one. The simplicity and therefore the expected reliability of holographic scanners may influence people to prefer holographic scanners. However many technical problems remain to be solved before holographic scanners can be widely used.
Since high resolution recording materials are not readily available in the infrared and laser diodes do not have a sufficient coherent length, it is difficult to make an infrared hologram for the LD directly. An infrared hologram for a simple object can be easily calculated by a computer. If the hologram is large and the spatial frequency is high (say, more than 1000 lines/mm), it is still an elaborated process to generate these holograms by an electron beam machine. Also because of the long writing time required for the hologram, the electron beam and the translation stage stability must be carefully maintained. The former is often affected by the static charge built up and the latter by temperature changes and mechanical vibrations. In this paper we describe a different approach to make the hologram. We first create a CGH for a visible wavelength for which high resolution recording media are available. The CGH is then used as the object to make the final IR hologram using visible wavelength. The CGH is designed in such a way that the wavefront distortion is precompensated. The CGH is drawn by a personal computer system which includes an MP 1000-01 plotter.
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