Optical System and Design

• Published in: Hyperspectral Satellites and System Design Vol. 1; pp. 223 - 298
• Main Author: Qian, Shen-En
• Format: Book Chapter
• Language: English
• Published: United Kingdom CRC Press 2020; Taylor & Francis Group
• Edition: 1
• Subjects: MTF Value; Spectral Range; Tertiary Mirror; VNIR Channel; Panchromatic Camera; Entrance Pupil; Telescope Design; Primary Mirror; Refracting Telescope; Stray Light; Keystone Distortion; Entrance Pupil Diameter; Prism Spectrometer; Polarization Sensitivity; Spectral Sampling; Polychromatic Diffraction MTF; HySI; SSI; Secondary Mirror; Imaging Spectrometer; Detector Array; Spectrometer Design; Grating Spectrometer; Optical Layout; Nyquist Limit
• ISBN: 9780367217907; 0367217902
• DOI: 10.1201/9780429266201-5

This chapter provides an “A to Z” description of the optical system and design of a spaceborne hyperspectral instrument. It starts with an overview of the optical system including (1) instrument front-end functions, (2) fore-optics, (3) spectrometers and focal plane arrays, (4) readout electronics, and (5) instrument structure. It then describes considerations of optical design related to the telescope and the spectrometer. Three kinds of telescopes are described followed by detailed design examples. Given the fact that all the spaceborne hyperspectral sensors are based on slit-scan spectrometers that use 2D detector arrays and operate in the pushbroom mode, this chapter describes the designs of various slit-scan spectrometers. After describing the principle of four types of generic grating-based spectrometers and different types of prism spectrometers, the chapter focuses on the description of Offner and Dyson forms of spectrometers and their performance comparison. Six design cases of slit-scan spectrometers for visible and near-infrared (VNIR) and shortwave infrared (SWIR) wavelength regions are demonstrated and compared. The chapter, then, describes the design of slit assembly and beam splitters, which are two common approaches to distributing the light collected by the fore-optics to the VNIR and SWIR spectrometers. The chapter also describes different designs of a depolarizer to significantly reduce polarization sensitivity of the optical system. Finally, the chapter provides detailed design examples of three imaging spectrometer systems. This chapter provides an “A to Z” description of the optical system and design of a spaceborne hyperspectral instrument. It begins with an overview of the optical system including instrument front-end functions, fore-optics, spectrometers and focal plane arrays, readout electronics, and instrument structure. External baffles are incorporated in the payload to prevent light from sources other than the scene from illuminating the pointing mirror. A pointing mirror which is controlled by the instrument controller is used to transfer the scene on Earth or a planet to the fore-optics. The function of the fore-optics is to use a telescope to image the ground scene onto the slit assembly of each spectrometer. The slit assembly should facilitate packaging of the spectrometers and the panchromatic camera. The slit assembly may have multiple slits for each spectrometer. The baffles in the fore-optics attenuate the out-of-field stray light and prevent optical cross talk within and between the spectrometers and the panchromatic camera.