ABSTRACT

The quality of the imagery reaching the end user from an Earth observation camera depends primarily on the imaging optics, the focal plane detector array and associated electronics, and the processing (applying corrections, map projection, etc.) of the raw data. The function of imaging optics is to transfer the radiance from the object space to image plane. In doing so, the optical system should ensure geometric and radiometric fidelity of the image, that is, the ability to maintain the shape, orientation, relative dimension, and relative radiance values as in the object space. The collecting optics for any imaging system is chosen from one of three basic configurations, which are (1) all refractive; (2) all reflective; and (3) a combination of reflective and refractive, called catadioptric. All three configurations have been used in spaceborne Earth imaging cameras. The selection of a particular configuration for a mission primarily depends on (1) the spectral range to be covered, (2) total field of view (FOV), and (3) aperture size. A variety of optical design software is available to optimize the configuration of imaging optics. A designer generally tries to get the best image quality within the FOV of interest and at the same time does a tolerance analysis, that is, how the image performance varies with changes in mechanical parameters and temperature change, to ensure practical realizability of such a system. For a spaceborne optical system, the design and system realization should take into account a wide range of environmental loads during fabrication, launch, and in space. These include mechanical and thermal stresses, and the radiation environment in space. It is required to pay utmost attention to optomechanical design to ensure that the shape and position of the functional elements of the system are unaffected under these loads, so that the system performance requirements are satisfied. The stringent mechanical and optical stability requirements to ensure high-quality imagery make the design and realization of the spaceborne imaging system a real challenge. In this chapter, we shall discuss some of these aspects.