Wide-angle optical systems with moderate spectral resolution, for monitoring the oceans from low Earth orbit

摘要

Monitoring of the oceans from satellite requires frequent updates - preferably with global coverage in one day, excluding effects of cloud. This demands optics covering swath widths up to about 3000km, within which spatial resolution in the order 250m or less is desirable for observation of coastal zones. Wide field angles, typically around 90°, are needed for optical systems operating from altitudes that are typical for polar orbiting satellites. At least 15 resolved spectral bands are needed in the visible and near-IR regions, requiring wide-field imaging spectrometers or designs using multiple filters. Other constraints on optical design include requirements for radiometric calibration, precise spatial registration of spectral bands, good control on stray light, and insensitivity to polarisation. The paper describes two design forms in which a single optical channel provides the complete wide-angle field, with appropriate allowances for calibration etc. In the first design, a wide angle telescope is followed by a spectrometer and an area-array detector. The spectrometer uses refractive dispersion for stray light control, and gives good spatial and spectral registration. In the second design, spectral resolution is provided by a set of filters with linear array detectors. In-field separation of detectors is used to avoid a need for dichroic beam splitters or dispersive optics; spatial registration in this case demands exceptional distortion correction, that takes account of Earth curvature. Both designs provide an external entrance pupil for location of calibration hardware.