Radiance Field-of-View Specifications, Chapter 4.

摘要

The field of view of EPIC Multitarget Radiance (radiometer) (EML) (radiance) instruments used for OSPREy applications, specified here in terms of the full view angle (FVA), must be optimized to accurately measure a variety of different radiometric sources: the direct solar irradiance when pointing the instrument at the Sun; the total radiance emanating from the sea surface, the sky radiance, and the radiance of a calibration plaque. The optimum FVA was determined based on the following considerations: (a) the FVA must encompass the entire solar and lunar disks when performing direct measurements under all observation conditions; (b) compliance with recommendations for sun photometers issued by the World Meteorological Organization (WMO); (c) an FVA large enough to allow accurate measurements of low-intensity sources (e.g., calibration plaques, the sea surface, and the lunar disk); (d) an FVA small enough to permit statistical filtering of surface and capillary wave effects when measuring the sea surface; and (e) a reasonable overall instrument length. The latter are needed to ensure accurate pointing and control of the sensor, particularly by the sun tracker; easier handling during servicing and the types of activities that take place during those events (e.g., monitoring the stability of the sensors with a portable source); and minimization of wind loading. After taking all considerations into account, it was concluded that an FVA of 2.5 deg combined with a slope angle of 0.7 deg is the optimal configuration. These geometric parameters were translated into a fore-optic design consisting of Gershun tubes with an overall length of 182 mm, a front aperture of 4 mm, and three internal baffles for stray-light suppression.