Performance of optical surfaces in space.

The Multispectral Scanner carried by the ERTS-1 satellite (launched on July 23, 1972) is a very high resolution line scanning imaging radiometer operating in the visible and near infrared spectral regions. The instrument contains optical surfaces that are exposed to both direct and reflected sunlight, and the reflectivity and transmission of some of these surfaces have decreased in orbit. Details of the optical system are explained, and data on the degradation are reviewed as a function of both spectral interval and time in orbit. The history of the scanner during testing (when hydrocarbons from an external source may have been deposited on the optical surfaces) is also examined, and a mechanism is postulated for the observed degradation in orbit.