Correlation of an optical system degradation with contamination on a critical surface

The loss of sensitivity of a radiantly cooled radiometer during space operation was investigated. A quartz crystal microbalance (QCM), mounted in the proximity of the instrument filter lens and held at its temperature, measured the contaminant at the location during a vacuum test. The temperature cycling of the lens and the QCM verified the contamination of the lens as the cause of the deterioration. Data on the instrument performance in space and in the chamber allowed estimation of the rate of contaminant accretion on the lens in space service. That rate was used to estimate the contaminant partial pressure. The nature of material was inferred by comparison of the partial pressures and the saturated vapor pressures of materials at the temperature of the contaminated surface. These data, in conjunction with data on fields of view of the lens, are used to identify the sources of contamination.