Root Cause Determination of On-orbit Degradation of the VIIRS Rotating Telescope Assembly

The Visible Infrared Imaging Radiometric Suite (VIIRS) is a sensor onboard the recently launched Suomi National Polar-orbiting Partnership (NPP) spacecraft. Shortly after launch, VIIRS was found to exhibit a pronounced decrease in the optical throughput of several bands, with the near-infrared bands being more affected than those in the visible. The anomaly investigation team performed several experiments that concluded that the primary source of the degradation was a throughput loss in the VIIRS rotating telescope assembly, most likely caused by ultraviolet light illumination. This paper will discuss the laboratory investigation that determined the root cause of the telescope degradation: UV photo-darkening of a tungsten oxide contaminant film that was inadvertently deposited on top of the telescope mirrors during the manufacturing process. We will present data from experiments conducted on witness mirrors manufactured along with the telescope, as well as other mirrors of the same type that were not contaminated.