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

The Visible Infrared Imaging Radiometer Suite (VIIRS) is a sensor onboard the recently launched Suomi 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 the primary source of degradation was throughput loss in the VIIRS rotating telescope assembly, likely caused by ultraviolet light illumination. This paper will discuss the laboratory investigation that determined the root cause of the telescope degradation to be UV photo-darkening of a tungsten oxide contaminant film that had been inadvertently deposited during the mirror 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.