Scatter due to Ultraviolet (UV) photopolymerization of molecular contamination on optical surfaces

Molecular contamination on optical surfaces is a serious issue for space payloads. For ultraviolet (UV) payloads, molecular contamination results in loss of throughput. Whereas, for visible (VIS) payloads loss of throughput due to molecular contamination is not very critical. However, UV photopolymerization of molecular contamination can create serious problems to VIS payloads with stringent scatter requirements. Visible Emission Line Coronagraph (VELC) on board ADITYA-L1 mission is one of them. For such payloads, quantifying the effect of molecular contamination in terms of scattered light is very critical. To do the same, UV exposure studies are carried out on optical surfaces with different levels of molecular contamination. Contamination levels are defined by the weight and mass loss of the contaminant. UV dosage levels on the optical surfaces is determined by the number of hours of UV exposure and UV irradiance levels. Variation in the scatter from the optical surfaces is measured pre and post UV exposures. Contaminated optical samples are exposed to the UV radiation levels equivalent to 125 days of mission life. Substantial increase in the scatter of the contaminated samples is observed due to the UV exposure. Increase in scatter is more rapid in the initial mission days (up to 45 days). This paper elaborates the experiments carried out and discusses the results from them.