Effect of Organic Contaminants on Satellite Optics

Transmittance Variation due to the Adsorption of Organic Molecules on Optical Glass Surfaces

Abstract

Organic gas adsorption onto optical surfaces equipped in satellites is one of the causes of signal degradation occurred in an orbit. To estimate the transmittance degradation caused by gas adsorption onto optical glass surfaces, the transmittance measurements were carried out in UV-VISUAL (200-800nm) and IR (1.7-15μm) wavelength ranges.Five kinds of glasses, SiO₂, BK7, Al₂O₃, CaF₂, and ZnSe, which are typically used for satellite optics, were selected as glass samples and three kinds of organic gas, 2-propanol, ethyl-acetate, and dichloromethane, which are dominant components of organic solvent, were selected as adsorption gases. In infrared wavelength range, several absorption bands correspond to vibration and/or bending transitions of functional groups included in adsorption molecules were detected. The strengths of these absorption bands decreased with glass temperature depression and completely disappeared at 233K. In UV-VISUAL wavelength, there is no local absorption features but broad transmittance degradations were detected. The relative spectral degradation for dichloromethane is similar to that for 2-propanol, but not similar to the case for ethyl-acetate. The differences of transmittance degradations among three kinds of adsorption molecules can be explained by the bonding structure of molecules. Namely, ethyl acetate has a π-bond, which can absorb visual light with π-π transition, and 2-propanol and dichloromethane have no π-bond but σ-bonds, which can absorb UV light. From the comparison of measured spectral transmittance degradations with sensor output degradations, it is found that the signal degradations of launched sensors are more similar to the transmittance degradation due to the 2-propanol or dichloromethane adsorption than that with ethyl-acetate. Moreover, we estimate the growth rate of the adsorbed molecular film thickness using the degradation data of orbiting sensor, MODIS/Aqua. To carry out our estimation, we adopted the assumption that the signal degradation is caused by the transmittance decrease of the sensor optics due to the gas adsorption. Our estimation shows that the growth rate of adsorbed molecular film during the period from 30 to 50 months after the launch is significantly lower than the growth rate just after the launch.