Contamination Analysis of Optical Telescope Assembly (OTA) April Electronic, Mechanical Components and Materials Engineering Group, Institute of Aerospace Technology (IAT), JAXA

Apr Impact of Contamination Molecules released into a vacuum environment from material surfaces and organic materials can adhere to other spacecraft surfaces Degradation of optical sensor by attenuation and/or scattering of light -> Degrades S/N ratio Change of thermo-optical properties -> Increases Solar absorption Surrounding pressure increase ->Electric Discharge Contamination on the critical surfaces must be predicted and controlled. Operational lifetime should be estimated according to the predicted contaminant deposition.

Apr OTA Contamination Control Requirement Significant heat flux enter the Sun pointing OTA. Excessive absorbed solar energy introduces deformation of the mirror. Therefore, Solar Absorptance of critical surfaces must be controlled under a certain level. Contamination control during ground process Material Selection Cleanliness control Vacuum bakeout End of life contamination budget ： 200 ～ 300 Å Contaminant deposition to be predicted analytically. Target outgassing rates of OTA components were distributed according to the contamination budget. Additional vacuum bakeout of components were performed from the analysis results.

Apr Predict Molecular contamination under thermal vacuum condition External contamination analysis Mathematical models for Outgassing Transport Absorption Key parameters Contaminant source temperature View factor Contamination sensitive surface temperature Outgassing Models Absorption Models Transport Models

Apr Analysis Conditions -Components of Uncertainty- Analysis Result Combined Uncertainty Over 100% Integrated outgassed Mass Outgassing rate Measured data :±80% Outgassing curve fit and Extrapolation:±35% Contaminant Source temperature Mass measurements Test sample preparation :±70% Contaminant transportation View factor calculation :±50% Geometry model Multiple reflection Reflectance Contaminant absorption Photodeposition rate:±80% Contaminant Density:±20% Contaminated surface temperature Re-sublimation temperature measurement Test sample preparation :±70% Outgass from minor volume materials Scattering with ambient gas Times of reflection Self scattering !! Uncertainty of each component is preliminary !!

Apr Case study in OTA Analysis Two analytical Tools Swales :Modified TRASYS JAXA :Thermal Desktop Operational time periods 3 Year /5 Year 3 on-orbit modes Decontamination Mode 1 Decontamination Mode 2 Operational Mode

Apr OTA Geometry Model Total:267 nodes Contamination Critical Surfaces :11 nodes Contamination Sources :85 nodes Top Door Heat Dump Window Secondary Mirror Primary Mirror CLU Lens Unit ModeTop Door Heat Dump Window Heater DM1Closed On DM2ClosedOpenOn OMOpen Off

Apr Analysis Conditions Contamination source temperature ; Highest temperature predicted Contamination sensitive surface temperature ; Lowest temperature predicted Solar Arrays and OTA components satisfy their target outgassing rates before launch. ModeContamination Source TransportationAnalysis case DM1,2Solar ArraysDirect fluxContaminant mass flux to HDW OMSolar Arrays OTA Components Direct flux Multiple reflection Deposition on critical surfaces

Apr Analysis Results -Operational Mode- Predicted Deposition [ Å ] 3 Year5 Year JAXASwalesJAXASwales Primary Mirror Secondary Mirror to to CLU Lens Decontamination Mode- Predicted mass flux [g/cm 2/ sec] DM1DM2 JAXASwalesJAXASwales Solar Array to HDW2.27E E-136.0E-13

Apr Predicted 3 Year Deposition

Apr Predicted 5 Year Deposition

Apr Limit of analytical prediction Hard to establish correlation with true value Because.... True Value is NEVER available The molecules contamination process is affected by many more factors. Crosslinking and Coloration induced by ultraviolet (UV) radiation SiO 2 production by atomic oxygen (AO) and siloxane Erosion by AO Only a part of parameters are now considered in analysis. The parameters have synergy effect each other. Samples contaminated on-orbit are barely observed.

Apr Conclusion OTA is appropriately designed to perform required contamination control. Case studies with two analytical tools produced the results of wide variety. In some analytical cases, the predicted deposition exceeded the end of life contamination budget. Analysis results have significant uncertainty. The operational life estimation should be verified using data obtained on-orbit operation to reduce uncertainty.