Contamination Analysis of Optical Telescope Assembly OTA

1
Contamination Analysis ofOptical Telescope
Assembly (OTA)

• April 18 2006
• Electronic, Mechanical Components and Materials
  Engineering Group,
• Institute of Aerospace Technology (IAT), JAXA

2
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 -gt Degrades S/N
  ratio
• Change of thermo-optical properties -gt
  Increases Solar absorption
• Surrounding pressure increase -gtElectric
  Discharge
• Contamination on the critical surfaces must be
  predicted and controlled.
• Operational lifetime should be estimated
  according to the predicted contaminant deposition.

3
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 budget200300Å
• 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.

4
External contamination analysis

• Predict Molecular contamination under thermal
  vacuum condition

• Mathematical models for
• Outgassing
• Transport
• Absorption
• Key parameters
• Contaminant source temperature
• View factor
• Contamination sensitive surface temperature

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Analysis Conditions-Components of Uncertainty-
Integrated outgassed Mass
Contaminant Source temperature
Outgassing curve fit and Extrapolation35
Outgass from minor volume materials
Analysis Result Combined Uncertainty Over 100
Test sample preparation 70
Mass measurements
Outgassing rate Measured data 80
Contaminated surface temperature
Geometry model
View factor calculation 50
Contaminant Density20
Times of reflection
Reflectance
Test sample preparation 70
Re-sublimation temperature measurement
Multiple reflection
Scattering with ambient gas
Photodeposition rate80
Self scattering
!! Uncertainty of each component is preliminary !!
Contaminant absorption
Contaminant transportation
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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

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OTA Geometry Model
Top Door

• Total267 nodes
• Contamination Critical Surfaces 11 nodes
• Contamination Sources 85 nodes

Heat Dump Window
Secondary Mirror
CLU Lens Unit
Primary Mirror
8
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.

9
Analysis Results
10
Predicted 3 Year Deposition
11
Predicted 5 Year Deposition
12
Limit of analytical prediction

• The molecules contamination process is affected
  by many more factors.
• Crosslinking and Coloration induced by
  ultraviolet (UV) radiation
• SiO2 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.

Hard to establish correlation with true
value Because.... True Value is NEVER available
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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.