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Responsible Parties: Page Content: William H. Kinard Page Construction: Thomas H. See

Significant progress has been made in understanding the various degradation mechanisms associated with optical materials flown on LDEF. Findings continue to show that most optical components survived quite well, and space-induced degradation was not a significant influence on most post-retrieval properties. Molecular contamination of exposed optical surfaces by outgassed material was the major influence on optical property degradation, leading to significant transmission loss in the ultraviolet spectral region.

The effects of optical scatter, caused by micrometeoroid and debris impacts, atomic oxygen, ultraviolet radiation, and contamination have been studied. Results show off-axis optical scatter due to contamination increased by about a factor of 10 on mirror surfaces. The potential exists for large increases in optical scatter for unshielded optics on the leading edge (ram) from meteoroid and debris impacts, but only small increases for Earth-looking cases. Recent ground-based experiments and modeling have increased understanding concerning the effects of space exposure on optical scatter and have become important spacecraft design tools.

Contaminant films and residue were widespread over LDEF and optical experiment surfaces, due to the decomposition and outgassing of several materials. At least two possible sources are materials on the vehicle itself, as well as those materials used in some of the experiments.

Four experiments flew fiber optics and a fifth experiment evaluated fiber-optic connectors. Four of these five experiments recorded on-orbit data using the NASA provided EPDS. Overall, the fiber optics performed well on-orbit, with little or no degradation to optical performance. Most environmental effects were confined to the protective sheathing. However, one fiber optic bundle was struck by a meteoroid or debris particle causing discontinuity in the optical fiber. Preliminary data have indicated the need for additional study of the temperature effects on fiber-optic performance. Post-flight testing performed on fiber optics flown on the Fiber Optic Exposure Experiment showed an increase in loss with decreasing temperatures, becoming much steeper near the lower end of their temperature range.

Four LDEF Experiments contained a variety of detectors. Most detectors were not degraded, with one exception. The triglycine sulfide had a 100% detectivity failure rate on both the control and flight samples.

Several types of optical sources were flown on LDEF, including solid and gas lasers, flashlamps, standard lamps, and LEDs. To date, the results indicate that most optical sources operated nominally except for two gas lasers (HeNe and CO₂) which would not fire during post-flight testing and a flickering deuterium lamp arc. During post-flight testing of the two gas lasers, no laser action could be obtained from the tubes. The characteristics of the tubes suggested that the mixture of fill gas had changed during the period between pre-flight and post-flight tests. This result is consistent with changes expected due to gas diffusion through the glass tube. The tubes were in good physical condition, and survived the launch and recovery phases without apparent degradation.

Micrometeoroid and debris impacts on optical surfaces caused localized pitting, punctures, cracking, crazing, and delaminations.

Spectral radiation from both solar and Earth albedo sources was indicated in the modifications of surface coating materials (chemical decomposition caused by ultraviolet radiation). This was particularly noticeable on an experiment located on the trailing edge were the holographic gratings had a 30% to 40% degradation of reflectivity from exposure to solar radiation and cosmic dust. Experimenters also noted that changes to coating interfaces as a result of infrared absorption may have contributed to mechanical stresses and failures from thermal cycling.

Atomic oxygen had a major effect in the oxidation of many physically "soft" materials, including optical coatings and thin films, as well as oxidation of uncoated, metallic reflective coatings (e.g., copper and silver). In general, "hard" uncoated optical materials were found to be resistant to the LEO environment.

Synergistic conditions of degradation resulted from the multiple and combined effects of environmental factors; for instance, UV and atomic oxygen attacked, changed, or even eroded away some of the overlaying contamination, modifying the broadband and spectral content of optical inputs to the sample beneath.



Over 50% of all LDEF's exterior surfaces were chromic acid anodized (CAA) aluminum. Extensive optical testing of LDEF's CAA aluminum-tray clamps was performed because of their wide distribution around the LDEF and representation of a complete spectrum of spaceflight environmental exposures. The tray clamps provided a complete picture of the spaceflight environmental effects on this surface treatment. Comparison of front-side (exposed), backside (shielded) and control clamps showed slight changes in the optical properties. However, the variation in absorptance and emittance have been attributed to the inherent variability in anodizing, to variations in measurements, and to the effects of on-orbit contamination deposited on tray clamp surfaces

An LDEF Optical Experiment Database was created (using Filemaker Pro database software) that provides for quick and easy access to available experimenter's optics-related findings. The database contains a file for each of the LDEF experiments that possessed optical hardware. Each file contains fields that identify the optical hardware flown, describe the environment seen by that hardware, summarizes experiment findings, and list references for additional information.

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