Hubble Presentations
Optical Component Degradation Assessment Part II - Surface Chemistry Analyses
Dr. June L. Tveekrem / Optics Branch, NASA Goddard Space Flight Center
Test Sequence and Rationale
- Nondestructive tests (e.g. microscopic inspection, photography) were performed first, then minimally destructive tests (e.g. X-ray Photoelectron Spectroscopy), then destructive tests (e.g. solvent rinse, scraping, cutting the mirror).
- The flight pickoff mirror was characterized first, then other optical elements exposed to the HST hub area were analyzed to see if they showed the same effects. The optics studied were: the flight pickoff mirror, the aperture window and several filters from the High Speed Photometer instrument.
- Since all optical elements proved to be contaminated with the same chemical species, later tests were performed on different optical elements, and the results were generalized to all the optical elements.
Micro-Photography Results
- The flight pickoff mirror, the aperture window, and the HSP filters were examined and photographed using a high-power microscope in phase-contrast (Nomarski) mode.
- The pickoff mirror showed a blue haze to the naked eye. Under the microscope, this haze was revealed to consist of numerous droplet-like features 1 to 2 microns in diameter.
- The aperture window did not appear contaminated or damaged, except for a crystalline defect in the exact center. This defect was present prior to launch.
- The HSP filters had a rough surface finish and several features which appeared to be manufacturing defects, but showed no visible contamination.
X-ray Photoelectron Spectroscopy (XPS) Results
- XPS consists of irradiating a surface with X-rays, which knock electrons out of the surface via the photoelectric effect, then measuring the energy of the emitted electrons. This allows the chemical elements present in the top 50 Angstroms of surface to be identified. The chemical bonds between an atom and its nearest neighbors can also be identified.
- Chemical elements present below the surface can be identified by sputtering away the top 50 Angstroms, then analyzing by XPS again. By repeating this process many times, a depth profile of the surface is obtained.
- Depth profile of pickoff mirror revealed that the Al + MgF2 coating was intact, but the surface was heavily contaminated with hydrocarbons, esters and silicones.
- The aperture window was also undamaged, but contained the same contamination as the pickoff mirror. The thickness of the contamination layer was 1/3 that off the layer on the pickoff mirror. Only the hub-facing side of the window was contaminated; the side facing into the WF/PC-I instrument was clean.
- Two HSP filters were analyzed; they contained the same contamination layer in the same thickness as the aperture window.
- The thickness of the contaminant was measured by sputtering a "square well" through the contamination layer on the HSP filter, then using an Atomic Force Microscope to measure the depth of the well. The result was 160 Angstroms.
- Since the aperture window and pickoff mirror were known to contain the same contaminant, the ratio of sputter times was used to deduce that the aperture window contained approximately 150 Angstroms of contamination and the pickoff mirror contained about 450 Angstroms.
- Because the contaminant did not come off in vacuum, and based on the shape of the reflectance degradation curve, an on-orbit UV-stimulated mechanism for depositing the contamination was suspected. The HST optical train is exposed to Earth-reflected UV for part of each orbit.
- The UV stimulation hypothesis is partly supported by the fact that contamination has not been found so far on hub-facing surfaces which were not exposed to UV. Investigations of such surfaces is continuing.
- To determine whether UV-assisted deposition and photopolymerization was possible, and to determine where the contamination might have come from, mass spectroscopy was performed next on the optics to better identify the chemical species of contaminants.
Surface Mass Spectroscopy Techniques
- The aperture window and the pickoff mirror were analyzed by Gas Chromatography / Mass Spectroscopy. In this technique, an area of the surface is rinsed with a strong solvent (methylene chloride), then the solution is injected into a gas chromatograph to separate the molecular types, then sent into a mass spectrometer for identification. This technique features high sensitivity, but can only detect soluble contaminants of relatively low molecular weight.
- The pickoff mirror was analyzed by Direct Probe Mass Spectroscopy and Pyrolysis Mass Spectroscopy. In these techniques, a small area of the mirror surface is scraped off, and the scrapings are heated to several hundred degrees Celsius to evaporate the sample. The evaporated molecules are directed into a mass spectrometer for identification. The advantage of this is that chemical species with high molecular weights can be detected.
- The HSP filter was analyzed by time-of-flight Secondary Ion Mass Spectroscopy. This technique consists of bombarding the surface with ions to knock molecules out of the surface, then sending the molecules into a mass spectrometer for identification. The advantage of this technique is that it is not necessary to previously remove or dissolve the contaminant from the surface. However, only the top few monolayers can be detected.
Surface Mass Spectroscopy Results
- The four mass spectroscopy techniques yielded results that were consistent with each other. The chemical species found, in order of abundance were:
- A very high molecular weight, polymerized hydrocarbon
- Poly dimethyl siloxane
- Di ethyl phthalate
- Di octyl phthalate
- Tri phenyl phosphine oxide
- Numerous low molecular weight hydrocarbon fragments
- The aperture window contamination was measured by XPS before and after solvent rinsing with methylene chloride. By comparing the relative strengths of the XPS peaks, it was estimated that 2/3 of the contamination layer was removed by the solvent.
- The reflectances and transmittance of the pickoff mirror and aperture window were remeasured after solvent rinsing. The aperture window results were inconclusive, but the pickoff mirror reflectance was partially restored.
Conclusions from Surface Analyses
- Reflectance degradation was caused by contamination, not optical coating damage.
- The pickoff mirror received about 450 Angstroms of contamination while the aperture window and HSP filters received about 150 Angstroms.
- The primary contaminant species detected were a polymerized hydrocarbon, polydimethylsiloxane, diethylphthalate and dioctylphthalate.
- The contaminant layer appears to be partially polymerized; some of it was removable by rinsing with strong solvents, but some was not. The part that was not removed was still enough to cause a significant reflectance degradation.
- Our leading theory is that the contamination happened during normal on-orbit HST mission operations; outgassed molecules from electronics and other hub-facing components impinged on optical surfaces and were partially polymerized there by exposure to earth-albedo UV.
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