Molecular film contamination is known to degrade the optical performance of space system components, including solar arrays and thermo-optical, second surface mirrors. In the form of a contaminant film, the resulting performance loss rate can be evaluated using traditional models for absorption and reflection. In recent years, however, some space-borne optical sensors have suffered severe and rapid performance degradation due to the formation of contaminant droplets that fog interior lenses, mirrors, and windows. Optical system analysts tasked with predicting the loss of throughput due to molecular film contamination have not addressed the impact of droplets in great depth. This paper investigates the conditions leading to the formation of films or droplets resulting from the outgassing products of typical spacecraft materials. A simplified view of surface energy and the wetting parameter are used to show that typical outgassed contaminants and optical substrates favor the formation of droplets. Therefore, analysis of throughput losses due to the scattering of droplets is critical. The droplets can be converted into films or extended islands when exposed to vacuum ultraviolet (VUV) radiation. This observation shows why droplets are rarely observed on external thermal control mirrors and solar arrays but might be considered highly likely in a low-VUV environment.
Previous studies have shown that molecular contamination outgassed from nonmetallic materials tends toward deposition on optical surfaces as droplets instead of nearly uniform thin films. Failure to consider the sources and effects of these droplets in an optical instrument omits large throughput losses due to scattering. This paper demonstrates that a simple treatment of optical system surfaces using vacuum ultraviolet (VUV) radiation reduces the formation of molecular contaminant droplets. VUV radiation exposure of a nominally clean silicon surface using a deuterium lamp suffices to remove hydrocarbon and carbonyl species that allow wetting of the surface by the contaminant. The throughput losses of the contamination due to droplet scattering can be reduced significantly.
This work presents further evaluation of the mechanisms driving the formation of molecular contaminant films and
arrays of droplets on silicon and other types of space system optical surfaces. A simple model is presented describing a
competition between the self-cohesive forces of a liquid-like droplet and the adhesive forces between the droplet and
surface. We show in this work that irradiation of the silicon surface prior to contaminant deposition increases the
adhesive forces, enhancing film formation. However, the surface states achieved by the VUV exposure cannot be
reproduced by simple approaches such as solvent wiping. Higher intensity VUV exposure produces a silicon surface that
allows film formation for even very pure contaminant analogs with high self-cohesion.
Mechanisms for molecular contaminant droplet formation are investigated. The tendency for droplet formation is
evaluated in terms of the surface tension of the liquid-like outgassed species and the surface energy of the collector.
Results are presented indicating that VUV irradiation of the surface prior to contaminant deposition eliminates some
droplet formation completely. This finding is discussed in terms of the removal of hydrocarbon and carbonyl-structured
compounds from oxidized silicon surfaces.
Previously, significant laboratory work has been performed on the photochemical deposition and darkening of molecular
contaminant films. Much of this work addresses single, purified molecular species to understand fundamental
photochemical processes. However, some of this work disagrees with other studies involving mixed, real spacecraft
materials. There are also points of disagreement with contaminated returned optics from the Hubble Space Telescope
where mixed contaminants were found. In this paper, we describe a method for vacuum depositing a controlled,
reproducible contaminant film containing two molecular species: tetramethyl-tetraphenyl trisiloxane (DC 704) and
dioctyl phthalate (DOP). We use this film to show differences in photochemical processes compared to a pure film of
DC 704. We show that some photopolymerization processes occur more slowly in a two-component, mixed film during
accelerated exposure to vacuum ultraviolet (VUV) radiation.
The effects of molecular film contamination on optical systems depend strongly on the film uniformity and thickness.
Molecular films of uniform thickness are responsible for light transmission losses through absorption. For example, a
partially darkened film of dioctyl phthalate 100 Å thick may cause losses of about 2% in the visible spectrum. However,
Ternet, et al, Villahermosa, et al, and others, have shown that scattering from droplets or "puddles" can cause
transmission losses of 30%. In this paper, we examine properties of the contaminant and surface that drive the formation
of smooth films and droplets. It is shown that surfaces play a strong, and sometimes dominant role in controlling film or
droplet formation. DC 704, a high purity, siloxane liquid, is shown to assume both droplet and smooth film character
depending on the surface.
The use of digital cameras and digital imaging software for the measurement of particle obscuration is discussed. Novel
calibration standards are used to evaluate the sensitivity and accuracy of commercially available digital cameras for
detecting microscopic dust particles and other contaminant features on surfaces. Lighting and illumination effects are
also illustrated and discussed. The digital image histogram of particles on a surface is shown to give good results for the
percent area coverage.
Novel light scattering properties of molecular films in a “droplet” configuration are presented and discussed. The illuminated films are shown to disappear when viewed at particular angles. The phenomenon is discussed in the context of Germer’s analysis of out-of-plane scattering from particles and surface micro-roughness.
The exterior optical surfaces of satellites are directly exposed to the harsh space environment. Here, a multilayer dielectric solar rejection filter was deposited on a silicon substrate and then subjected to electron and proton irradiation, simulating an orbital environment. Following the exposure, damage was observed that was attributed to dielectric breakdown. Optical and scanning electron microscopy revealed extensive pitting as a result of this exposure. The typical size of dischrage pits was 50 - 100 microns at the surface, extending to the substrate material, where a 10 micron diameter melt region was found. Pit damage occurred at pre-existing coating defects and was accelerated by pre-exposure to proton radiation. Pitting was not observed on similar samples that had also been overcoated with a conductive thin-film.
Real-time instruments based on surface acoustic wave (SAW) resonators are now seeing greater application for measuring the accumulation of nonvolatile residues (NVRs) on contamination sensitive surfaces. In this paper, we study the use of a desiccant, or dry GN+-2) to remove volatile films from the SAW sensing surfaces, with the intent of leaving the NVR behind. Using water as moderately volatile model material, the SAW device was capable of indicating monolayer growth in agreement with the expected frequency change. The drying agent was successful in removing all water from the SAW device. Additionally, the SAW device was capable of detecting different regimes of desorption kinetics. In trials of several candidates, only one example of NVR could be deposited, most likely a phthalate from flexible tubing heated beyond its working temperature. The deposit was so large that it overwhelmed subsequent observations of water desorption.
Laboratory measurements of photochemical deposition rates of outgassing products from Tefzel insulation have been conducted. We show that outgassing products from Tefzel insulation photodeposit under conditions of surface temperature and arrival rates for which bulk condensation will not occur. Normalized to the sample size, the photodeposition rate exceeds the reported condensable material outgassing rate. The result reported here strongly support the conclusion that photochemical deposition of contaminants from Tefzel is potentially a significant mechanism for degradation of thermal control surfaces on spacecraft.