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.
Purging is a common scheme to protect sensitive surfaces of payloads and spacecraft from airborne
contaminant intrusion during ground assembly, integration, and launch vehicle encapsulation. However, the
purge for space volumes must be occasionally interrupted. Thus it is important to gain insights into the
transport of ambient particles penetrating through vent holes and entering the interior of a confined space
system, such as a space telescope, during a purge outage. This study presents experimental work performed
to measure time-dependent aerosol concentration changes during a purge outage. The laboratory results from
the aerosol experiments were compared with a mass balance based mechanistic model which had been
experimentally validated for aerosols ranging from 0.5 to 2 μm. The experimental data show that the steady-state
aerosol concentration inside a simulated space telescope (SST) is governed by the surrounding particle
concentration, SST air exchange rate, and the particle deposition rate.
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.
To understand the dynamics of airborne particulate intrusion into a space telescope, a mechanistic model based on mass balance was developed to predict the extent to which ambient particles penetrate through vent holes and enter the interiors after the purge is off. This work describes the mathematical modeling analysis, supplementing with results from laboratory measurements using a cylindrical chamber as a simulated space telescope. It was found that the characteristic time for airborne particles to reach a saturation level, after the purge is off, can be characterized by the air exchange rate and particle deposition rate inside the confined space volume. The air exchange rate, measured using a tracer gas technique, is associated with the natural convection and air flow turbulence intensity adjacent to the chamber. During the purge outage, the steady-state airborne particle concentration inside the space telescope is governed by the ambient particle concentration, air exchange rate, and particle deposition rate.
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.
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.
Perhaps the most pernicious types of spacecraft contaminants are photochemically deposited ('solarized') molecular films. The magnitudes of the effects of these films on thermal control and solar photovoltaic surfaces are difficult to predict with high reliability. This uncertainty has two primary origins. Spacecraft contaminant films are not made of pure, well characterized materials, and, once they are deposited, they can become further darkened by energetic radiation in the natural space environment. This paper presents the results of a laboratory study aimed at gaining a greater understanding of the impact photochemical deposition on spacecraft and other optics. Photodeposition results and analyses of the ultraviolet and visible transmission spectra of films from several organic precursor molecules are reported. The major differences among contaminant film types have been found to be in the initial photodeposition propensity, rather than in the ultimate optical properties of the films. Models of the effects of the photodeposits on thermal control surfaces are presented.
Spacecraft function in a hostile environment of sunlight, charged particles, micrometeroids and debris, and their own self-induced contamination environment. Contamination can occur during fabrication and ground processing, or by very long term outgassing and transport processes on orbit. One of the most deleterious effects of contaminant films is that they increase the solar absorptance of optics, such as thermal control mirrors and solar cell cover slips. This paper will discuss the role of vacuum ultraviolet induced photochemistry in the deposition of contaminant films during the multi-year life of a spacecraft. Laboratory and spaceflight measurements leading to a one-photon "Langmuir" type model for the deposition mechanism will be presented. Measurements of visible and ultraviolet optical properties of photodeposited films will be described. The implications of these process for terrestrial optical systems, including a case history of similar effects in an ultraviolet laser system studied in this laboratory will be discussed.
A series of vacuum simulation tests were conducted to characterize the possible contamination of payload surfaces due to separation of the Pegasus fairing. This paper presents the test philosophy, experimental approach and the contaminant levels found, compared to MIL-STD- 1246B. These tests show that the Pegasus fairing separation using the frangible base joint and nose bolt nut results in Level 500 payload surfaces. The source of Zn particles near the bottom of the test payload needs to be found.