The Particle Analysis Cameras for Shuttle (PACS) Experiment was flown on Mission STS61C (Columbia) in January 1986. This experiment involved a pair of cameras in a stereo viewing configuration and an associated strobe light flash to permit particle observation during the entire orbit. Although only one camera functioned properly, significant trends and particle counts were still obtained from the film data. We report here the preliminary analysis and conclusions from that mission.

Siloxane contaminated second-surface-mirrors were exposed to the geosynchronous solar environment and to laboratory irradiation designed to simulate the solar environment. Af-ter two an one-half years of space exposure, the solar absorptance (as) of the contami-nated (600Å) mirrors has almost tripled (Aas=0.145). Ge9synchron9us results show that doubling the siloxane film thickness from 300 angstrom (A) to 600Å produced as values greater by only 40%, suggesting that a major portion of the solar radiation is absorbed in the external 300Å thick layer. To compare with flight data and calibrate our production facility, samples were irradiated in 10-6 torr vacuum with 10 KEV electrons and radiation from two ultraviolet sources: a krypton lamp at 123.6nm, and a deuterium lamp at 180 - 450 nm. Accelerated lab results are similar to the on-orbit data, and verify that the geo-synchronous solar environment can be simulated for a given type and thickness of contaminant.

oated mirrors used in optical instrument payloads designed for use at ultraviolet wave-lengths face the risk that molecular contamination, in small amounts, can degrade reflectivity. This can be particularly damaging if a combination of solar ultraviolet radiation and organic gases are present, resulting in photopolymerization of hydrocarbon tars onto a mirror surface. Shuttle flight data has suggested that high energy atomic oxygen may be used to clean a previously contaminated mirror. A passive flight experiment, relying only on before and after reflectivity measurements, was designed and built to investigate the effects of molecular contamination in space. A number of mirror samples were prepared with a variety of surface treatments. These included contamination of the mirrors with Dioptyl Phthalate, ultraviolet irradiation of a portion of the contaminated samples, and scratches on the mirror surfaces. The samples were mounted in scaled-down telescope barrel configurations to accurately simulate the effects of the exposure of a full-size astronomical telescope to ambient atomic oxygen flow. A total of nine small optical assemblies were mounted on a plate. The plate was then mounted on a Shuttle Payloads of Opportunity Carrier pallet, facing out of the Shuttle bay, and flown as part of the Hitch-Hiker-G payload on the STS-61C flight of January 1986. The results of this experiment are presented in this paper.

The pressure buildup on spacecraft surfaces, facing the direction of motion, results in a contaminant cloud comprised of surface recombined ambient species, ambient/surface contaminants and the incoming ambient gases. The surface reemitted gases produce large column densities of potentially optically active gas species that do not require outgassing, engine, vent or leak contaminant sources. These gases can create unacceptable levels by the simple act of placing a surface on orbit which intercepts the ambient species. This enhanced gas cloud density also changes the mean free path of contaminant molecules injected into this cloud and alters the resulting return flux to surfaces. A new modeling approach (RAMDEN) is presented that allows gas buildup predictions to be made in a short time (minutes to hours) as opposed to days for Direct Monte Carlo Simulation (DMCS). Predictions are made for disks, solar arrays and shuttle surfaces. The impact of this effect on current shuttle contamination models and developing space station models is addressed.

The Extreme Ultraviolet Explorer will perform an all-sky survey and spectros-copic observations over the wavelength range 80 - 900A. Hydrocarbon and particulate contamination will potentially affect the throughput and signal to noise ratio of the sig-nal detected by the instruments. We have developed a witness sample program to allow us to investigate and monitor the effects of specific contaminants on EUV reflectivity. Witness samples were intentionally contaminated with thin layers of Balzer's P-3 pump oil. An oil layer 150A thick was applied and found to evaporate over 8 hours. The EUV reflectivity and imaging properties were then measured and found to be acceptable for grazing angles between 5 and 30 degrees. In a second test, layers 500Å thick were deposited and then allowed to evaporate in vacuum; once the oil had evaporated to at least 350Å, the final sample reflectivity was degraded less than 10 percent, but the image was degraded severely by scattering. An outline of the contamination control program is also presented.

A method is derived for estimating the relationships between clean room air cleanliness class (FED-STD-209B), exposure time and surface cleanliness level (MIL-STD-1246A). The basis of the method is extrapolation of Hamberg's data on particulate fallout rates observed in clean rooms (J. Envr. Sci. May/June, p.15, 1982). The results are presented in simplified form to estimate clean room requirements for different surface cleanliness levels. A discussion is provided on the particle size distributions observed by Hamberg and Shon (30th Ann. Mtg., Institute for Environmental Sciences).

The dislodgement, venting, and redeposition of particles on a surface in the shuttle bay by the vibroacoustic, gravitational, and aerodynamic forces present during shuttle ascent have been investigated. The particles of different sizes which are displaced, vented, and redistributed have been calculated; and an estimate of the increased number of particles on certain surfaces and the decrease on others has been indicated. The average sizes, velocities, and length of time for certain particles to leave the bay following initial shuttle doors opening and thermal tests have been calculated based on indirect data obtained during several shuttle flights. Suggestions for future measurements and observations to characterize the particulate environment and the techniques to limit the in-orbit particulate contamination of surfaces and environment have been offered.

High performance satellites and critical surfaces aboard advanced long-life spacecraft require effective contamination control measures to maintain performance stability and to ensure data accuracy. Contamination is said to exist and be of concern if a spacecraft or launch vehicle produced material interferes with the intended performance of a surface or sensor. This paper will discuss the development of contamination requirements for space-borne optical instrumentation. In addition, a comparison of instrument contamination requirements to measured instrument degradation levels for mission activities will be presented. If maintained, these requirements ensure against undesirable instrument degradation due to the contamination environment created by fabrication, launch, mission activities, ambient atmosphere and the instruments themselves.

The instrumentation payloads of the Space Station polar platforms consist primarily of Earth Observing System (Eos) instruments and National Oceanic and Atmospheric Administration (NOAA) instruments. The accuracy, lifetime, and mission effectiveness of the instrumentation is adversely affected by the degradation of contamination sensitive surfaces. This paper describes the analysis and results of a study undertaken to select a propellant system for the polar platforms from a contamination standpoint, by evaluation of the potential contamination to three platforms, from three candidate propellant systems. In addition, this paper reports the results of a study which determined both the molecular and particulate contamination requirements for 37 Eos/NOAA instruments. These requirements provide an upper bound for acceptable levels of contamination.

Second surface mirrors (metallized fused silica), commonly used for critical spacecraft thermal control applications, require low solar absorptance and high thermal emittance. Particulate contamination on these surfaces increases the solar absorptance and consequently degrades the thermal performance of the spacecraft. Quantification of the effects of particulate contamination from manufacturing through deployment will permit an assessment of effects on mission performance. Aerojet ElectroSystems Company (AESC) has developed a theoretical model to predict second surface mirror optical degradation resulting from particulate contamination at coverages less than 1% obscuration. Experimental verification of the model has been performed using well characterized, uniformly monodispersed particles of 4-12 μm size distribution. Both white latex spheres and nickel particles with solar absorptances in the range 0.1 to 0.5 were used to investigate the effects of opaque particles which are characteristic of typical spacecraft contaminants. The experimental data obtained in this study indicated that the Aerojet model serves as a valid means of quantitatively correlating second surface mirror optical properties with particle obscuration for small coverages. The quantitative prediction capability resulting from this investigation permits establishment of maximum permissible levels of particle accumulation with respect to mission performance impact. This, in turn, is useful for formulating contamination control strategies during prelaunch activities where particle fallout rates can be monitored and controlled.

A systematic analytical investigation has been performed by The Boeing Aerospace Company under the sponsorship of Air Force Rome Air Development Center to identify molecular and particulate contaminants that affect the optical sensor of a space-based surveillance system during prelaunch, launch, deployment, and lifetime operation of the system. This paper reports results of this investigation and includes discussion on contamination sources and species, contaminant transport and deposition mechanisms, and their impact on optical sensors in the ultraviolet, visible, and infrared regimes. The effects of cryodeposits on infrared sensor surfaces and organic deposition on ultraviolet sensors are addressed. Methods of contamination prevention and control are also summarized. The results of this investigation show that contamination of optical sensors can reduce or eliminate a sensor's ability to detect and identify targets. This performance degradation can occur directly or indirectly as a result of contamination, depending on the system design. Some effects of contamination include stray light scattered into the field-of-view, false targeting on particles, attenuation of signal, and warming of optical train surfaces due to increased absorptance of contaminated radiators and other surfaces. These effects are categorized and presented in the form of molecular/particulate deposition and free molecules/particles in the field-of-view.

Scattering by condensible molecular contamination on mirrors can be modeled by assuming the contaminant has not uniformly covered the surface, but has coalesced into spherical particles. BRDF values representing scatter from these particles can be calculated by Mie theory. The model suggests that particles formed from the equivalent of a 10Å layer of molecular contaminant could give BRDF (1°) of approximately 5 x 10^-4, for 10.6μ light. Thicker layers of molecular contamination could give BRDFs that begin to degrade stray light rejection in space telescopes.

The NASA Space Station is being designed to accomplish a number of different goals. The design includes the incorporation of basic truss structure and systems, pressurized manned modules for habitation and experiments, attachment accommodations for external payloads, a servicing facility, and several orbiting platforms for payloads. The contamination control implications of such a vast endeavor are tremendous. The NASA/Goddard Space Flight Center (GSFC) serves as the primary user interface for the Space Station. In addition, the GSFC is responsible for Space Station Attached Payload Accommodations, the Co-Orbiting and Polar Orbiting Platforms, the Customer Servicing Facility, and the Flight Telerobotics Servicer. A main priority of the Space Station is to accommodate users and to service users. The primary users of the Space Station include materials processing experiments, life sciences experiments, astrophysics payloads, Earth observation payloads, and plasma physics experiments. There will be an initial set of users identified for the Initial Operating Capability (IOC) Space Station and a host of additional users identified for future growth of Space Station. As one can imagine, each of the users aboard Space Station has its own unique set of science and performance requirements. Correspondingly, each user has its own contamination sensitivities and requirements. This paper will provide a summary of the contamination sensitivities and requirements,of a number of planned Space Station users in all science categories. The paper will also address Space Station and user plans for accommodating these needs and ensuring that contamination requirements are met.

The Upper Atmosphere Research Satellite (UARS) is a large (10,000 lbs, 170" diameter, 30' high) NASA/Goddard Space Flight Center spacecraft that will study aspects of the chemistry, dynamics and energy balance of the atmosphere above the troposphere. It will carry ten instruments operating in wavelenghts ranges spanning from the UV to the microwave region. While some of the instruments are only marginally affected by contamination, instruments that are known to be typically sensitive to contamination (like UV solar pointing and cryogenic instruments) will be on the UARS. The potential for cross-contamination is very high. The instruments are being developed by different organizations (each of them having different facilities and operational procedures), will be integrated at the spacecraft contractor site, and will share the same environment during the system performance tests and, obviously, the same flight environments. The goal of the UARS contamination control effort is to allow all the instruments to operate at the desired level of performance, and to achieve this within a non-unlimited budget. Given the mentioned operational scenario and the heterogeneity of the individual instrument requirements, it is easy to appreciate that the control of contamination for the UARS is an extremely complex and challenging task. In this paper, the instrument sensitivities and science requirements are described, and the analytical work leading to the derivation of the contamination control requirements is outlined.

Telescopes mounted on space platforms may be subject to performance degradation by the condensation of gaseous species on cryogenically cooled optical surfaces. Attenuations of the flux of impinging contaminants by a purging counterflow of helium (He) gas at the telescope entrance aperture have been calculated by a Monte Carlo procedure that follows individual trajectories selected at random. The calculations involve no significant approximations and should be quite accurate. In particular, multiple-collision trajectories are treated, employing the best available information on the atomic and molecular collision cross sections. Extensive calculations on protection of the Shuttle-mounted CIRRIS I and CIRRIS IA telescopes are presented which illustrate the extent of protection afforded.

Previous studies of metallic mirror surfaces using surface-plasmon resonance (SPR) showed contamination present after 3M and Universal strippable coatings were applied and removed.' In this paper, we examine contamination by moisture and 3M-coating residue, and methods to reduce this contamination using ultrasonic cleaning in acetone and isopropyl alcohol. We also discuss the sensitivity of the silver film surface to roughening during ultrasonic cleaning.

It has long been known that optical surfaces aboard spacecraft and research rockets can suffer irreversible degradation due to various forms of contamination present in the spacecraft environment. Sources of contamination include: outgassing and subsequent deposition of foreign materials (hydrocarbons, silicones, etc.), bombardment by reaction by-products of thruster firings (including potentially reactive chemical etching species), liquid, gaseous and particulate venting emissions and the residual contaminants from ground processing and handling. While detrimental to any experiment employing optical surfaces, experiments operating in the vacuum ultraviolet (VUV) and experiments employing cooled optical surfaces are generally most seriously affected. It is also possible that VUV reflectivities and transmittances of optical components in space flight and space station experiments may go through a condition of temporary degradation in the course of a mission without being noticed by a postflight calibration resulting in erroneous data. In response to this problem, a self-contained, space qualifiable optical contamination monitor has been developed to provide in-flight and/or real time monitoring of VUV reflectivity and transmittance. Time lining measurement results with other spacecraft events will permit, for the first time, the ability to evaluate the direct effects of various spacecraft operations on critical optical surfaces.

This paper will review the relative sensitivities and practicalities of the common surface analytical methods that are used to detect and identify unwelcome adsorbants on optical surfaces. The compared methods include visual inspection, simple reflectometry and transmissiometry, ellipsometry, infrared absorption and attenuated total reflectance spectroscopy (ATR), Auger electron spectroscopy (AES), scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), and mass accretion determined by quartz crystal microbalance (QCM). The discussion is biased toward those methods that apply optical thin film analytical techniques to spacecraft optical contamination problems. Examples are cited from both ground based and in-orbit experiments.

A test facility has been developed for in-situ measurement of the thermo-optical and electrical effects of molecular contamination deposited on sensitive spacecraft surfaces. The Contamination Effects Test Facility (CETF) consists of three separate vacuum chambers interconnected by gate valves through which test sample surfaces may be moved as needed by various vacuum manipulators. Deposition of contamination occurs in one chamber, where surface electrical properties can also be measured. In the second chamber, a wide range of thermo-optical properties can be measured by use of a unique ellipsoidal-mirror reflectometer. The third chamber maintains a vacuum environment around the test sample while the chamber is transported to facilities for solar ultraviolet (UV), electron, and proton irradiation of the sample at orbital intensities. By keeping atmosphere away from the contaminated surface at all times during the effects measurement and irradiation stages, the CETF provides a more realistic space simulation that avoids the possible effects of oxygen and water on the thermo-optical or electrical properties of the contaminant deposits. For testing of the volatile species produced by rocket propulsion systems, which are condensible only at cryogenic temperatures, continual vacuum capability precludes rapid icing due to atmospheric water vapor.

This paper will review the particulate control program applied to the design, construction and testing of the Optical Telescope Assembly. Allowable levels of contamination were derived from system performance requirements using performance predictive models. These levels were used as guidelines in establishing control and monitoring procedures for the assembly process. Measurements taken at critical junctures indicated that cleaning of the optics was required. Cleaning operations were performed at the latest stage of assembly possible. Subsequent measurements verified the effectiveness of the procedures. Optical surface tests, conducted after Space Telescope environmental testing, revealed that the optical surfaces were well within acceptable limits.

A major new facility was required to meet the contamination control requirements of the Hubble Space Telescope (HST) Program during the Assembly and Verification phase of the program. The Vertical Assembly and Test Area (VATA) facility would have to maintain a volumetric cleanliness level of better than 10,000 for all operations of assembly and verification testing of the HST. The assembly of this 43 foot long by 14 foot and 10 foot diameter telescope of 25,000 pounds was expected to require access for 30 to 50 people at a time and would be conducted with all major subassemblies being retained in a vertical attitude. This resulted in the need for a building with an inside height of ninety (90) feet, and 20 ton bridgecrane with a hook height of seventy-six (76) feet and a Vertical Assembly Test Stand (VATS) of five stories (platform levels). It was decided that, to provide the most effective clean room environment, a horizontal flow system would be required. The room had to be long enough also so that incoming equipment and hardware could enter the opposite end of the room (downstream of the HST) without degrading the assembly and test area. The facility (55 ft. wide by 90 ft. high by 120 ft. long) incorporated a fifty-five (55) foot wide by ninety (90) foot high High Efficiency Particulate Air (HEPA) filter wall (non-DOP tested), a pre-filter system, a downstream balance chamber system to divert the air through an overhead return cavity and back through the filter system and a separately filtered makeup air system (also used for cooling purposes on the HST during testing). Lockheed Missiles and Space Co., Inc. developed this facility, in Sunnyvale, California for the HST Program and its operation has met and exceeded all requirements in maintaining the strenuous cleanliness levels needed for the HST assembly and verification testing. This paper will discuss the requirements and the development of this facility.

This paper summarizes the results of several contamination analyses performed in support of Hubble Space Telescope (HST) system development. The discussion topics encompass several flight-phase operational and contamination events: prelaunch standby, on-orbit closed-door and open-door operations, HST interior outgassing, and ram-induced pressure rise in the Aft Shroud (AS) region. A number of refined analytical models (computer codes) have been used to carry out these analyses. The results indicate that the contaminant build-up during prelaunch and flight operations will be well within the HST contaminant allowable limits (about 50% the budget allocation). Specific findings are as follows: (a) no significant contamination of HST primary mirror (PM) and secondary mirror (SM) due to orbiter purge air will occur during prelaunch standby even with a worst-case scenario; (b) using a diffusion flux model and from available data on payload bay (PLB) sources, the deposition rates on the HST PM and SM have been predicted to be 0.4 angstrom/hr and 2.7 angstroms/hr, respectively, during on-orbit closed-door operations; (c) particulate contamination based on a scenario involving two astronauts during extravehicular activity (EVA) in conjunction with instrument change-out has been found to be negligibly small; (d) certain lubricating oils, if deposited on and not removed from HST interior surfaces, could cause significant contamination on HST critical optics; however, this potential contamination has been eliminated by HST thermal hakeout; (e) by utilizing a Monte Carlo model, the ram-induced pressure rise in the AS region has been predicted to be insignificant and would not violate the electric breakdown requirements.

The HST program scheduled a series of operational verification tests prior to launch. Some of these major tests include thermal vacuum/thermal balance, modal and acoustic testing. This paper addresses the preliminary work done to prepare for the acoustic testing operation. Acoustic cleaning of large assemblies is usually the first in a series of steps for cleaning a large space structure. Very little work has been done in the area of contamination analysis to determine particle redistribution after a large structure undergoes acoustic excitation. This paper will cover 4 experiments that were carried out in the Large Vehicle Environmental Test Cell #1 at Lockheed Missiles and Space Company in Sunnyvale, California. The experiments looked at particle redistribution, fallout from the test cell, acoustic cleaning as a precleaning tool for small particles and the effects of orientation of a model structure undergoing acoustic excitation.

This paper describes analyses that were performed in support of the Hubble Space Telescope (HST) particulate contamination control effort. The specific problems addressed include extension of available particle removal data to launch acoustic and random vibration conditions, development of an engineering model for transport of suspended particulates by airflow and in the presence of vehicle acceleration, turbulent diffusion, and migration of particulates over vibrating surfaces, and integration of the various models into a code that could be used to generate contamination level estimates for the HST primary mirror and other critical surfaces for the HST mission phases. The overall redistribution calculations were made assuming a specified initial contaminant distribution in terms of MIL STD 1246Å levels, and using predicted vibration data for the various HST surfaces and mission phases. As expected, the effects of airflow were found to be significant, particularly for the larger particles. Particles smaller than about 20 microns did not participate appreciably in the redistribution.

Molecular outgassing from spacecraft nonmetallic materials and surface contaminants can greatly affect optical surfaces if such outgassing sources condense onto the critical surfaces. Thermal vacuum bakeout of spacecraft hardware has proven to be an effective method of outgassing flight materials and depleting contaminant hydrocarbons from hardware associated with critical optical systems. This paper describes the thermal vacuum bakeout program used for the Hubble Space Telescope components at Lockheed Missiles & Space Co. (LMSC) prior to their integration with the telescope primary and secondary mirrors. Rationale for adopting a bakeout program from experimental data and analytical modelling are presented in this paper. Development of criteria based on telescope performance requirements is discussed. Selection of appropriate bakeout instrumentation and location, including Temperature-controlled Quartz Crystal Microbalance sensors and Residual Gas Analyzer, is described. The development of chamber cleaning and verification techniques plus chamber operational controls are shown to be a crucial part of the bakeout program. Outgassing curves for the five major subassemblies designed and fabricated at LMSC are included to demonstrate the effectiveness of the thermal vacuum bakeout program.

There are three primary factors which drove the low pressure venting performance of the Hubble Space Telescope (HST) aft shroud. The first two arise from design requirements. An important portion of the HST mission is to image very dim objects in deep space. Thus, stray light from the exterior environment into the optical path must be held to an extremely low level. Light leakage into the aft shroud must be constrained to approxi-mately 7.0 X 10-09 of the incident solar flux. In order to achieve this level, light rays incident on the external apertures of the aft shroud vents must be made to reflect many times off of the blackest possible surfaces (baffles) before they reach the interior of the aft shroud. Unfortunately, light rays (photons) and molecules of gas in the free molecular flow regime (collisionless) behave somewhat similarly - a 'random walk' through the vent with diffuse reflections from the walls of the vent passage and its baffles. Thus baffled vent configurations which effectively limit light leakage also tend to severely restrict the efficiency of free molecular flow through the vent.

A comprehensive contamination control program was developed and implemented at LMSC for the Hubble Space Telescope (HST) Thermal Vacuum Test. The test program involved monitoring HST sensitive optical systems using both active (realtime) and passive molecular contamin-ation samples. Data and test conditions were monitored realtime to assure that molecular contamination accumulation would not threaten the telescope primary optics. The contamina-tion control program utilized six Temperature-controlled Quartz Crystal Microbalance (TQCM) sensors, nine magnesium fluoride-coated aluminum over glass optical witness samples (OWS), and particulate and nonvolatile residue (NVR) fallout samples. The program utilized wit-ness samples which were baselined before and analyzed after the vacuum test to characterize the particulate contamination levels and optics reflectance changes induced by the thermal vacuum environment. This paper describes the instrumentation and active and passive monitoring techniques used for the HST Thermal Vacuum Test as well as the contamination results and analyses. Rationale for selection of the sampling methods and locations relative to the spacecraft surfaces is included.

The Particulate Optical Test (POT) was designed to record and measure particulate contaminants on the Hubble Space Telescope (HST) primary mirror. The objective of the test was to quantify the primary mirror particulate contamination prior to launch. The test consists of taking dark-field photographs of the primary mirror from in front of the telescope's aperture door. These photographs are subsequently digitized and analyzed to produce the areal coverage estimates. The estimated particulate areal obscuration is approximately 1.0% of the primary mirror surface in the usable region. This level of contamination is within the budget value of 2.5% and indicates that there was little increase in particulate contamination during the HST assembly process or the acoustic test period.

The Hubble Space Telescope (HST) will be the first permanent National Astronomical Observatory in space 310 miles above the earth. To meet the scientific objectives of its mission the Ritchey-Chretien optical system must meet stringent contamination control criteria to prevent obscuration and scattering losses. The HST was assembled, tested and prepared in the 10K laminar flow clean room to meet these contamination control concerns. Once testing and preparations for shipment are complete the HST will be ready for its momentous 5,000 mile trip from Lockheed Missiles and Space Co., California to the Kennedy Space Center for launch. Measuring 43 feet in length and 14 feet in diameter, the HST will weigh in at 25,000 pounds (see Figure 1). Due to the size of the HST it cannot be transported by air but must be shipped by sea to the launch site. During this trip the HST will be in six different containers/locations for a minimum of six months. Environments vary from the Vehicle Assembly and Test Area clean room at Lockheed ( a horizontal laminar flow Class 10K clean room) to the Vertical Processing Facility (a nonlaminar flow 100K clean work area), see Figure 2. The HST contamination control budget for degradation of the primary and secondary optics only permits 0.1% for reflectance loss at 1216 angstroms and 0.1% due to particulate obscuration due to transportation and launch activities. The key to protecting the HST optics is the protective cover system and the internal gas purge.

The resonances associated with the coupling of light into a surface plasmon propagating along a metal surface are exceedingly sharp and narrow as a function of incidence angle. The fields associated with the wave directly at the metal surface are high, and so the plasmon propagation is strongly affected by even slight changes in the surface conditions; this causes changes in the resonances. These changes can then be used as indicators of altering surface conditions. The growth of tarnish layers, the stability of coatings, and the degree of surface contamination are some of the effects that have been studied in this way .1,2

A short review of irreversible computer models of monolayer growth is presented together with some preliminary results obtained from a recently developed reversible model.

This experiment is designed to measure the performance degradation of a cryogenic high straylight rejection telescope during its fabrication, assembly and test phases. The main thrust of this experiment is to obtain experimental data to define the point (s) in the process at which contamination occurs and to identify the remedial steps to correct the problems. The CIRRIS I (Cryogenic Infrared Radiance Instrumentation for Shuttle) telescope and its cryogenic chamber are utilized in this experiment as a representative cryogenic sensor in support of the SPIRIT II program (Spatial Infrared Telescope). Both CIRRIS I and SPIRIT II are programs through Utah State University and the Air Force Geophysics Laboratory (AFGL), Hanscom Air Force Base, Bedford, Massachusetts. This contamination study program is being performed for the Rome Air Development Center (RADC), and the Air Force Geophysics Laboratory.

We are studying various approaches to cleaning contaminants off of optical surfaces at cryogenic temperatures. The techniques we are reviewing must be relatively efficient, portable, and not degrade the optical quality of the surface being cleaned, i.e., they must remove only the surface contamination layer, and leave the substrate untouched. Various techniques that we have considered include removal by electromagnetic waves by discharge-produced species, and by electron or ion bombardment. We have considered electromagnetic waves, generated by devices such as lasers or heat lamps, spanning a wavelength range from microwaves to soft X-rays. Proper wavelength selection offers great opportunities for efficient removal of selected contaminants. Discharge-produced species include such things as electronically excited metastables, slow ions, atoms, and free radicals. Although production efficiencies for energetic species in discharges are not high, some species contain sufficient energy to remove many contaminant molecules in one metastable-surface collision. In addition, the technique is surface specific, and should not lead to significant heating or damage of the substrate. Electron and ion bombardment techniques are widely used in a number of surface-cleaning applications. Great care, however, must be exercised in using these techniques to avoid damaging the optical quality of the surface We will present a review of the various techniques giving the strengths, weaknesses, and efficiencies of each. In addition, we have begun some experiments using selected techniques to test their efficacy. The experiment involves cleaning a thin layer of ice off a mirror surface main-tained at liquid nitrogen temperature in a vacuum chamber. An off-axis scattering technique is used to determine the degree of contamination of the surface, and, after the cleaning procedure, how well the surface quality has been restored.

The feasibility of using low energy (<30 eV) oxygen ions to clean optics has been demonstrated, It was found that the removal rate of a hydrocarbon film is between.10-23 and 10-24 cm /ion. These rates imply that a one meter diameter optic with a 1000 Å layer of organic contaminant can be cleaned with 10 mole (0.32 milligram) of oxygen impinging on the surface. The removal rate varied with the thickness of the film; the lower layers of the film were more difficult to remove than the outer layers. The effect of the ions on the scatter of a gold mirror was also studied. It was determined that the ions increased the scatter by less than a factor of two.