Contamination control is of critical importance for the success of most aerospace programs. Thermal control surfaces, solar arrays, and optical devices may be adversely affected by even minuscule levels of molecular and/or particulate contamination. There is a wealth of data available on the subject of contamination and its effect on specific spacecraft systems. However, what is rarely discussed is how one: (1) quantifies the level of contamination that must be maintained in order for the system to function properly, and (2) enforces contamination control to ensure compliance with requirements. This paper summarizes recent efforts to addresses these specific issues which have culminated in the development of a handbook on contamination control that illustrates process and methodology while providing direction to more detailed references as needed.

The moderate resolution scanning spectroradiometer (MODIS) is a multiband scanning instrument containing low scatter optics whose performance depends on low particulate and molecular contamination levels. The derivation of the MODIS instrument cleanliness requirements is discussed, along with the contamination control necessary to ensure that the requirements are met and the performance of the MODIS instrument is not degraded. Representative cleanliness monitoring results from the protoflight model, currently being integrated and tested at Santa Barbara Remote Sensing (SBRS), are also presented.

The Hubble Space Telescope was designed to be periodically serviced on-orbit during its 15 year mission. Servicing carriers have been designed for these servicing missions and were previously flown during the Hubble Space Telescope Servicing Mission 1, Space Transportation System 61, December 1993. In preparation for the Hubble Space Telescope Servicing Mission 2, the Hubble Space Telescope contamination control philosophy was reviewed to determine its applicability to reflown hardware. The contamination control program currently in place for the Hubble Space Telescope Servicing Mission 2, evolved from the Hubble Space Telescope Servicing Mission 1 contamination control program. The challenge of the Hubble Space Telescope Servicing Mission 2 contamination control program was to maintain the integrity and outgassing certification of the reflown hardware while accommodating configuration changes to the hardware. Environmental control of the hardware, materials screening and outgassing certification of added hardware were the important features of the program.

The orbiters undergo periodic maintenance during which extensive mechanical and electrical system inspection, upgrades and/or refurbishment are performed. For sensitive payloads, new materials added to the orbiter midbody (payload bay) area is of concern for the first flight after an orbiter maintenance down period (OMDP). Discovery, (OV- 103), recently underwent an OMDP, and the Hubble Space Telescope Servicing Mission 2 will be its first flight after OMDP. Due to the contamination sensitivity of the Hubble Space Telescope, a program was implemented to monitor the nonvolatile residue and particle levels of the midbody during the OMDP.

The Hubble Space Telescope (HST) has been designed to accommodate changeout and/or repair of many of the primary instruments and subsystem components, in an effort to prolong the useful life of this orbiting observatory. In order to achieve the science goals established for this observatory, many HST instruments must operate in regimes that are greatly influenced by the presence of on-orbit propagated contaminants. To insure that the required performance of each instrument is not compromised due to these contaminant effects, great efforts have been made to minimize the level of on-orbit contamination. These efforts include careful material selection, performing extensive pre-flight vacuum bakeouts of parts and assemblies, assuring instrument assembly is carried out in strict cleanroom environments, performing precision cleaning of various parts, and most recently, the incorporation of a relatively new technology -- molecular adsorbers -- into the basic design of future replacement instruments. Molecular adsorbers were included as part of the wide field/planetary camera 2 (WFPC-2) instrument, which was integrated into the HST during the servicing mission 1 (SM1) in 1993. It is generally recognized that these adsorbers aided in the reductio of on-orbit contamination levels for the WFPC-2 instrument. This technology is now being implemented as part of the basic design for several new instruments being readied for the servicing mission 2 (SM2), scheduled for early 1997. An overview of the concept, design, applications, and to-date testing and predicted benefits associated with the molecular adsorbers within these new HST instruments are presented and discussed in this paper.

The use of quartz crystal microbalances (QCMs) for the measurement of the amount of mass flux that is deposited on a surface for space applications has historically been limited to the use of crystals having a resonant frequency of, at most, 15 MHz, because of the difficulty of working with the small dimensions in thickness that are associated with such crystals. This has limited the lower mass flux measurement to approximately 10^-11 g/cm²-s, or 0.20 angstrom/Hz, if the condensate density is near unity. Until recently, this has been a sufficiently precise measurement of molecular flux to satisfy the needs of the experimenter. However, the growing need for the precise measurement of, for instance, the erosion/deposition rate of ion thrustors, the erosion rate on low-orbit satellites and the precise measurement of outgassing over long periods of time, has necessitated increasingly lower mass flux measurement, translating into higher mass sensitivities. With the trend toward reduced satellite size, there is a corresponding need to dimensionally miniaturize the QCMs in order to place them into even smaller spaces. A new series of QCMs and TQCMs (thermoelectrically-cooled QCMs) with crystal frequencies upward to 25 MHz, has recently been developed. These are not only physically much smaller than earlier models, but also extend the mass sensitivity range upward by a factor of 4.84 over the 15 MHz theoretical value of 5.102 multiplied by 10⁸ Hzcm²g, lowering the limit of discernible condensate thickness measurement to approximately 0.04 angstrom/Hz. The new TQCMs also increase the effective (Delta) T, i.e. the temperature differential between the hot and cold sides of the Peltier in the TQCMs from 86 degrees Celsius to 120 degrees Celsius, causing the lower temperature of the crystals to be between minus 75 degrees Celsius and minus 100 degree Celsius when the QCM is operated at ambient temperature. Tests conducted under simulated space environments using these new miniaturized QCMs are the subject of this paper.

A 50-cm cryogenic mirror at one end of an aluminum telescope was successfully cleaned by nitrogen snow in a series of demonstration tests. The mirror was maintained below 70 Kelvin under vacuum during the cleaning, with a 15 Kelvin cold cap pumping the nitrogen gas to maintain a realistic space environment. The snow was produced by an assembly of 6 nozzles and valves attached to the exterior of the telescope. The nozzles protruded less than 1 cm into the telescope and were well outside the mirror diameter. Contamination of the mirror was produced by silica and alumina dusts propelled into the telescope by special velocity-moderating sources. Cleaning effectiveness was measured by scatter of 10.6-mu laser light at 2 degrees from three spots on the mirror surface. All scatter system components were exterior to the telescope, with only small holes for the passage of laser radiation. The clean mirror BRDF of 5 multiplied by 10^-4 sr^-1 was raised as high as 3 multiplied by 10^-2 sr^-1 by the contamination process and subsequently reduced to the original level by one or two seconds of nitrogen snow spraying. Nitrogen snow cleaning under vacuum proved much more effective than carbon-dioxide snow cleaning of the same mirror in air.

The fraction of neutral molecules transmitted through a restrictive passage in molecular flow can be highly dependent on the shape of the reservoir from which the gas originates. A particular reservoir shape was investigated to determine its effect on the angular distribution of molecules entering two types of simple restrictive passages. It was determined that a reservoir consisting largely of two-dimensional shells imparted a bimodal distribution to the reservoir gas, resulting in lemniscate flux distributions for the gas entering a baffle network. A Monte Carlo code was developed and compared to analytical transmission probabilities calculated by Clausing. After establishing the validity of the code, the effects of lemniscate and Lambertian flux distributions for molecular flow were simulated for two-dimensional channels and right- angle bends for various length-to-height ratios. It was determined that the shape of the entrance distribution can play an important role in the calculation of transmission probabilities.

The industry-standard, free-molecular contamination code MOLFLUX (molecular flux) developed for NASA has been used for many years by most aerospace contractors to predict on- orbit spacecraft surface contamination levels. Recently, MOLFLUX (version 2.0) which was written for a VAX computer was converted for use on a PC running Windows NT. Both the graphical-user-interface (GUI) menuing portion (written in C) of MOLFLUX), and the physics portion (written in FORTRAN) were fully converted. Subsequent to the conversion task, an effort was initiated to validate the PC version of MOLFLUX for two major spacecraft systems. A new free-molecular contamination code recently developed at TRW, CONFLUX (continuous flux) is being used to validate MOLFLUX. CONFLUX provides a 'nearly-exact' solution to the 'reduced' free- molecular contamination problem by permitting molecules to bounce from surface-to-surface with no bounce limit. CONFLUX is also double-precisioned for higher accuracy. Excellent comparisons have been made between MOLFLUX and CONFLUX on the AXAF and EOS spacecraft systems (discretized into hundreds of surface elements). Both models are also being verified against simple systems having closed-form analytic solutions. In addition the S-cubed environmental work bench (EWB) is being tested on the EOS spacecraft system. All comparisons are still in a preliminary state.

This paper describes a finite-element numerical approach for predicting the time-dependent particulate motion of released particulate that complements the standard particulate redistribution model for predicting contamination levels of exposed payload surfaces during the ascent-phase. The particulate redistribution model is used to give a spatially and time averaged estimate of the contamination level during ascent that is considered to be very conservative. All particulates are assumed to be retained in the fairing/payload volume with none venting out of the volume. The new numerical model takes more comprehensive approach and attempts to estimate the time-dependent particulate surface-impingement flux by considering the key physical mechanisms believed to govern the gas/particulate transport problem. It solves the time-dependent fairing/payload gas transport (venting) and particulate transport problem during the ascent-phase. The gas transport model is used only up to point where gas flow no longer significantly impacts the partiality motion. Thus, only the continuum fluid regime is considered in the model. It is possible to treat particulate release beyond this point, but it has not been considered in the modeling effort described in this paper. Booster acceleration, gas motion during venting and particulate drag (due to gas flow) are treated. Released particulate trajectories and particulate impingement fluxes are predicted, and subsequent surface contamination levels are estimated. Results are presented for both Pegasus/TOMS and EOS during ascent. The final contamination levels predicted are considered to be the initial on-orbit particulate contamination levels.

In an orbital environment, outgassed, offgassed, ambient, and vented molecular species can cause spacecraft performance degradation by direct impingement or by scattering with each other in the gas phase before and after reemitting from the spacecraft surfaces. This can cause detrimental conditions for the spacecraft surfaces and sensors who view through this cloud. Likewise particulates released by the spacecraft, or nearby systems, can deposit on surfaces, intercept optical lines-of-sight or recontact the vehicle later on in the mission, causing surface damage. This paper presents two models that have improved capability over previous similar models. The integrated spacecraft environments model (ISEM) can be used to predict the transport of spacecraft generated molecular species as they collisionally interact with each other and the ambient molecular environment. An ISEM analysis produces a three dimensional mapping of molecular density and velocity by molecular species, within a modeling volume defined by the user. This model is the next generation of the SPACE and Molflux contamination models and was developed through NASA funding. The orbital particulate trajectory model (OPT) can be used to predict the particulate locations of released particulates as a function of time relative to the spacecraft coordinate system as well as an Earth coordinate system. In this paper the capabilities, applications, validation, and availability of the (ISEM) and the (OPT) model are discussed.

in mid-1994 the Space Telescope Science Institute (STScI) questioned the Hubble Space Telescope (HST) Project at NASA's Goddard Space Flight Center about the feasibility of using HST to observe the planet Venus. To assess the mission safety of such an observation, a worst-case analysis of the effects of exposing the inner forward portion of the HST's light shield to direct sunlight was performed. The results, the specifics of the actual observation plan, and the implementation of operational protections formed a rationale for proceeding with the science observations on January 24, 1995.

The total pressure sensor (TPS) is one of ten contamination sensors aboard the midcourse space experiment (MSX) satellite. The TPS measures both the natural and spacecraft induced pressure environments. This paper presents a first look at the TPS data from the early operations phase of the MSX mission. Flight data are show to be in good agreement with the external contamination model predictions for MSX. TPS fluctuations are shown to be consistent with the venting characteristics of the Spirit III cryogenic cover. Data are presented which characterize and confirm the tumbling nature of the receding Spirit III cover upon its release. Finally, flight data over an orbital period are shown to conform to a bimodal pressure profile.

Results are reported on the measurements obtained from the neutral mass spectrometer (NMS) during the early operations phase of the midcourse space experiment (MSX) mission. During this phase the dominant species present around the spacecraft was argon that originated from the sublimation of the solid argon cryogen used to cool the SPIRIT III aperture door. The major contaminant species was water vapor which outgassed from spacecraft surfaces that were in the NMS field of view. Organic species were generally below the detection limit. Inorganic species were detected that may have come from the NMS cover during deployment. In addition to the on-orbit results, the operation and specifications of the NMS are briefly described along with ground-calibration data.

The xenon flashlamp is one of a suite of instruments that monitor the particulate and gaseous contamination environments of the midcourse space experiment (MSX) spacecraft. The near-field particulate measurement comprises the high intensity xenon flashlamp that illuminates a volume of space in the field of view of the UVISI wide field of view visible imager (UVISI IVW). Radiation scattered by illuminated contaminant particles is imaged by the IVW. The intensity of the radiation is related to a particle's size and composition. The particle's track yields information about its velocity and trajectory. From ground calibration data we estimate a sensitivity to detect particles smaller than 1 micrometer and to determine cross-field velocities from 1 mm/sec to 50 m/sec. The visible radiation measurement of the particulate environment provided by the xenon flashlamp and UVISI IVW is complemented by multiband IR, UV, and visible measurements by other MSX sensors. The early mission data from this experiment will quantify the relationship between ground contamination control measures, the on-orbit contamination environment, and the performance history of on-orbit sensors.

The krypton radiometer (KR) is one of a suite of instruments that monitor the gaseous and particulate contamination environments of the midcourse space experiment (MSX) spacecraft. The krypton radiometer measures the local water density in a volume of space approximately 50 cm from the spacecraft near its +X/+Y/+Z corner. The instrument comprises an array of krypton VUV lines source lamps that dissociate water and a near UV radiometer that detects the chemiluminescence from the OH dissociation products. Ground calibrations indicate that the instrument has sufficient sensitivity to detect water densities as low as 1.5 multiplied by 10⁷ molecules cm^-3. Water is the primary outgassing species during the early part of a spaceflight. Water deposition is also a particular concern to cryogenic sensors, such as the spatial infrared imaging telescope III (SPIRIT III) on this spacecraft. As the mission progresses, we will correlate the KR measurements of the water density with measurements by the neutral mass spectrometer, total pressure sensor and cryogenic quartz crystal microbalance. Using the MSX external contamination model we will create a complete description of the MSX water environment including outgassing, return flux and deposition, and effects.

The midcourse space experiment (MSX) satellite was launched into a 903 Km, 99.4-deg orbit April 24, 1996. It carries imaging spectrometers and radiometers that operate in the UV, visible, and infrared spectral ranges. In addition, it carries several contamination measuring instruments that are being used to characterize the contamination environment on, in, and around the satellite. Five are quartz crystal microbalances (QCMs), four of which are temperature- controlled (TQCMs). They are located on various external surfaces of the spacecraft and are operating at minus 40 degrees Celsius to minus 50 degrees Celsius to measure the condensation of silicone and organic molecules. One is a cryogenic quartz crystal microbalance (CQCM) which is located adjacent to the SPIRIT III infrared cryogenic telescope primary mirror. Its temperature followed the mirror which cooled from 28 to 20 K during the first week of operation. All QCMs recorded deposition in the 10 - 20 ng/cm$2)-day (1-2 angstrom/day) range. Thermo-gravimetric analyses on the QCMs provided insight into the amount and species of contaminants condensed. Data from the QCMs and other instruments in the contamination experiment (CE) suite played an important role in determining when it was safe to open covers on some of the optical instruments.

Particles generated from spacecraft surfaces will interfere with the remote sensing of emissions from objects in space, the earth, and its upper atmosphere. We have previously reviewed the sources, sizes, and composition of particles observed in local spacecraft environments and presented predictions of the optical signatures these particles would generate and presented predictions of the signatures of these nearfield particles as detected by spacecraft optical systems. Particles leaving spacecraft surfaces will be accelerated by atmospheric drag (and magnetic forces if charged). Velocities and accelerations relative to the spacecraft x,y,z, coordinate system allow the particle to move through the optical sensors' field-of-view after they leave the spacecraft surfaces. The particle's trajectory during the optical system integration time gives rise to a particle track in the detected image. Particles can be remotely detected across the UV-IR spectral region by their thermal emission, scattered sunlight, and earthshine. The spectral-bandpass-integrated signatures of these particles (dependent upon size and composition) is then mapped back onto the UV, visible, and IR sensor systems. At distances less than kilometers, these particles are out of focus for telescoped imaging systems. The image produced is blurred over several pixels. We present here data on the optical signatures observed after the mechanical doors covering the MSX primary optical sensors are removed. This data represents the first observations by these sensors on-orbit, and must be treated as preliminary until a more careful review and calibration is completed. Within these constraints, we have analyzed the data to derive preliminarily positions and trajectories.

Initial results from the midcourse space experiment (MSX) ion mass spectrometer (IMS) is presented. The IMS is designed to measure the concentration of contaminant and ambient ionospheric ions from 1 - 56 amu. In addition, the IMS is used to infer the floating potential of the satellite. Ambient ions observed in the IMS data include H⁺, He⁺, N⁺, and O⁺. The instrument was operated in a 'contaminant ion' mode during the early-operation phase (first seven days) in which the instrument was placed in its most sensitive state. This was done to search for contaminant ions, produced through charge exchange reactions between neutral water vapor and ionospheric O⁺ ions moving at 7.5 km/s relative to the satellite. Mass peaks near those expected for H₂O⁺ were observed. Finally, the initial results on the satellite charge inferred from changes in the ion current collected as a function of the stopping potential are presented. It is found that the satellite potential floats between minus 6 to minus 10 V with respect to the plasma potential.

The sources of molecular deposits on a temperature- controlled quartz crystal microbalance (TQCM) attached to the shuttle bay wall at some distance from the payload and pointing out of the bay have been investigated. Explored as possible sources are: (1) the outgassing of the shuttle tiles treated with the water proofing di-methyl-ethoxy- silane (DMES) compound, scattered back by the ambient molecules into the bay; (2) The ambient scattered return flux of the outgassing from the RTV 560 silicone which is used as the adhesive for the tiles; (3) the ambient scattered return flux of the payload outgassing; (4) the return flux from outgassing source originating near, below, or adjacent to the package containing the TQCM; and (5) the self-scattering of venting plumes and outgassing above the monitor package depositing on the TQCM. In all of these sources, the magnitudes of the outgassing, of the leakage rates, and venting had to be estimated in the absence of data. The resulting analyses which can be useful for similar investigations, indicates that the outgassing from the tiles, the RTV, the payloads, or the shuttle may not have been the sources of the deposit (about 1 (mu) g/cm² during the period from bay door opening and payload release from the bay). It appears more probable that the origin is the self- and/or ambient-scattered return fluxes originating near, below, or internal to the monitor package.

A spacecraft was instrumented with five calorimeters and four temperature controlled quartz crystal microbalances. The calorimeters which are fitted with identical second surface silvered fused silica mirrors of the type frequently used for sun-illuminated spacecraft radiators, serve as contamination effects detectors. Two of the minus 50 degree Celsius TQCMs are located near four of the calorimeters. Over a four year period, the five calorimeter temperatures have increased because accumulated deposit of molecular contamination (from the outgassing spacecraft) has increased the solar absorptance of the mirrors. One, with a significant field of view of the spacecraft, has increased far more than the others which have either little or no spacecraft surfaces in their fields of view. The increases in calorimeter temperatures have been converted into increases in solar absorption. The ratios of these quantities range between 8 multiplied by 10^-3 and 1.4 multiplied by 10^-2 solar absorptance units per (mu) g/cm² deposit. These values are consistent with industry practice of designing thermal systems with the expectation that sunlit surface solar absorptance will increase about 0.01 for every 100 angstroms of molecular contamination accumulated. Since the calorimeters were significantly warmer than the TQCMs, an even larger value of this ratio can be supported for conservative design of critical thermal systems.

A new European Space Agency (ESA) flight instrument attached to the exterior of the MIR Space Station is providing a better understanding of the effects of the space environment. The instrument is measuring, real time, the impacts and trajectory of hypervelocity particles, the atomic oxygen flux and any contamination deposition/effects during the course of the mission. The ESA mission, EuroMir '95, began in September 1995 and was completed in March 1996. Active data from the quartz crystal microbalances confirm the existence of a severe gaseous environment. The mission has also allowed for an EVA which will return passive materials to earth for subsequent laboratory analyses. These data are considered quite germane due to the similarity in orbital altitude and inclination of the Mir and Alpha Space Stations.

In preparation for the Hubble Space Telescope (HST) second servicing mission, hardware which was assembled a decade earlier was refurbished and cleaned to meet a requirement more than an order of magnitude cleaner than the original requirement. The fine guidance sensor (FGS) radial bay module is located in ]close proximity to the HST science instruments; therefore the condemnation sensitivity of the second servicing mission science instruments necessitated the establishment of new FGS contamination requirements. These new requirements are based on a critical optics temperature of minus 88 degrees Celsius; the original FGS outgassing requirements were based on protecting the HST primary mirror, which has an average temperature of plus 10 degrees Celsius. A contamination reduction plan was devised, implemented, and refined, resulting in partial deintegration of the FGS, the use of molecular adsorbers, and the use of a bakeout temperature within 1 degree of Celsius of the maximum survival temperature of the hardware. Final contamination measurements are within 3% of the predicted levels and meet the second servicing mission contamination requirements.

After the first Hubble Space Telescope (HST) servicing mission, the WFPC-1 and HSP instruments were returned to earth. Three optical surfaces from these instruments were analyzed in detail. They were the WFPC-1 pickoff mirror, the WFPC-1 aperture window, and the HSP filter assembly, all of which faced the central hub area of the HST. Hub-facing optics were of particular interest because any degradation in their performance might indicate a changed environment within the telescope itself. The pickoff mirror reflectance and aperture window transmittance were both found to be severely degraded in the far UV. The cause of the reflectance loss was contamination; the pickoff mirror was covered with a contaminant film about 450 angstrom thick, and the aperture window and HSP filters each had about 150 angstrom. The contamination contained multiple chemical species, some of which had been photopolymerized by exposure to earth-albedo UV. A UV-stimulated deposition and polymerization mechanism was posited. This contamination process is not expected to happen, however, for current and future instruments in HST. The HST components outgassed for 3.5 years before the first servicing mission, so the contaminants are no longer present in any appreciable quantity. Steps are being taken to ensure that any new equipment installed in the HST will not outgas. Over 2.5 years of operation, neither the WFPC-2 instrument nor the corrective optics module (COSTAR) has shown performance degradation in the UV There is also no evidence that the primary or the secondary mirror of the HST has changed.

The physical processes involved in the permanent deposition of contaminant molecules on sunlit spacecraft surfaces have been explored in greater depth by a series of measurements in a ground test facility. Solar simulation was provided at narrow line wavelengths of 0.12, 0.18, and 0.25 (mu) by the use of low-pressure hydrogen and mercury lamps instead of the broader-spectrum krypton and xenon lamps used in previous studies. Surface coverage was varied from sub- monolayer to bulk both by surface temperature change and by changes in the incident molecular flux. Photofixing of contaminants deposited on cooler surfaces in the dark was measured by warming the surfaces after periods of ultraviolet irradiation. For the model contaminant DC-704, photodeposition rates at 0.12 (mu) were about 6 times larger than 0.18 (mu) . This calls into question the practice of comparing ultraviolet simulators to the solar ultraviolet in terms of total photon incidence below 0.2 (mu) . Proper ground simulation of contaminant photodeposition may require the use of Lyman-alpha lamps for certain materials. The results of this work also indicate that even at constant surface coverage of unreacted contaminants, the photodeposition rate is strongly dependent on surface temperature.

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.

Contamination of bare and coated substrates leads to strong degradation of their optical properties. We have developed an apparatus allowing precise positioning of a sample and simultaneous mapping of absorptance and scattering of a selected area on its surface. Thus we have characterized different cleaning procedures of bare substrates by mapping surface absorption. We have shown that cleaning processes displace polishing and cleaning residues without completely removing them. However, in all cases, multicomponent glasses (such as BK7) have a mean surface absorptance higher than that of fused-silica. Then we have studied coated substrates. For a coating on a multicomponent glass we have shown evidences of a photoinduced absorption in the substrate due to solarization during the evaporation process. Moreover metallic ions migrate from the glass substrate and gather at the film interfaces. We have developed a method of front and back illumination of samples in order to separate each part of the in-depth absorptance of single layer coated glasses that is to say bulk and interface absorptances of the film, and substrate absorption due to solarization. In all our measurements, absorptance of the substrate's solarized area prevails and, in the layer, absorptance at the interfaces is much higher than that of the bulk material. On contrary, for low absorbing films deposited on fused-silica substrates our results show that absorption of the film simply adds to that of the bare substrate. That is why we can observe precise correlation between absorption mappings of the same area on the bare and on the coated substrate.

Contamination-induced degradation of optics often leads to enhanced absorption, which presents a serious limit for many important applications, especially those associated with high power laser systems and/or large aperture components. For this reason many different techniques were developed during the last decade for weak absorption studies, which include laser calorimetry, photoacoustic spectroscopy, as well as various photothermal techniques. In this paper recent progress is presented for the surface thermal lensing (STL) technique, a novel photothermal method which is demonstrated to be an ultra-sensitive tool for monitoring contamination effects on optical components. Compared with the various conventional photothermal methods, such as the photothermal deflection technique, STL drastically reduces the experimental complexity but retains the advantages of being sensitive, accurate and non-contact, and capable of in-situ monitoring of optical absorption down to the sub-ppm level. Experimental data with emphasis on absorption measurements and contamination studies of optical thin films are presented.

Stellar Optics Research International Corporation (SORIC) has compiled one of the world's largest resources of spectral reflectance and bidirectional reflectance distribution function (BRDF) data for black, white and reflective surfaces and materials. Most of the data has come from formerly proprietary, private sources in the U.S.A., Canada, and the former U.S.S.R., as well as from international, publicly accessible, and private research papers and reports. From this databank, SORIC is creating a series of full-featured, interactive database products available for use with PCs. The first data module that will be released is for BRDF data and the properties of black and white surfaces and materials commonly employed within or viewed by ground-based and space-based instrument applications. The surfaces and materials serve the functions of: (1) stray light control, (2) thermal control, (3) calibration, and (4) visual target cues. The associated BRDF data module can be used with stray light analysis packages, or in conjunction with the black and white surfaces and materials data module. This paper presents an update on the overall database project, and its data modules.

MERIS is a passive optical instrument, that will fly on the first polar orbiting Earth observation mission ENVISAT. The development of this instrument is currently carried by an international team led by AEROSPATIALE. The instrument primary mission goal is to monitor bio-optical ocean parameters on a large scale. Secondary goals of MERIS include atmospheric investigation on cloud and aerosols parameters and on land surface processes. The instrument will acquire 15 spectral images, programmable in width and position with a spectral sampling interval of 1.25 nm within the visible spectral range of 390 nm to 1040 nm. MERIS images will have, a swath width of 1100 km and spatial resolution of 300 m. The high radiometrical (1 to 5%) and spectrometrical performances (1 nm) will be provided by an on board calibration system. This paper describes the straylight design of the instrument and associated performances in order to obtain the high level of radiometric resolution.

A coherent heterodyne detector was built that provides high angular resolution and near-specular angle measurements of the bidirectional scattering distribution function (BSDF). It extends the range of covered spatial frequencies by more than one order of magnitude towards lower frequencies.

The performance of a star sensor is often limited by the stray light level on the detector and by non-uniformity and slopes of the corresponding irradiance. One of the major contributors to this unwanted noise is the baffle, which is frequently designed as a single equipment: its geometry is derived using simple rule to block first and second level scattering of stray light flux coming from external sources at off-axis positions defined by the mission. The geometry of the baffle is constrained by mechanical engineering requirements which confine its overall dimension. Better system performances are obtained when the early design of the baffle can take into account the angular response of the camera.

The fine error sensor on the Lyman FUSE instrument has been analyzed for stray light. It was concurrently analyzed using both APART/PADE and ASAP. This paper shows the results of the stray light analysis, shows some limited BRDF measured data, and presents the baffle design of the instrument. A comparison of the strengths and the weaknesses of the two stray light analysis codes is also given.

The moderate resolution imaging spectroradiometer (MODIS) will be one of the primary instruments observing the earth on the Earth Observing System (EOS) scheduled for launch at the end of this decade. Its 36 spectral bands ranging from 0.4 to 14.4 microns will image the entire earth. The radiometric requirements are such that the contributions to the optical point spread function from optical ghosts, scattered light and cross-talk in the very small spectral filters, located near the focal planes, must be characterized very well and kept to a minimum. This paper describes a series of Monte Carlo simulations of the propagation of light through the optics in order to determine the extent of the ghosting and scattered light. These results are also compared with experimental data.

Space-borne instruments must be designed with proper regard for the opto-mechanical properties of the materials that are actually used in the final construction of the instrument. An optical design that demonstrates excellent imaging properties in the computer model may be corrupted by the very materials used to fabricate and mount those optics, yielding actual performance that is less than that promised by the original design. In this paper, we review lessons learned in the retro-fit of the Hubble Space Telescope with the 'COSTAR' corrective optics, and how we are applying what we learned on COSTAR to the next generation instruments STIS and NICMOS. A compendium of BRDF data for materials used on all four instruments is summarized in the appendices of this paper and is offered on DOS diskette.

Experiments have been performed to measure the optical properties of several black surfaces after baking in air. Ten pieces of aluminum sheet measuring about 25 mm square were processed to produce Martin Black (MB). The final processing step in making Martin Black is 'sealing' by immersion in hot water. Another ten pieces of aluminum sheet were processed to produce enhanced Martin Black (EMB). EMB is sealed by immersion in steam. Four samples each of MB and EMB were not baked and served as references for the changes that might be caused by baking. The remaining samples were baked in an oven for one hour each. The baked and unbaked Martin Black coupons were characterized by a variety of optical tests including spectral reflectance in the visible and near infrared (0.4 to 2.5 micrometer) and middle infrared (5 micrometer to 20 micrometer) and directions scattering in the visible. Data in the waveband from 2 to 5 micrometer were very noisy and are not reported here. Scattering tests were performed with a BRO FASCAT automated scatterometer using HeNe laser light at 632.8 nm. Baking either Martin Black or enhanced Martin Black in air at temperatures up to 340 degrees Celsius has no consistent effect on reflectance: the material remains black and diffusely reflecting. Baking causes small changes in reflectance but there is no pronounced effect due to the baking process. Baking Martin Black does not produce enhanced Martin Black.

Martin Black and enhanced Martin Black samples were heated above 620 K and also cooled to 77 K while the directional reflectance and emittance were measured in the spectral region of 1.35 to 26 micrometer. Little emittance variation was found for both surfaces below 300 K. From 77 K to 315 K Martin Black emittance was 98.5% or greater from 7.5 to 24 micrometer. Similarly, enhanced Martin Black emittance was 96% or greater. Furthermore, these conditions apply up to 620 K. Significant reflectance variations below 7.5 micrometer were observed at ambient temperature after baking samples at 620 K both in air and vacuum environments. Reflectance variations as a function of temperature from 300 to 620 K were measured. Humidity and vacuum exposure effects on the surface reflectance properties were also investigated. Post-backed sample reflectance near 5 micrometer was extremely sensitive to ambient air exposure.

Ball Aerospace is currently designing and building several instruments with strict stray light requirements in the ultraviolet (UV) region. Available BRDF data in this spectral region is very limited, so Ball modified a UV monochrometer to measure the BRDF of critical surfaces. Measurements with the monochrometer were also possible in the visible, so results in the visible were compared to results using a TMA scatterometer. Results from the two instruments compared reasonably well and were used to generate correction factors for the UV BRDF data. Surfaces measured included graphite fiber reinforced cyanate (GFRC), Desoto black paint on GFRC, spectralon, and an aluminum diffuser.

Grazing incidence reflections as a source of stray light are a problem which continues to beleaguer optical systems and instrumentation. These reflections tend to be specular and are a primary cause of ghosting. Traditional means of blackening (absorption) fail miserably. Techniques of scattering the undesirable/problem light into a larger (and more benign) solid angle, while successful, are often impractical. Furthermore, while these techniques excel at reducing ghosting, they typically redirect significant light into the diffuse background, reducing the SNR. Black flocking combines the advantages of absorption and scattering. Historical disadvantages of flocking are its poor durability and the difficulty of applying flock to irregular surfaces. Presented here, is the technique of electrostatic application, which overcomes these shortfalls. BRDF (bi-directional reflectance distribution function) measurements of black flocking are presented and comparisons made with other blackening techniques. An example of this technique is shown where it is used to improve a low-light spectrographic instrument. Finally, proposed specifications for the application of (black) flocking are made for use in optics.

A number of common building materials and finishes are found to be phosphorescent. To avoid this as a source of stray light, it is suggested that with sensitive low-light level systems, material selection be done judiciously and with cognizance of this phenomenon. Luminous radiance and decay properties for various materials are presented. The common causative ingredient of phosphorescent materials is identified as titanium di-oxide (TiO₂), an increasingly popular and widespread white pigment. Techniques used to discover and check questionable articles are presented.

Narcissus is a stray light problem for infrared imaging sensors. Control of narcissus is a requirement for designing scanning sensors and narcissus analysis tools are available in some optical raytrace programs. These tools have not been optimized for staring sensors. Narcissus is assumed to be unimportant in staring array designs because the shading effects can be removed by offset correction of the detector array data. This would be sufficient for sensors which meet the following conditions: (1) calibrated at the entrance aperture, (2) operate at a constant optical housing temperature, (3) no movement of lenses for focus or change in field of view. Narcissus may be noticeable for sensors not meeting these conditions. We have developed procedures for applying existing narcissus analysis tools to staring sensors. A staring array prototype FLIR has been analyzed. Laboratory tests have confirmed the narcissus analysis.

The lack of measured surface scatter data at extreme ultraviolet (EUV) wavelengths has been commented upon by a number of authors. The need for such data arises primarily in the area of stray light design and analysis. Most stray light software requires knowledge of the bidirectional reflectance distribution (BRDF) for relevant surfaces. Without this measured data, quantitative assessment of stray light effects is difficult, making the confident prediction of instrument performance almost impossible. The Goddard EUV scatterometer was designed to perform such measurements, and the geometry, important design issues, and performance are discussed. This instrument is capable of plane-of-incidence BRDF measurements at EUV wavelengths between 58.1 nm and 121.6 nm, with a lower measurement limit of approximately 10^-5 sr^-1, and is able to accommodate angles of incidence between 10 and 75 degrees. The scatterometer can measure scatter to within 1.5 degrees of the specular beam, and the scatter angle can be measured to within 0.1 degree. BRDF data is presented for the commonly used visible-diffuser material white Spectralon SRS-99 at 121.6 nm; and for common baffle surfaces Martin Black, black Spectralon SRS-02, and an evaporated Cu black at wavelengths of 58.4 nm and 121.6 nm and angles of incidence of 15 degrees and 45 degrees. The BRDF distribution of a 3000 line/mm Al diffraction grating is also presented.

For some stray light applications, it may be advantageous to use a black applique rather than a conventional black coating. Appliques consist of a free-standing sheet of black material and an adhesive or other means for attaching the applique to a substrate. In this paper the optical scatter in the visible and infrared of black appliques from Battelle, Dupont, Edmund Scientific, Energy Science Laboratory, Inc. (ESLI), Rippey and Rodel is reported and compared to Martin Black. The Rippey and Rodel appliques are sold as polishing cloths for the semiconductor industry, whereas the ESLI applique was originally developed as a low sputter yield coating. The Battelle applique consists of a carbon loaded polyurethane film with a surface which is heat molded into a micro-grooved pattern. The ESLI applique consists of high aspect ratio fibers mounted in an adhesive base and was the blackest applique of all those investigated. For an incidence angle of 10 degrees, a scattering angle of 45 degrees and a wavelength of 632.8 nm, the BRDF of the best ESLI applique was 3 multiplied by 10^-4, compared to 1 multiplied by 10^-3 for the best Battelle applique, 1.5 multiplied by 10^-3 for a representative Martin Black sample and 1.8 multiplied by 10^-3 for the Edmund applique. The Battelle applique is quasi-diffuse due to its surface microstructure, with a higher BRDF (2-5 multiplied by 10^-3) at scatter angles less than 15 degrees For a wavelength of 10.6 micrometer, an incidence angle of 7.5 degrees and a scatter angle of 45 degrees, the BRDF of the ESLI coating (1 multiplied by 10^-3) was slightly higher than Martin Black (8 multiplied by 10 ^-4), with the Battelle applique exhibiting strong dependences on scatter angle and groove orientation. In the 2 - 14 micrometer spectral range, the directional hemispherical reflectance of the ESLI coating at a 20 degree incidence angle is below 0.45% and only weakly dependent on incidence angle to 60 degrees. In- plane and cross-plane BRDF measurements at 3.39 micrometer are reported on a 'biased' ESLI coating which is designed for use at near grazing incidence. In-plane BRDF measurements at wavelengths of 0.6328 and 10.6 micrometer are reported for most of the appliques studied.

As part of a study being performed for the U.S.A.F. Phillips Laboratory, Ball Aerospace has extensively tested a relay mirror that flew on the Hubble Space Telescope (HST) as part of the University of Wisconsin's high speed photometer (HSP). In this paper, a review of the history of the HSP mirror is presented, including: (1) pre-flight and post- flight optical testing, (2) bi-directional reflectance distribution function (BRDF) performance before and after the post-flight removal of contaminants, and (3) chemical fingerprints of the contaminants. The BRDF data from the HST/HSP relay mirror are summarized in this paper and are also made available on DOS diskette.