The contamination control of telescopes with the straylight-rejection capability is reviewed to identify the degradative effects of contaminant phenomena on the measurements. Three levels of optical contamination are discussed including bidirectional reflectance distribution function (BRDF), point-source rejection ratio (PSRR), and nonrejected earth radiance (NRER). Measurements of degradation to low-scatter surfaces are set forth for the Zip telescope during storage and for the Cirris 1A telescope performance. PSRR measurements indicate that the Cirris 1A degraded by a factor of 15 during ground testing. A portable external BRDF station is described that measured cryogenic BRDF and BRDF degradation over the life of the Cirris 1A telescope. The optical contamination measurement described are concluded to be important to both determining the causes of degradation and optimizing telescope performance.

The Cosmic Background Explorer (COBE) was recently launched and is now acquiring spectacular science data on the cosmic structure of the universe. The instruments developed to obtain this science data are extremely sensitive to contamination. The task of the project scientists and the contamination engineers was to develop specific contamination requirements which were linked to the mission science objectives. A description of the mission the instruments the science goals the derivation of the subsequent contamination requirements an overview of the contamination control program and a brief summary of contamination program results will be presented. The COBE Project was initiated to answer basic questions concerning the formation of the universe: What was the primeval explosion that started the expanding universe? What made it uniform on such a gigantic scale? What started the formation of galaxies? What caused them to be arranged in giant clusters of galaxies? Is there a center or an edge of the universe?'' The answers to many of these ambitious questions are now being obtained by the scientific instruments on board COBE: the Diffuse Infrared Background Experiment (DIRBE) the Far Infrared Absolute Spectrophotometer (FIRAS) and the Differential Microwave Radiometers (DMRs). The DIRBE is a cryogenically cooled instrument with a primary mirror highly sensitive to particulate scattering. The FIRAS is a cryogenically cooled spectrophotometer with a sky horn also intolerant of particulate scattering. The DMRs are radiometers that have antennae with narrow throats which can become obscured due to the presence of particles. The cryogenic cooling of the DIRBE and FIRAS led to the need for relatively strict molecular requirements and the scattering and obscuration issues led to the need for extremely rigid particulate requirements. Analytical studies and testing results were used to quantify cleanliness levels that would allow for the achievement of the mission objectives. 1. THE MISSION The COBE shown in Figure 1 is a 19 foot long 13 foot diameter spacecraft weighing 5000 pounds. The spacecraft was launched in November 1989 into a near polar circular orbit 559miles above the Earth''s surface. To achieve this orbit COBE was launched on a Delta rocket from the Vandenburg Air Force Base. The COBE''s spin axis was oriented 94 away from the sun and was directed outward from Earth. To shield the instruments from the earth sun and moon''s light COBE carried a sunshade. Each instrument in the three instrument complement works in unison as will be briefly described in the following sections. 16 / SPIE Vol. 1329 Optical System Contamination: Effects Measurement Control 11(1990)

A stringent contamination control plan has been developed for the optical components of the Extreme Ultraviolet Explorer instruments whose performance in the 80900 A wavelength range is highly sensitive to particulate and molecular contamination. The contamination control program has been implemented over the last three years during assembly test and calibration phases of the instrument. These phases have now been completed and the optics cavities of the instruments have been sealed until deployment in space. We discuss various approaches used during ground operations to meet optics'' contamination goals within the project schedule and budget. We also present the measured optical properties of EUV witness mirrors which remained with the flight mirrors during ground operations. These were used to track optical degradation due to contamination from the cleanroom and high vacuum test chamber environments. 1.

This paper focuses on the determination of cleanliness levels the contamination control plan and highlights of how the plan was implemented through the build and test of the CLAES instrument. The hardware development addressed both molecular and particulate contamination concerns however this paper will concentrate on particulate contamination because of its greater impact on CLAES performance. A primary noise source is off axis scatter from the earth surface. The stratospheric altitude of interest is 10 to 60 kilometers resulting in low altitude detetion of only 0. 2 degrees above the hard earth surface. This necessitated careful attention to straylight controls throughout the design fabrication assembly and ground test of the instrument. 2.

The data from the Wide Field Planetary Camera (WFPC) are analyzed to determine the absolute source-emission parameters and contaminant reemission parameters of outgassing mass accumulation. The data correspond to the mass accumulated on a -100 C quartz crystal microbalance and then desorbed at -50 C as a function of time. The parameters investigated include contaminant mass, first-order rate constants, and characteristic energies. The data are modeled in terms of a minimum number of species characterized by source-rate and reemission-rate parameters. The modeling technique is found to produce adequate fits, and the contaminants that accumulate at -100 C and desorb at -50 C do not accumulate at -50 C. The data analysis is found to be an adequate technique for assessing outgassing parameters for the prediction of WFPC internal contamination.

The far-ultraviolet (FLJV) performance of optical imaging instruments using cooled detectors is extremely sensitive to molecular contamination. Conventionally stringent cleanliness standards for material screening hardware fabrication and assembly level test and integration are inadequate for instruments which require a stable FUV performance. A multifaceted contamination control strategy has been developed for the second generation Wide-Field and Planetary Camera (WFPC-2) to improve the FIJV stability by several orders of magnitude compared to the first camera (WFPC-l). This strategy involves: Improved on-orbit boil-off capability of the detector optics added internal shielding and instrument venting in-process subassembly vacuum bakeout at elevated temperatures material substitution sample testing in ultra-clean vacuum facility and internal instrument contamination transport modelling. A science performance goal of 1 photometric accuracy at 1470 Angstrom over an extended time (of at least 30 days) has been established as a contamination control target for WFPC-2. The WFPC-2 is currently planned to be launched by the Shuttle in mid-1993 and replace the WFPC-l which was recently launched with the Hubble Space Telescope (HST). 1.

Experimental investigations are described for ion-beam sputtering and RF-plasma sputtering to determine the effectiveness of the methods for removing contaminants from an optical surface. The effects of ion-beam sputtering are tested with an ion gun and measured by mounting a 5-MHz quartz-crystal microbalance on a sample holder and simulating spacecraft contamination. RF-plasma sputtering involves the application of an alternating electric field to opposing electrodes immersed in a low density gas, and is tested with the same setup. The energy dependence of the sputtering yields is measured to determine whether the different contaminants are removed and whether the mirror surface is affected. Ion-beam sputtering removes all contaminants tested, but also affects the mirror surface at high energies. When the correct DC bias is applied, RF sputtering can remove the contaminants without removing the metal-mirror surface.

IR laser heating is the subject of tests to determine its effectiveness as a method of removing optical contaminants with special attention given to on-orbit decontamination. A variety of mirrors are contaminated in a vacuum at a temperature of about 100 K with a BRDF diagnostic monitoring used to measure cleanliness before and after contamination and after laser cleaning. Laser treatments with CO2 and Nd:YAG lasers are investigated for contaminants such as H2O, CO2, and dust. The contaminants usually degraded the BRDF by a factor of about 2, and the laser treatments are generally able to return the BRDF to the precontamination level. The Nd:YAG laser treatment relies on heating the mirror surface and is not as effective and applicable as that of the CO2 laser. Successful cleaning can be achieved at temperatures of 35-300 K with thick contaminant films without damaging or distorting the mirror surface.

Recent results on ion beam cleaning experiments of super-polished cryogenic mirrors are presented. A variety of contaminants such as air, water, ammonia, and hexane have been studied and some of the experimental parameters necessary to remove these were determined. The mirrors were exposed to a controlled amount of a known contaminant and/or mixtures of known contaminants. Dual-wavelength (0.633- and 10.6-micron) bidirectional reflectance distribution function measurements were made on the clean, contaminated and ion beam cleaned surfaces. These data, along with quartz crystal microbalance measurements and mass spectroscopic data, are used to compare the effect of contaminants and ion beam removal of these contaminants on the scatter properties of various mirror surfaces.

Mirrors have been examined using various surface analytical techniques following experiments to remove contaminants using either a CO2 laser or an argon ion gun. Optical microscopy, scanning electron microscopy, and noncontact surface profilometry were used to obtain morphological information about the mirror surface. X-ray photoelectron spectroscopy, energy dispersive X-ray analysis, and gas chromatograph/mass spectrometry were used to investigate the composition of the mirror surface.

The cleaning of mirrored surfaces by a patented ultraviolet laser-cleaning process is effective in removing molecular and particulate contaminants. The cleaning process, developed for use in the semiconductor industry, uses a pulsed excimer laser, typically operating at 308 nm, as an energy source. The raw-beam intensity profiles is homogenized to a ''top-hat'' profile to ensure precise, controlled removal of contaminants. The technology has been shown to be effective in removing, without damage to the surface, such contaminants as water (liquid or frozen), dioctyl phthalate, diffusion pump oils, fingerprints, silicon-dioxide particles, dust, and many other potential contaminants from quartz (bare and antireflection-coated), nickel, gold, beryllium, polymeric, and other surfaces. The cleaning process is equally effective in ambient and cryogenic/vacuum environments. Beam-energy densities of less than 0.5-J/sq cm pulse are generally sufficient to achieve complete cleaning. Examination of cleaned surfaces by optical, electrooptical, and light-scattering measurement techniques show that the cleaning process returns the substrate surface quality to the original level.

Studies on the effectiveness of the jet flush in removing particle fallout and Arizona-standard fine dust on polished optical substrates have been carried out at ambient pressure and vacuum. These studies have shown that the CO2 jet flush is a viable method for removing contaminants from optical surfaces with no damage to the surface. The studies also show that the jet flush has potential for use as an on-orbit cleaning device for space optics.

The paper describes a size-selective fallout monitor that can be employed to assess the degradation of sensitive optical surfaces as well as facilitate the analysis of particle types. The device combines a vertical elutriator and a quartz crystal microbalance, and only particles greater than a specific size can pass through an upward laminar flow generated in the device. The larger particles cause a frequency shift in the crystal oscillator, thereby permitting the measurement of the fallout associated with the contamination of optical instruments.

The principle of operation of the surface acoustic wave (SAW) piezoelectric crystals used as microgravimetric sensors in mass microbalances is discussed. Special attention is given to a SAW 200-MHz crystal developed for measuring molecular deposition on spacecrafts, whose operating frequency does not depend on the thickness of the crystal. The frequency stability of the 200 MHz SAW device is better than 5 x 10 exp -9, which corresponds to a lower limit-of-detection of 3 x 10 exp -12 g for a signal-to-noise ratio of 3. A block diagram of the 200 MHz SAW mass microbalance and a schematic diagram of SAW resonator are presented together with performance data of this device.

A surface acoustic wave (SAW) crystal has been tested to demonstrate its usefulness in a space environment. The testing was done in a vacuum space chamber. The temperature of the heat sink on which the SAW was supported was varied over the anticipated temperature range that the SAW would be subjected to in space (from 100 C to LN2 temperature). The repeatability of frequency, the stability of the device and the long term drift were important features of the test. An empirical determination of mass sensitivity (mass/frequency change) of the SAW was accomplished by measuring the water-vapor outgassing rate of the chamber walls with not only the SAW but also a 10-MHz QCM with a known mass sensitivity. This test not only measured the mass sensitivity by comparison but it also allowed the measurement of the linearity of the SAW''s total dynamic range.

A Total Integrated Scatter (TIS) system was built to test the viability of a TIS instrument to be used in space to monitor damage to optical and thermal control surfaces due to the low earth environment. The systems accuracy and repeatability in detecting changes in the surface quality of various space materials after exposure to atomic oxygen was tested. A method for distinguishing roughening of a surface from dust contamination is described.

A conceptual design and supporting analyses are presented for an in situ, on-orbit particulate-contamination monitor for the AXAF High Resolution Mirror Assembly (HRMA), a six-shell Wolter-Type-I grazing incidence telescope. The monitor technique is based on the measurement of light scattered perpendicularly from the mirror surface from a near-IR laser beam inserted into the HRMA optics spirally at grazing incidence. Results of a proof of concept experiment indicate sensitivity of scatter to the fraction of a mirror surface obscured by particles (CF) to be 0.077/sr/CF, compared with a theoretical prediction of 0.096/sr/CF. A noise-equivalent CF (NECF) of 6.2 x 10 to the -7th limited by area sampling statistics was shown to be achievable in the laboratory configuration, from which an NECF below 5 x 10 to the -7th is projected for a flight implementation of the technique. Surface illumination efficiencies of 0.63 and 0.51 for the HRMA inner paraboloid and inner hyperboloid, respectively, are shown to be achievable. The monitor design employs commonly available space qualified optical components, is low in cost, and will afford continuous monitoring of HRMA contamination levels throughout the AXAF mission, from assembly on the ground through end of life in space.

Molecular and particulate contamination of spacecraft optical surfaces can be extremely detrimental to optical system performance degrading system throughput and increasing stray light background. Data that characterize the impact of various contaminants especially cryo- and photo-deposited molecular films on transmission reflection and scatter from optical surfaces are needed for allocation of spacecraft contamination budgets. A facility has been developed to measure the effects of molecular and particulate contaminants on optical component performance. The TRW Optical Scatter and Contamination Effects Facility (OSCEF) is capable of measuring the Bidirectional Reflectance/Transmittance Distribution Function (BRDF or BTDF) and specular reflectance/transmittance in ambient (ex-situ) as well as cryogenic vacuum (in-situ) environments. Light sources for scatter and reflectance/transmittance measurements include Argon ion HeNe Nd:YAG and CO2 lasers at wavelengths from 351 nm to 10. 6 um. Ex-situ scatter measurements can be performed on hardware to 30 cm diameter (1 meter with some reconfiguration) over nearly 4irsr1 of scatter angle space and to within 0. 3 degrees of the specularly reflected beam. In-situ measurements can be performed on 12. 5 mm diameter witness samples at temperatures from 20 K to 373 K while contaminants from representative spacecraft materials are cryo- and/or photo-deposited onto the sample surface. Contaminant layer thickness is monitored by a thermally controlled quartz crystal microbalance (TQCM) located adjacent to the witness sample. A xenon continuum lamp (1450 to 1 800 A) is available for photochemically

The Jet Propulsion Laboratory (JPL) Molecular Contamination Investigation Facility (MCIF) is a new facility built to support the effort to improve the Wide Field Planetary Camera''s far-ultraviolet performance. However the MCIF is expected to produce generic data on the accumulation at low collection temperatures of outgassing products from standard spacecraft materials and components and to provide specific data for future JPL spacecraft and instruments. The MCIF comprises two independent stainless steel vacuum systems each roughed by a molecular drag pump and ultimately pumped by an ion pump. Each system contains a sample isolation chamber cell with a regulated heat exchanger (-20 to 165C) for the sample to be measured. The detection system includes a residual gas analyzer and three quartz crystal microbalances (QCM) which look back at the sample isolation chamber. Two of QCM''s are thermoelectrically controlled and heat sunk to a regulated heat exchanger the third a cryogenic QCM is heat sunk to a LN2 heat exchanger. This arrangement permits the simultaneous measurement of outgas accumulation at three pre-selected surface temperatures in the range -180 (the CQCM) or -55 (the TQCM''s) to 80C. Each system has an automatic temperature control unit and all necessary temperature sensors. There is also a computerized automatic data acquisition system for each vacuum system. The operation of the MCIF will be discussed and typical results will be presented. 2.

Measurements of the BRDF values of naturally collected mirror surfaces contaminated with particles are compared with predictions from Mie scattering theory. The principles of the scattering theory are set forth, and the test apparatus is described which includes an IR scatterometer and a visible scatterometer. The three metal mirrors are contaminated by normal laboratory air for a period of 95 hours, and the particles are characterized by count and size distribution for the Mie scattering calculation. The results of the Mie data reduction are compared to the experimental measurements, and the measured BRDF is found to be in fairly good agreement with the calculations. The scattering from the particles is primarily forward scattering which degrades the BRDF significantly, but because the particle optical properties do not greatly affect the forward scattering calculations, the measurements are close to predicted values.

The design of a scatterometer used for in situ studies of optical surface contamination is reviewed, and dual-wavelength (0.633, 10.6 micron) BRDF measurements are presented which were made on molecular contaminants adsorbed onto cryogenically cooled mirrors. The results are compared to others obtained from the literature, and the BRDF increase from contaminants is analyzed using a simple theory based on Rayleigh scattering.

Based on 1) experience gained from material outgassing tests and evaluation of the Wide Field Planetary Camera (WFPC) instrument and 2) consideration for cost/schedule impacts related to performing and utilizing the results of outgassing tests a set of new standard outgassing methods is proposed for screening materials considered for use in space instruments with cooled sensors/optics/detectors. The existing outgassing standard ASTM E-595 is inadequate and inappropriate for selecting materials to be used in and around optics that are cooler than about 20 C. This paper describes some of the background assumptions caveats observations and facts that have led us to propose a set of specific test protocols. The sensitivity of any given spaceborne optical instrument to molecular contamination is obviously a function of many variables. With instruments of varying sensor/optics/detector temperatures how can one hope to standardize material outgas screening? Related to this issues of test/data conservativeness and compromise will be briefly described. Materials and processes issues such as vacuum bakeouts and diffusion as related to molecular outgassing are described. In addition a few material outgassing test results will be presented for a variety of test conditions. Selected results from JPL multiple Quartz Crystal Microbalance (QCM) testing are summarized. Standardization of more applicable and stringent outgassing tests than covered by ASTM E- 595 will go a long way toward improving the performance quality and reliability of future optical flight hardware. We

The method and setup developed to test optical support materials for the HST are presented in terms of general applications to the ambient offgassing of contamination-sensitive instruments. An ex situ vacuum UV spectrophotometer is employed to analyze optical witness samples for optical degradation related to offgassing contaminants that are condensed onto the samples. A table of the ambient offgassing-test results lists the weight loss, wavelengths, and presence of visible films on 15 materials. The offgassing initial requirement of less than 12 ppm and/or the requirement after testing of less than 0.1 ppm are not fulfilled in the cases of seven materials. The testing procedure is found to be an efficient method for screening nonmetallic materials that support spaceborne optical systems. The offgassing data can support standard outgassing data to screen candidate materials effectively.

This paper discusses a computerized data base derived from 3053 selected data sources on the subject of spacecraft contamination. The Contamination Data Base was developed at Boeing Aerospace Electronics between June 1986 and November 1988 under contract to the Air Force Wright Aeronautical Laboratories. A detailed description of the Contamination Data Base itself or a detailed review of the literature is beyond the scope of this paper. Instead the paper focuses on a discussion of the data base and how it was assembled a broad characterization of the data available an assessment of areas where data are lacking or deficient and brief conclusions and recommendations. Since completion of the contracted effort many publications have been released which address some of the most severe deficiencies. The most important recommendation is that a follow-up data base effort is required. 1.

An approximate molecular scatter model based on the Bhatnagar-Gross-Krook (BGK) theory for a binary system was developed to predict the plume return flux and molecular density distribution due to spacecraft thruster firing during orbital flight. This binary model considers intermolecular collisions involving a plume species and an ambient species and defines the onset of the BGK molecular flow to be on a plume translational breakdown surface based on density decay considerations (Bird''s criterion). With the assumption of average local collision frequency quantities the BGK quasi-linear differential equation can be decoupled into a selfscatter (collision between plume molecules) equation and an ambient scatter (collisions between plume and ambient molecules) equation both amenable to relatively straightforward numerical integration schemes. A sample molecular scatter solution for a small hydrazine monopropellant thruster (with 5-lb thrust) is included in this analysis. 1.

The transport of molecules, under vacuum conditions, from a source surface to a receiving surface is of major concern from the perspective of spacecraft contamination control. The transport phenomena involved is a complex mechanism comprising the physical characteristics of each surface, the properties of contaminant species participating, and the temperatures of both surfaces. Because of both the complex nature and the limited data available to describe such a phenomena, contamination modeling usually requires that a highly simplified engineering approach be undertaken. One area where this is particularly true is in the representation of the surface accommodation of incident molecules. When a molecule in the gas phase collides with the surface of a receiver it can either "stick" to that surface or be scattered away. Molecules accommodated by this surface become thermally equilibrated to the receiver temperature while the material that is not accommodated retains its original energy and undergoes specular reflection. The ratio of this thermally accommodated mass to the total incident mass is known as the "accommodation" or "sticking" coefficient. Most of the current theory and experimental work performed to date has been restricted to the accommodation coefficients of the rare gases in contact with metal surfaces3'10"1. UnfortUnately, the results generated by these studies cannot be made very useful to spacecraft contamination engineers who are predominantly interested in environments where contaminants are typically limited only to water and long-chain hydrocarbons. Because of this deficiency most current spacecraft contamination analyses are forced to rely on general mathematical expressions that consider the sticking coefficient to be only a direct function of the temperature gradient between the emitting and receiving surfaces. The major shortcoming of the simplified method presently in use is that it may provide an inadequate representation of the actual molecular transport occurring between surfaces. The purpose of this paper is, therefore, to study the nature of the transport mechanisms involved in the adsorption of high molecular weight gases on typical spacecraft surfaces, the overall concept of the sticking coefficient, and the quantitative and qualitative theory involved. In addition, this paper will examine some of the existing molecular accommodation data as it relates to spacecraft applications, as well as present new experimental data gathered by the Contamination Control Section of the Goddard Space Flight Center (GSFC). All this information will then be correlated and used to verify the accuracy of the most common sticking coefficient equations in use by contamination analyses.

A model is developed to predict the deposition of contaminants on the CCD sensors of the Wide Field Planetary Camera to determine expected performance parameters. Contaminant deposition due to internal instrument sources is considered as is the mechanism of transport to the CCDs themselves. The Contamination Analysis Program (CAP) model considers deposition and reemission kinetics, nodal containment sources, and internodal shape factors related to line-of-sight transport. Indirect transport is also accounted for in the CAP model by considering the effect of diffuse reflection at internodal exchanges. The effective total transport factors for each node-to-node exchange is precalculated to reduce the effective number of nodes and reducing run time of the CAP program. The method is applied to four distinct conditions and is found to be suitable for the analysis of internal self-contamination. Cold traps, venting, and vacuumlike design are factors which are found to be important for cold-sensitive sensors.

A fast atomic oxygen source has been used to bombard samples of the hydrocarbon materials graphite and polyethylene. Infrared fluorescence is readily observed above both surfaces that is consistent with the expected primary combustion products CO C02 and OH. Emitter production efficiencies (defined as the ratio of emitters to incident 0atoms) for both materials have been estimated from the observed radiation intensities. These are 4O2 for CO and for CO2 for both samples and ''iO2 for OH for polyethylene. 1.

Measurements of optical spectra of surfaces undergoing bombardment by N2 and N2+ jfl an ultrahigh vacuum environment provide information related to the origin of spacecraft glow and erosion. This work is complimentary to other measurements also carried out in our laboratory in which we utilize 0 and 0+ beams . These. efforts are part of a broad program whose goal is the understanding of interactions between surfaces and low-energy charged and neutral particles. 1.

A laboratory study has been performed by Boeing Aerospace Electronics under the sponsorship of the Air Force to investigate the effects of contamination on the electrostatic charging phenomena of spacecraft thermal blankets for the Ulysses mission. Induced contamination by the upperstage PAM-S Star-48B motor nozzle post-burn outgassing was studied for three types of electrically conductive thermal blankets: indium-tin-oxide-coated electrodag-coated and aluminized Kapton. Blanket samples were irradiated with 50 eV electrons at temperatures ranging from 25C to -100C and contaminant deposition thicknesses ranging from 200 to 3000 A (assuming specific gravity of 1. 0). The experimental results show that the charging characteristics of all three contaminated blankets are similar. Charge accumulation was observed to be a strong function of contaminant deposition temperature and a non-linear function of primary electron flux. 1.

Two aromatic hydrocarbons bibenzyl (BB) and dodecahydrotriphenylene (DTP) have been studied in an ongoing contaminant effects measurement program. Photochemical deposition of these molecules did not proceed quickly under conditions which result in the deposition of dark tenacious films from phthalate siloxane and alkene precursor molecules. Additional measurements show that DTP probably does not deposit photochemically at a substantial rate because the quantum yield for photodeposition is small not that a DTP molecule does not reside on the surface long enough to absorb light. The initial sticking coefficient of DTP appears to depend on surface temperature. Films of DTP scatter visible and near-ultraviolet light very efficiently which is consistent with the observed heats of vaporization and desorption for this molecule. 1.

The efficacy of pulsed CO2 lasers is demonstrated for remote removal of contaminants from cryogenic, low-scatter mirrors in a simulated space environment. A gold-overcoated, nickel-coated aluminum mirror was housed in a vacuum dewar and cooled to either 90 K with liquid nitrogen or 34 K with liquid-helium cryogen. Admitting a controlled leak of room-air contaminated the mirror. Bidirectionl reflectance distribution function (BRDF) measurements at 3, 6, and 9 deg monitored the amount of contamination on the mirror. Upon contamination, the mirror BRDF generally increased by an order of magnitude or more. Using an x-y scanner to raster a pulsed CO2-laser beam across the face of the mirror, successful cleaning of sizable areas (25 sq cm) of the mirror was demonstrated at both temperatures. In all cases the laser-cleaning technique returned the BRDF of the mirror to precontamination levels and showed no evidence for contaminant redeposition within the cleaned area.