KEYWORDS: Mirrors, Deformation, Particles, Space mirrors, Ion irradiation, Ionizing radiation, Telescopes, Monte Carlo methods, Electrons, Finite element methods
Dimensional changes of mirrors under spaceborne irradiation must be considered in the context of increasingly narrow tolerances. Among those factors impacting the optical figure, thermal transients are one of the main reason accounting for deformation, thus this is the reason why low CTE material (ZERODUR®) and/or heat diffusive material are exclusively considered for these applications. Radiation induced compaction can play a role in dimensional stability. This compaction effect is due to the local change of density induced by the collision of energetic particles (e.g. protons and electrons trapped in the Van Allen belt) with the mirror substrate. To address this perceived problem, we performed numerical evaluation of the irradiation levels expected over four different orbits (LEO, GEO, Sun Synchronous, L2) as seen by a ZERODUR® primary mirror. From these orbits, we then selected the harshest Earth-Orbiting conditions, namely those at GEO, and performed an equivalent lab-based e- and H+ irradiation. The effects of radiation induced deformation (compaction) onto the coupons have been then measured interferometrically for change. Finally, using our laboratory irradiation results and following a rigorous mechanical approach, we propose a upper bound for the expected deformation that would be expected with a baffled spaceborne lightweighted telescope. The negligible deformation derived in this study are consistent with decades of successful missions using ZERODUR® mirrors.
Spaceborn optics are exposed to extreme environmental conditions that may have short- and long-term effects on their performance. The periodic day/night transition over the orbit can induce strong temperature transient that may affect the optical figure due to e.g. high thermo-optic coefficients. Besides radiative heating, the ionizing radiation (high energetic e-, H+ and photons) can also damage the optics via solarization and/or compaction. Due to their low thermo-optical coefficients combined with low density, SCHOTT IRG InfraRed Glasses are a material of choice for astrospace applications. In this study we also show that these glasses are resilient to ionizing radiation. IRG 22, 24, 25, 26 and 27 chalcogenide glasses from the SCHOTT portfolio were irradiated by means of 60Co gamma irradiation at the ESA ESTEC irradiation facility to an estimated ionizing dose of 10 kGy. The comparison of the transmittance and refractive index prior and after irradiation showed only marginal changes.
The low thermal expansion glass ceramic ZERODUR® has a long and successful history in spaceborne optical applications. This material is especially used where precise shape invariance is required, i.e., for the mirror substrates dimensional stability when subject to temperature gradients and transients. In space, temperature may not be the exclusive driving force impacting the form stability; influence from ionizing radiations require considerations. The real impact of ionizing radiation on ZERODUR® has become a matter of reconciling, on the one hand, in situ experience, e.g., that the secondary mirror of low Earth orbit (LEO) Hubble Space Telescope crossing the South Atlantic Anomaly or the overall optics of geosynchronous equatorial orbit (GEO) Chandra X-ray Observatory are not reporting any specific problems related to dimensional stability at the optical form level. On the other hand, finite element simulation based on early lab experiments of ZERODUR® compaction are suggesting the opposite. This debate was brought to the forefront with the SILEX mission, where radiative ageing models were significantly overestimating the deformation experimentally observed on the lab replicas and were in even stronger disagreement with the observations collected over the mission. It has been speculated that an erroneous form factor in the physical model used to derive the phenomenological compaction law was responsible for these discrepancies. Following this hypothesis, we readdressed the effect of ionizing radiation induced by γ, electron, and proton fluences on ZERODUR® compaction. For each of these, we present and discuss the irradiation source, the experimental setup, the sample design, and the measurement procedure as well as the observations. Consistent with the feedback gathered over many different space missions, we confirm that the compaction observed is significantly smaller than the estimations available in the prior literature.
The decrease of transmittance due to electromagnetic radiation is the so-called solarization. This effect is especially well documented for energetic UV light. Typically, these radiations generate color-centers in the glass that act as absorption sites whose spectral characteristic and magnitude depend strongly on the composition. In a recent work, we demonstrated that high power blue laser light, can also lead to solarization of optical glass. In recent years blue laser based solutions (wavelengths around 450 nm) became more and more present in industrial and commercial applications. Blue laser light has in deed unique capabilities for material processing of copper gold or aluminum. Due to their very high power densities modern blue laser diodes are also a technology of choice for illumination system of high performance digital projection, e.g. for cinema and event applications. Optical glasses are widely used in optical systems of blue light laser applications. From now on, these glasses must meet the stability requirements challenged by the steadily increasing power of blue laser, de facto understanding and mitigating this solarization phenomena is now a prime technical challenge. However, until recently only limited data were available on the specific solarization behavior of optical glass under high power blue laser radiation. To this end, SCHOTT has established a dedicated laser irradiation setup to thoroughly characterize blue laser solarization effects of optical glass. Strategies have been developed to achieve blue laser solarization stable glass for demanding applications. In the present work, we shows the stability of different optical glasses against blue laser solarization and discusses the results of the stabilization approach. Aspects like saturation level of the effect, power density and wavelength dependence are discussed for N-BK7 as an example.
Spaceborne optical systems should ideally stay dimensionally stable under the conditions met over the orbital mission, e.g. stable under strong thermal gradients resulting from the instrument alternatively exposed to the sun then shielded by the terrestrial umbra and penumbra. In this situation, materials exhibiting a low coefficient of thermal expansion (CTE) mitigate the effect of thermal transients. However, deformations of the optics are not only due to thermal effects but can also be induced by environmental ionizing radiations resulting in compaction. Here we investigated the compaction of the glass-ceramic mirror material ZERODUR® under different types of ionizing radiations (γ, e-, H+) and for different deposited doses representative of the environment met over typical orbits. A phenomenological model binding the compaction to the estimated deposited dose based on the experimental data is proposed and compared with the data published.
For the past four decades, ZERODUR® glass ceramics has flown on many satellites including the prominent space missions METEOSAT, Hubble Space Telescope, CHANDRA, and LISA pathfinder. Firstly, this unique material is chosen as mirror substrate for high precision optics due to its extreme thermal stability. Its near-zero thermal expansion over a wide temperature range of several tens of Kelvin is homogenous within the entire volume. This enables control of the ZERODUR®’s surface profile at the 10-picometer level. SCHOTT has conducted comprehensive material characteristics studies on the mechanical properties and the radiation environment effects experienced in realistic orbits to define the suitability of ZERODUR® to survive both high loads during launch and to support long-term missions, respectively. Besides these advantageous material properties, this glass-ceramics can be extremely light-weighted by precision grinding enabled by the new SCHOTT competence center for machining and metrology. In this paper, we will continue to trace the recent developments of ZERODUR® glass ceramics as space material in the context of selected missions flown since 2009 and those anticipated. Major milestones such as the LISA pathfinder mission are discussed. Furthermore, we outline the viable use of ZERODUR® for great observatory architectures for IR/O/UV spaceborne telescopes. The objective of this paper is to supplement an earlier review of the use of ZERODUR® for space applications in the past decade and to summarize the latest progress and results of material studies toward very efficient production capabilities for high-performance light-weighted mirror substrates.
IR/O/UV’s astrophysics imposes highest requirements on the stability of mirrors filling 6-m in aperture. We will discuss success factors and ZERODUR®’s singular ability to address these, both for a 6-m monolith and for 6-m segmented mirror. Not only low CTE over a large temperature range, but also extreme homogeneity of CTE is needed. Homogeneity is best achieved with a monolithic substrate, without bonding or fusing of different melts. For a segmented approach, methods refined for making 949 ZERODUR® segments of the ELT ensure unparalleled consistency of pieceto-piece material characteristics.
The light flux of laser based digital projectors is constantly growing. Further, the setup gets more compact. This leads to increasing spatial dependent power densities inside color channel controlling optical systems of high-end laser cinema projectors. It was surprisingly found that high power blue laser irradiation leads to solarization of optical glass, resulting in a decrease of transmission over time. Therefore, SCHOTT started to develop blue laser solarization stable optical glass. A blue laser irradiation setup and spectral photometer measurement facility was established to characterize the solarization effect on optical glass. First very promising results of the development show that it is possible to significantly increase the stability of optical glass against blue laser solarization. This paper discusses the actual status of the development of blue solarization stable optical glass used for digital projection. Solarization results are shown as a function of time and wavelength. Additionally the intensity dependence and saturation of blue laser solarization is addressed.
The dominant selection characteristic between candidate mirror materials with acceptable strength and “polishability” is dimensional stability. 250K to 290K represents the temperature of many spaceborne telescopes. Furthermore, pointing and orbital motion, changing solar view factors create thermal transients. Traditionally, stability has been passively managed either with high thermal diffusivity materials (e.g. Al, Be, SiC) or low thermal expansion materials (e.g. ZERODUR®, ULE, ClearCeram and Carbon Fibers). Recently Kyocera introduced a high thermal diffusivity material, Cordierite CO720, with its Coefficient of Thermal Expansion (CTE) passing from negative to positive near 23C, postulating this would provide simultaneous low expansion and high diffusivity. We examine this postulate, noting both CO720’s CTE(T)’s high slope and Zero CTE at a warmer temperature than typical for space telescopes. Our conclusion from FEM simulations is that CO720 does not change the trade space
Blue laser light has unique capabilities for material processing of copper gold or aluminium. The energy absorption efficiency at 450 nm in gold is orders of magnitude higher than the absorption at typical infrared wavelengths. Modern blue laser diodes enable very high energy densities. For focusing the laser beam usually fused silica optics are used. Fused silica is known for the very high solarization stability over wide range wavelengths but optical designs are limited to its specific optical position (refractive index and Abbe number). It would be beneficial for the optical design in terms of performance and flexibility to use optical glass for such applications. Only limited data is available on the solarization behavior of optical glass for high power laser radiation at 450 nm. This paper discusses the requirements on optical materials used for blue laser processing applications regarding long time stability aspects, showing recent results in the development of solarization stable glass optical glasses.
Recent comprehensive measurements of ZERODUR®’s response to realistic levels of ionizing radiation will be reported. This will compliment an update of ZERODUR®’s “history in space”[1] as implemented in both space agencies in Europe and in the United States. Specific recommendations for use of ZERODUR® in various orbit environments will be suggested.
ZERODUR surfaces resulting from machining to spherical or aspherical forms have been investigated by visible light microscopy to identify subsurface damage (SSD). The rotation table peripheral grinding method has been used. Surface roughness (SR) of the different finished surfaces has been measured by a profilometer. We then examine the correlation between subsurface damage and roughness. This correlation allows for a precise determination and reduction of the SSD, enabling subsequent process steps such as polishing to continue with finer tool sizes resulting in a more efficient overall process.
ZERODUR®, a glass-ceramic by SCHOTT, has a successful 50-year heritage of stability in critical mirror substrates of astronomical payloads used in various orbits. This heritage includes the Hubble Space Telescope (secondary mirror) that celebrated its 30 year-in-space anniversary, and the Chandra Great Observatory (all mirrors). Although ZERODUR® exhibits a strong record of dimensional stability under orbital ionizing radiation, prior dosage measurements exhibit considerable disagreement on the magnitude of dimensional change under irradiation, and extrapolation to the level of realistic dosages presents large uncertainty. This has resulted in users of ZERODUR® needing to apply large error budget allocations for radiation effects. Our intent is to conduct rigorous measurements, matched to compaction theory, and extend measurement into lower doses realistic to mirrors in telescopes. Detailed models have been constructed, coupons designed in collaboration with the irradiation facility of the metrological institute Physikalisch-Technische Bundesanstalt (PTB) and precision optical metrology defined by Arizona Optical Systems (AOS). We will describe the models, the coupons manufacturing, the irradiation plan, and the plan defining measurement of curvature and shape change due to irradiation. The measurement data will be reconciled against the FEM results to refine accuracies in modeling. Accurate measurement in a stable set-up enables quantitative results at realistic dosages.
Observational astronomy has sought better telescopes with higher resolution from its beginning. This needs ever-larger mirrors with stable, high-precision surfaces. The extremely low-expansion glass ceramic ZERODUR® has enabled such mirrors for more than 50 years with significant improvements in size and quality since then. We provide a survey of the progress achieved in the last 15 years. Equally important as the thermal expansion coefficient CTE is its homogeneity. The CTE variation in 4-m mirror blanks lies below 5 ppb / K in radial and axial directions on large and short scales. Improved measurement capabilities allow reduced bias, which in the past made variations look greater than they were. Isotropy and uniformity of ZERODUR are outstanding. A method for lifetime calculation increases reliability considerably with respect to mechanical loads. The production and metrology capability and capacity are greatly extended. Surface figure and texture of large blanks allow starting directly with polishing. Filigree structures with up to 90% weight reduction are well-suited for space mirrors. The progress with low thermal expansion and its measurement, the insensitivity of ZERODUR against ionizing radiation in space, and outstanding application examples are presented in the first part of our review.
Observational astronomy has striven for better telescopes with higher resolutions from its start. This needs ever-larger mirrors with stable high precision surfaces. The extremely low expansion glass ceramic ZERODUR® enables such mirrors with more than 50 years of significant improvements in size and quality since its development. We provide a survey of the progress achieved in the last 15 years. The narrowest coefficient of thermal expansion (CTE) tolerance is now ±7 ppb / K. It is possible to adapt the material for lowest expansion to temperature-time courses given for special environments. At cryo temperatures, expansion is low and adaptable. Improved measurement capabilities allow for absolute CTE uncertainty of 3 ppb / K and reproducibility of 1 ppb / K (2σ). The influence of ionizing radiation on the surface figure integrity is subject to new investigations. Improved measurement capabilities increase the reliability of structure designs. Some outstanding examples are given for applications of ZERODUR in astronomy and in the very important high technology industry. The progress of thermal expansion homogeneity, the mechanical strength of ZERODUR, production capabilities in melting, precision machining, light-weighting, and dimensional metrology is presented in the second part of the paper.
Demanding new environmental requirements for earth observation, and for astrophysics, may impose ultrastability requirements on telescope mirrors. Dimensional stability of the mirror material under environmental boundary conditions is as important to the success of a mission, if not more so, than considerations of strength and stiffness. Among these characteristics is the response of the mirror material to variable thermal stimulus. It is found that not only the mirror material’s coefficient of thermal expansion (CTE) is important, but also the homogeneity of the material characteristics. Furthermore the match of thermal characteristics between the mirror material and those of metering structure contributes to maintaining a manageable error budget for operations under various view factors to sun and earth.
Satellites operating on LEO and GEO trajectories are subject to the effect of ionizing space radiations, mostly electrons, that are concentrated in the Van Allen belts. These ionizing radiations are responsible for accelerated ageing (especially compaction, i.e. local material density variation), which is thought to be detrimental for the optical figure of the embedded optical devices. The studies made on this topic during the last four decades, are proposing very different phenomenological power laws description of this effect. However, the simulated deformations derived from these laws are in partial disagreement with the observations made at the laboratory, moreover they do not account for the absence of problems reported during the space missions embedding ZERODUR material. In order to elucidate these mismatches, we defined a new experimental approach suited for the description of the compaction phenomenon for doses corresponding to typical astrospace missions. An overview of the preparatory simulation work for the design of the irradiation environment, for the design of the build-up shielding material as well as of the design of the target samples will be presented. This study will also give a short description of the experimental irradiations sequence as well as the high precision metrological approaches used in order to determine the changes induced in the ZERODUR.
ZERODUR has been and is still being successfully used as mirror substrates for a large number of space missions. Improvements in CNC machining at SCHOTT allow to achieve extremely light weighted substrates incorporating very thin ribs and face sheets. This paper is reviewing data published on the interaction of space radiation with ZERODUR. Additionally, this paper reports on considerations and experiments which are needed to confidently apply an updated model on ZERODUR behavior under space radiation for extremely light weighted ZERODUR substrates.
In a continuous effort since 2007 a considerable amount of new data and information has been gathered on the bending
strength of the extremely low thermal expansion glass ceramic ZERODUR®.
By fitting a three parameter Weibull distribution to the data it could be shown that for homogenously ground surfaces
minimum breakage stresses exist lying much higher than the previously applied design limits.
In order to achieve even higher allowable stress values diamond grain ground surfaces have been acid etched, a
procedure widely accepted as strength increasing measure.
If surfaces are etched taking off layers with thickness which are comparable to the maximum micro crack depth of the
preceding grinding process they also show statistical distributions compatible with a three parameter Weibull
distribution. SCHOTT has performed additional measurement series with etch solutions with variable composition
testing the applicability of this distribution and the possibility to achieve further increase of the minimum breakage
stress.
For long term loading applications strength change with time and environmental media are important. The parameter
needed for prediction calculations which is combining these influences is the stress corrosion constant. Results from the
past differ significantly from each other. On the basis of new investigations better information will be provided for
choosing the best value for the given application conditions.
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