Spin-cast ZERODUR mirror substrates of the 8 m class and lightweight substrates for secondary mirrors

For about 20 years SCHOTT has been supplying the glass ceramic ZERODUR, a material with very low thermal expansion. Besides many other applications, ZERODUR is excellently suited for the manufacture of mirror substrates for telescopes. About 80% of all telescopes in the western world with mirror diameters >1.8 m have been equipped with ZERODUR during the last 10 years. The development of modern astronomical telescopes is aimed at larger primary mirrors and lighter secondary mirrors.New techniques have been developed by SCHOTT for manufacture of thin monolithic mirror blanks of more than 8 m in diameter. The development of thin meniscus shaped shells using the spin-casting technique was successfully completed last year. During a test production several mirror substrates up to 4.1 m in diameter and down to 57 mm in thickness could be produced. The know-how has been acquired for the fabrication of mirror substrates of more than 8 m in diameter by the spin-casting technique.SCHOTT has also performed considerable developmental work in the field of lightweighted ZERODUR mirror substrates which can be generated using different techniques: forming of the lightweighted structure during casting, fusion of individual components to a total structure and lightweighting of a massive block by various mechanical machining methods.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.Registered trademark of SCHOTT Glaswerke, Mainz.     

High-pressure properties of diaspore,AlO(OH)

The structural compression mechanism and compressibility of diaspore, AlO(OH), were investigated by in situ single-crystal synchrotron X-ray diffraction at pressures up to 7 GPa using the diamond-anvil cell technique. Complementary density functional theory based model calculations at pressures up to 40 GPa revealed additional information on the pressure-dependence of the hydrogen-bond geometry and the vibrational properties of diaspore. A fit of a second-order Birch–Murnaghan equation of state to the p–V data resulted in the bulk modulus B ₀ = 150(3) GPa and B ₀ = 150.9(4) GPa for the experimental and theoretical data, respectively, while a fit of a third-order Birch–Murnaghan equation of state resulted in B ₀ = 143.7(9) GPa with its pressure derivative B′ = 4.4(6) for the theoretical data. The compression is anisotropic, with the a-axis being most compressible. The compression of the crystal structure proceeds mainly by bond shortening, and particularly by compression of the hydrogen bond, which crosses the channels of the crystal structure in the (001) plane, in a direction nearly parallel to the a-axis, and hence is responsible for the pronounced compression of this axis. While the hydrogen bond strength increases with pressure, a symmetrisation is not reached in the investigated pressure range up to 40 GPa and does not seem likely to occur in diaspore even at higher pressures. The stretching frequencies of the O–H bond decrease approximately linearly with increasing pressure, and therefore also with increasing O–H bond length and decreasing hydrogen bond length. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

Asteroidal agglutinate formation and implications for asteroidal surfaces

A large number of shock recovery experiments that address the ease of impact melt formation as a function of peak shock pressure lead to the conclusion that impacts at 5 km/sec into fragmental, porous surfaces will produce agglutinate-type glasses; no shock melts are produced at these velocities in dense silicate target rocks. While agglutinitic glasses dominate lunar surface soils, they are virtually absent in gas-rich, brecciated meteorites. This apparent paucity—if not complete lack—of agglutinate-type glasses is also inferred from remote IR-reflectance spectroscopy. The need to identify mechanisms that inhibit agglutinate formation on asteroidal sufaces was recognized previously and was predominantly attributed to lower projectile velocities and different gravitational environments.We will argue in this paper that additional mechanisms may be required. Specifically we propose that spall processes at a target's free surface play a major role in asteroidal surface evolution. At 5 km/sec collision velocity, a target (R_T) to projectile (R_P radius ratio of $\text{R}{}_{\mathrm{<mtext>T</mtext>}}\text{R}{}_{\mathrm{<mtext>P</mtext>}}\text{≈ 100}$ delineates the boundary between an “infinite half-space” and a “finite”-sized target. In the first case, collisional energy is expended in a pure cratering regime; in the latter, additional displacement of target material in the form of spallation products occurs. The spall volume may exceed the crater volume by an order of magnitude. Therefore fragmental impact deposits on small planetary bodies may be entirely controlled by spall products, rather than crater ejecta. Because tensile failure occurs at <0.2 GPa stress, spall velocities are measured in meters per second (contrary to crater ejecta) and therefore spallation products are efficiently retained even in low gravitational environments. Spall products are also more coarse grained than crater ejecta; they are also highly biased toward petrographically “unshocked” (<0.2 GPa) rocks.Thus asteroidal surface deposits should be more coarse grained and less shocked than lunar ones—consistent with meteorite evidence and remote-sensing observations. Because spall volume exceeds crater ejecta volume, the total growth rate of asteroidal surface deposits is accelerated, leading to relatively short surface residence times of individual meteorite components, another significant difference between lunar and asteroidal surface materials.     

On the Influence of Surface Heterogeneity on the Bowen-Ratio: A Theoretical Case Study

Summary The influence of surface heterogeneity on spatial distribution, temporal development, and on the domain-average of the ratio between sensible and latent heat-flux (Bowen-ratio) is investigated for synthetic landscapes of differing degrees of surface heterogeneity. In so doing, simulations are performed applying a 3-dimensional non-hydrostatic mesoscale model. The synthetic landscapes consist of patches of sandy loam covered by mixed forest and loamy soil covered by grass. The results of the numerical experiments substantiate that land-surface distributions will non-linearly influence the Bowen-ratio if patches of equal type exceed a certain size. Moreover, the heterogeneity of the upwind region may play a role. Similarity coefficients show that the surface type dominating a landscape does not necessarily determine the mean Bowen-ratio representative for this area. Thus, when applying the strategy of dominant surface type, the margin of error in the regional Bowen-ratio depends on the horizontal resolution of the model or on available data. Received April 21, 1999 Revised December 10, 1999     

The construction of oxygen isotope chronologies from tree-ring series sampled at different temporal resolution and its use as climate proxies: statistical aspects

Tree-ring oxygen stable isotope data series from conifers growing on the Dachstein Plateau (Austrian Alps) were selected to demonstrate the applicability of the serial pooling method using shifted 5-year tree-ring blocks for summer temperature reconstruction. The addressed method allows the construction of long isotope chronologies with significant climate correlation and well preserved climate sensitivity applying the irreducible sample replication of five trees. The linear regression model for temperature reconstruction is verifiable and the predicted data are well correlated with instrumental data, especially reproducing the long-term temperature trend. However, the reduced mean variance leads to loss of extreme years, which can be regulated by the combination of one data series in annual resolution with five shifted 5-year block data series. This significantly improves the variance of the mean chronology, sufficiently to identify extremes. Therefore, we recommend the use of mixed data sets as a compromise between essential sample replication and economic considerations.     

Timing of Variscan HP-HT metamorphism in the Moldanubian Zone of the Bohemian Massif: U-Pb SHRIMP dating on multiply zoned zircons from a granulite from the Dunkelsteiner Wald Massif, Lower Austria

In an attempt to better constrain the timing of Variscan HP-HT metamorphism in the SE Bohemian Massif we have dated zoned zircons from a garnet-kyanite granulite of granitic composition from the Dunkelsteiner Wald Massif, Lower Austria, by means of sensitive high-resolution ion microprobe (SHRIMP) technique. In order to combine isotopic information with crystal growth textures, CL and BSE images were systematically taken from the dated zircons. A characteristic threefold concentric zoning was found in many zircons. This involves pre-Variscan protolithic cores followed by two distinct metamorphic/anatectic overgrowth shells of Variscan age. The inner overgrowth shell is characterized by a weak CL but bright BSE signal, and yields high contents of uranium (0.1 to 0.2 wt.%). A pooled U-Pb Concordia age for this zone is 342.0?±?3.0?Ma (n?=?11, MSWD?=?0.12). The second, outer, overgrowth shell is always bright in the CL image, dark in the BSE image, and has generally low uranium contents (mostly <500?ppm). A pooled U-Pb Concordia age for this zone is 337.1?±?2.7?Ma (n?=?11, MSWD?=?0.22). These results imply that the Variscan HT crystallisation history of the Moldanubian granulites took place over a period of a few million years and was not an extremely rapid subduction-exhumation process. SHRIMP measurements in the protolithic cores yield a cluster of (sub)concordant ages between ??390 and 460?Ma and a few outliers at higher ages mostly represented by cores in cores. Core domains, which are large, homogeneous and with undisturbed igneous oscillatory zoning, yielded preferentially ages between 430 and 460?Ma. We therefore consider that granitic protolith formation took place at that time. The still older inner cores are interpreted as inherited into the granitic melt.     

Vertical distribution and growth performance of Baltic cod larvae – Field evidence for starvation-induced recruitment regulation during the larval stage?

Besides variable egg survival, previous studies suggested that the larval stage may be the most critical phase in determining Baltic cod recruitment variability, and that larvae need to conduct an ontogenetic vertical migration from hatching depths (>50 m) to upper layers with increased food availability in order to initiate first feeding, improve their nutritional condition and growth, and avoid starvation. Recently, detailed information on the stage-resolved vertical distribution of main Baltic copepod species, including the preferred larval Baltic cod prey species Pseudocalanus acuspes, has become available. Therefore, the vertical distribution of Baltic cod larvae in August 2007 and their depth-dependent nutritional condition and growth were investigated. RNA–DNA based methods were used to estimate growth, including a novel approach to estimate growth performance by relating observed specific growth rates (SGR) of field caught larvae to temperature-dependent reference growth rates (G_ref) for fast-growing laboratory reared fish from the literature. This standardization to G_ref was found to have a great potential to improve investigations on the growth and ecology of larval fish. The need for early larvae to migrate to shallower layers was corroborated, while larger size classes were found at increasingly greater depths. This may reflect a continuation of the ontogenetic vertical migration in order to follow increasingly larger prey items at greater depths and to save energy in cooler waters below the thermocline. Larval growth generally declined with increasing depth, but the decline in growth became less pronounced in larger size classes. This indicates that larger larvae were better in coping with the ambient environment and the available prey field at greater depths. Generally, Baltic cod larvae grew poorly compared to larvae from other studies, which is discussed in relation to differences in predation and a possible food–temperature trade-off for larvae in the highly stratified Baltic Sea. Comparison with earlier results showed a higher frequency of starving larvae and lower frequencies of larger larvae after the first-feeding stage in 1994 and 1995. As this was a period of low Baltic cod recruitment despite favourable conditions for egg survival, it is concluded that larval starvation mortality has a high potential to contribute to recruitment variability in Baltic cod.