The Infrared Imaging Spectrograph (IRIS) for TMT: Volume phase holographic grating performance testing and discussion

Maximizing the grating efficiency is a key goal for the first light instrument IRIS (Infrared Imaging Spectrograph) currently being designed to sample the diffraction limit of the TMT (Thirty Meter Telescope). Volume Phase Holographic (VPH) gratings have been shown to offer extremely high efficiencies that approach 100% for high line frequencies (i.e., 600 to 6000l/mm), which has been applicable for astronomical optical spectrographs. However, VPH gratings have been less exploited in the near-infrared, particularly for gratings that have lower line frequencies. Given their potential to offer high throughputs and low scattered light, VPH gratings are being explored for IRIS as a potential dispersing element in the spectrograph. Our team has procured near-infrared gratings from two separate vendors. We have two gratings with the specifications needed for IRIS current design: 1.51-1.82{\mu}m (H-band) to produce a spectral resolution of 4000 and 1.19- 1.37 {\mu}m (J-band) to produce a spectral resolution of 8000. The center wavelengths for each grating are 1.629{\mu}m and 1.27{\mu}m, and the groove densities are 177l/mm and 440l/mm for H-band R=4000 and J-band R=8000, respectively. We directly measure the efficiencies in the lab and find that the peak efficiencies of these two types of gratings are quite good with a peak efficiency of ~88% at the Bragg angle in both TM and TE modes at H-band, and 90.23% in TM mode, 79.91% in TE mode at J-band for the best vendor. We determine the drop in efficiency off the Bragg angle, with a 20-23% decrease in efficiency at H-band when 2.5 degree deviation from the Bragg angle, and 25%-28% decrease at J-band when 5{\deg} deviation from the Bragg angle.Comment: Proceedings of the SPIE, 9147-33

Quantitative Assessment of Deformation-Induced Damage in a Semisolid Aluminum Alloy via X-ray Microtomography

Semisolid tensile testing combined with X-ray microtomography (XMT) was used to characterize the development of internal damage as a function of strain in an aluminum-magnesium alloy, AA5182. Novel techniques were developed to allow the quantification of both the size evolution and orientation of the damage to determine mechanisms controlling the early stage growth and localization. During the initial stages of semisolid deformation, it was observed that strain was accommodated by both the growth of as-cast porosity and the detection of new damage-based voids. As the volume fraction of damage increases, the growth of voids occurs in an orientation perpendicular to the loading direction, both through expansion within the grain boundary liquid and void coalescence. The damage then localizes, causing failur

Dynamics of the Freezing Front During the Solidification of a Colloidal Alumina Aqueous Suspension: In Situ X-Ray Radiography, Tomography, and Modeling

International audienceIce templating of colloidal suspension is gaining interest in material science because it offers the possibility to shape advanced materials, and in particular porous ceramics. Recent investigations on this peculiar process show that a correlation between the morphology of the frozen suspension and the velocity of the freezing front do exist. The dynamics of the freezing front of a colloidal suspension of alumina is investigated in the present study by experimental tests, finite element numerical analysis and theoretical analytical calculations. The experimental tests are carried out by in situ X-ray radiography (investigation of the dynamics of the freezing front) and tomography (investigation of the resulting morphology of the frozen suspension). The finite element model is a continuous properties model; it is used for investigating the dynamics and the shape of the freezing front. The analytical model is based on the two-phase Stefan problem. We propose a solution for the dynamics of the solidification front based on the calculation of the diffusivity as a function of the particle fraction and local temperature

Fast reconstruction of 3D volumes from 2D CT projection data with GPUs

cited By 0International audienceMeso-F.E. modelling of 3D textile composites is a powerful tool, which can help determine mechanical properties and permeability of the reinforcements or composites. The quality of the meso F.E. analyses depends on the quality of the initial model. A direct method based on X-ray tomography imaging is introduced to determine finite element models based on the real geometry of 3D composite reinforcements. The method is particularly suitable regarding 3D textile reinforcements for which internal geometries are numerous and complex. An analysis of the image's texture is performed. A hyperelastic model developed for fibre bundles is used for the simulation of the deformation of the 3D reinforcement. © EDP Sciences, 2016

The InfraRed Imaging Spectrograph (IRIS) for TMT: Reflective ruled diffraction grating performance testing and discussion

We present the efficiency of near-infrared reflective ruled diffraction gratings designed for the InfraRed Imaging Spectrograph (IRIS). IRIS is a first light, integral field spectrograph and imager for the Thirty Meter Telescope (TMT) and narrow field infrared adaptive optics system (NFIRAOS). We present our experimental setup and analysis of the efficiency of selected reflective diffraction gratings. These measurements are used as a comparison sample against selected candidate Volume Phase Holographic (VPH) gratings (see Chen et al., this conference). We investigate the efficiencies of five ruled gratings designed for IRIS from two separate vendors. Three of the gratings accept a bandpass of 1.19-1.37 {\mu}m (J band) with ideal spectral resolutions of R=4000 and R=8000, groove densities of 249 and 516 lines/mm, and blaze angles of 9.86 and 20.54 degrees, respectively. The other two gratings accept a bandpass of 1.51-1.82 {\mu}m (H Band) with an ideal spectral resolution of R=4000, groove density of 141 lines/mm, and blaze angle of 9.86{\deg}. We measure the efficiencies off blaze angle for all gratings and the efficiencies between the polarization transverse magnetic (TM) and transverse electric (TE) states. The peak reflective efficiencies are 98.90 +/- 3.36% (TM) and 84.99 +/- 2.74% (TM) for the H-band R=4000 and J-band R=4000 respectively. The peak reflective efficiency for the J-band R=8000 grating is 78.78 +/- 2.54% (TE). We find that these ruled gratings do not exhibit a wide dependency on incident angle within +/-3{\deg}. Our best-manufactured gratings were found to exhibit a dependency on the polarization state of the incident beam with a ~10-20% deviation, consistent with the theoretical efficiency predictions.Comment: Proceedings of the SPIE, 9147-34

Long-term denudation rates from the Central Andes (Chile) estimated from a digital elevation model using the black top hat function and inverse distance weighting : implications for the neogene climate of the Atacama Desert

A methodology for determining long-term denudation rates from morphologic markers in a Digital Elevation Model (DEM) is checked by a comparative study of two drainage basins in the Precordillera of the Central Andes. In both cases the initial configuration of an incised pediment surface has been restored by using two different methods: the Black Top Hat (BTH) function and the Inverse Distance Weighting (IDW) interpolation. Where vertical incision and hillslope erosion are recorded, the 1DW appears to be the most adequate to reconstitute the pediment surfaces. Conversely, where only vertical incision is observed, the BTH describes more precisely the former pediment surfaces and it is easier to solve. By subtracting the DEM from the reconstructed marker we calculated an eroded volume, and estimated its uncertainty by considering Root Mean Square Error (RMSE) and DEM grid error. For the last similar to 10 Myr we obtained long-term denudation rates of 7.33 +/- 1.6 m/Myr in the San Andres drainage basin and 13.59 +/- 1.9 m/Myr in the El Salado drainage basin. These estimations are largely in agreement with other reported estimates of long-term denudation rates in the Atacama Desert. Comparison with long-term denudation rates reported in a wide range of climatic regimes suggests that our estimates cannot be explained by the current rainfall in the Precordillera. However they could be explained by a rainfall similar to that reported 40 km to the east in the Puna. This suggests that during the time span concerned the geomorphologic evolution of the study area, this evolution is dominated by an orographically controlled rainfall pattern. The preserved pediment surface and the small long term denudation rates determined in this study also indicate that the Precordillera was never reached by humid tropical air masses and precipitation as currently observed in the Altiplano during the summer months

Estimation des changements de la ligne de rivage de la zone côtière sablonneuse de Kénitra au Maroc

Les plages du littoral de Kénitra ont connu des modifications au cours de ces quatre dernières décennies. La mise en valeur économique de certaines plages par des aménagements touristiques et l’extraction massive de sables pour les travaux d’aménagements urbains sont à l’origine d’une déstabilisation des échanges transversaux de sédiments. Ils sont accentués par la succession de périodes de sécheresse et par la multiplication de construction de barrages sur le bassin versant du Sebou. Le préambule à une meilleure gestion de ces plages est la compréhension de leur comportement passé vis-à-vis des contraintes naturelles et anthropiques. Cette démarche s’appuie essentiellement sur les missions aériennes de 1963 et de 1993. Le Modèle Numérique de Terrain (MNT), d’une précision de ± 10 cm en altitude issu de ces missions et le suivi du positionnement du trait de côte par un système d’analyse numérique (DSAS), ont permis d’estimer les taux de changement.Mots-clés : Littoral, basin versant de Sebou, Maroc, photographies aériennes,MNT, DSAS