A novel image reconstruction method applied to deep Hubble Space Telescope Images

We have developed a method for the linear reconstruction of an image from undersampled, dithered data, which has been used to create the distributed, combined Hubble Deep Field images -- the deepest optical images yet taken of the universe. The algorithm, known as Variable-Pixel Linear Reconstruction (or informally as "drizzling"), preserves photometry and resolution, can weight input images according to the statistical significance of each pixel, and removes the effects of geometric distortion both on image shape and photometry. In this paper, the algorithm and its implementation are described, and measurements of the photometric accuracy and image fidelity are presented. In addition, we describe the use of drizzling to combine dithered images in the presence of cosmic rays.Comment: Invited paper, to appear in Applications of Digital Image Processing XX, ed. A. Tescher, Proc. S.P.I.E. vol. 3164, in press; 6 pages, 4 included figures, SPIE LaTex style file include

Transition to plastic motion as a critical phenomenon and anomalous interface layer of a 2D driven vortex lattice

The dynamic transition between the ordered flow and the plastic flow is studied for a two-dimensional driven vortex lattice, in the presence of sharp and dense pinning centers, from numerical simulations. For this system, which does not show smectic ordering, the lattice exhibits a first order transition from a crystal to a liquid, shortly followed by the dynamical transition to the plastic flow. The resistivity provides a critical order parameter for the latter, and critical exponents are determined in analogy with a percolation transition. At the boundary between a pinned region and an unpinned one, an anomalous layer is observed, where the vortices are more strongly pinned than in the bulk.Comment: 7 pages, 7 figure

Continuous melting of a driven two-dimensional flux lattice with strong pins

The phase diagram of a driven two-dimensional vortex lattice in the presence of dense quasi-point pins is investigated. The transition from the crystal to the liquid is found continuous at intermediate inductions. The correlations in the pseudo random force that allow for an uncomplete unbinding of the dislocations is proposed as a key mechanism to account for the continuous transition.Comment: 7 pages, 8 figure

Magnetic critical properties and basal-plane anisotropy of Sr2_2IrO4_4

The anisotropic magnetic properties of Sr$_2$IrO$_4$ are investigated, using longitudinal and torque magnetometry. The critical scaling across $T_c$ of the longitudinal magnetization is the one expected for the 2D XY universality class. Modeling the torque for a magnetic field in the basal-plane, and taking into account all in-plane and out-of-plane magnetic couplings, we derive the effective 4-fold anisotropy $K_4 \approx$ 1 10$^5$ erg mole$^{-1}$. Although larger than for the cuprates, it is found too small to account for a significant departure from the isotropic 2D XY model. The in-plane torque also allows us to put an upper bound for the anisotropy of a field-induced shift of the antiferromagnetic ordering temperature

A New Method for Band-limited Imaging with Undersampled Detectors

Since its original use on the Hubble Deep Field, "Drizzle" has become a de facto standard for the combination of images taken by the Hubble Space Tele- scope. However, the drizzle algorithm was developed with small, faint, partially resolved sources in mind, and is not the best possible algorithm for high signal-to-noise unresolved objects. Here, a new method for creating band-limited images from undersampled data is presented. The method uses a drizzled image as a first order approximation and then rapidly converges toward a band-limited image which fits the data given the statistical weighting provided by the drizzled image. The method, named iDrizzle, for iterative Drizzle, effectively eliminates both the small high-frequency artifacts and convolution with an interpolant kernel that can be introduced by drizzling. The method works well in the presence of geometric distortion, and can easily handle cosmic rays, bad pixels, or other missing data. It can combine images taken with random dithers, though the number of dithers required to obtain a good final image depends in part on the quality of the dither placements. iDrizzle may prove most beneficial for producing high-fidelity point spread functions from undersampled images, and could be particularly valuable for future Dark Energy missions such as WFIRST and EUCLID, which will likely attempt to do high precision supernova photometry and lensing experiments with undersampled detectors.Comment: Accepted for publication in PASP. This version has an added short discussion of analytical technique

The Way Forward: From Sanctions to Supports

The New York City Working Group on School Transformation brought together education practitioners, school reformers, policy-makers, advocates, and parent and student leaders to propose alternatives to the school closings policy of the New York City Department of Education (DOE). (See the list of Working Group members in Appendix 1.) The group was initiated by the New York City Coalition for Educational Justice and coordinated by the Annenberg Institute for School Reform following the fall 2011 conference Effective Alternatives to School Closings: Transforming Struggling Schools in New York City. This report presents the Working Group's conclusions about the limitations of school closings and a set of recommendations for systemic responses to the needs of struggling schools

Anisotropy and interaction effects of strongly strained SrIrO3 thin films

Magneto-transport properties of SrIrO$_3$ thin films epitaxially grown on SrTiO$_3$, using reactive RF sputtering, are investigated. A large anisotropy between the in-plane and the out-of-plane resistivities is found, as well as a signature of the substrate cubic to tetragonal transition. Both observations result from the structural distortion associated to the epitaxial strain. The low-temperature and field dependences of the Hall number are interpreted as due to the contribution of Coulomb interactions to weak localization, evidencing the strong correlations in this material. The introduction of a contribution from magnetic scatters, in the analysis of magnetoconductance in the weakly localized regime, is proposed as an alternative to an anomalously large temperature dependence of the Land\'{e} coefficient