The Asymmetric Pupil Fourier Wavefront Sensor

This paper introduces a novel wavefront sensing approach that relies on the Fourier analysis of a single conventional direct image. In the high Strehl ratio regime, the relation between the phase measured in the Fourier plane and the wavefront errors in the pupil can be linearized, as was shown in a previous work that introduced the notion of generalized closure-phase, or kernel-phase. The technique, to be usable as presented requires two conditions to be met: (1) the wavefront errors must be kept small (of the order of one radian or less) and (2) the pupil must include some asymmetry, that can be introduced with a mask, for the problem to become solvable. Simulations show that this asymmetric pupil Fourier wavefront sensing or APF-WFS technique can improve the Strehl ratio from 50 to over 90 % in just a few iterations, with excellent photon noise sensitivity properties, suggesting that on-sky close loop APF-WFS is possible with an extreme adaptive optics system.Comment: 5 figures, accepted for publication by PAS

Kernel-phases for high-contrast detection beyond the resolution limit

The detection of high contrast companions at small angular separation appears feasible in conventional direct images using the self-calibration properties of interferometric observable quantities. In the high-Strehl regime, available from space borne observatories and using AO in the mid-infrared, quantities comparable to the closure-phase that are used with great success in non-redundant masking inteferometry, can be extracted from direct images, even taken with a redundant aperture. These new phase-noise immune observable quantities, called Kernel-phases, are determined a-priori from the knowledge of the geometry of the pupil only. Re-analysis of HST/NICMOS archive and other ground based AO images, using this new Kernel-phase algorithm, demonstrates the power of the method, and its ability to detect companions at the resolution limit and beyond.Comment: 7 pages, 4 figures, 2011 SPIE conference proceeding

Bootstrap Blues

Meet David*. In mid-January, he came to the small town Iowa elementary school where I work. David has attended more schools in the two years since he started school than I have in my lifetime. In fact, the school he just moved from only has four days of attendance listed on his record. David moves so often because he’s homeless. His situation is not what we may stereotypically think of as “homeless”—you wouldn’t see him on the streets or even in soup kitchens. Instead, David stays with his mother, and they couch surf from one home to another from week to week. David and his mother are part of a mounting statistic that tells us that 41 percent of the homeless population includes families

A computational study of 13-atom Ne-Ar cluster heat capacities

Heat capacity curves as functions of temperature were calculated using Monte Carlo methods for the series of Ne_(13-n)Ar_n clusters (0 <= n <= 13). The clusters were modeled classically using pairwise additive Lennard-Jones potentials. The J-walking (or jump-walking) method was used to overcome systematic errors due to quasiergodicity. Substantial discrepancies between the J-walking results and those obtained using standard Metropolis methods were found. Results obtained using the atom-exchange method, another Monte Carlo variant for multi-component systems, also did not compare well with the J-walker results. Quench studies were done to investigate the clusters' potential energy surfaces. Only those Ne-Ar clusters consisting predominately of either one or the other component had lowest energy isomers having the icosahedral-like symmetry typical of homogeneous 13-atom rare gas clusters; non-icosahedral structures dominated the lowest-energy isomers for the other clusters. This resulted in heat capacity curves that were very much different than that of their homogeneous counterpart. Evidence for coexistence behavior different than that seen in homogenous clusters is also presented.Comment: 45 pages, 11 Figures, figures in .gif format files. Journal of Chemical Physics, AIP ID number 513730JC

Magic number behavior for heat capacities of medium sized classical Lennard-Jones clusters

Monte Carlo methods were used to calculate heat capacities as functions of temperature for classical atomic clusters of aggregate sizes $25 \leq N \leq 60$ that were bound by pairwise Lennard-Jones potentials. The parallel tempering method was used to overcome convergence difficulties due to quasiergodicity in the solid-liquid phase-change regions. All of the clusters studied had pronounced peaks in their heat capacity curves, most of which corresponded to their solid-liquid phase-change regions. The heat capacity peak height and location exhibited two general trends as functions of cluster size: for $N = 25$ to 36, the peak temperature slowly increased, while the peak height slowly decreased, disappearing by $N = 37$; for $N = 30$, a very small secondary peak at very low temperature emerged and quickly increased in size and temperature as $N$ increased, becoming the dominant peak by $N = 36$. Superimposed on these general trends were smaller fluctuations in the peak heights that corresponded to ``magic number'' behavior, with local maxima found at $N = 36, 39, 43, 46$ and 49, and the largest peak found at $N = 55$. These magic numbers were a subset of the magic numbers found for other cluster properties, and can be largely understood in terms of the clusters' underlying geometries. Further insights into the melting behavior of these clusters were obtained from quench studies and by examining rms bond length fluctuations.Comment: 15 pages, 17 figures (PDF format

My Life As A Labelmaker

It’s easy to label to people. I find it particularly easy at Gettysburg College. When I assign a label to someone, it’s like it appears in big red letters across their forehead. Sometimes my snap judgment comes from what they’re wearing. Salmon colored pants? FRATERNITY, BRO, PREP. Sometimes it comes from what they say. “Dude that chick’s a femi-nazi.” MISOGYNIST, PRIVILEGED, JERK. My judgment comes from all sorts of different places but the important part is that my initial judgment sticks. It sits there, tattooed on people’s foreheads, staring at me, and it’s the only thing I see from that point forward. [excerpt

Parallel J-W Monte Carlo Simulations of Thermal Phase Changes in Finite-size Systems

The thermodynamic properties of 59 TeF6 clusters that undergo temperature-driven phase transitions have been calculated with a canonical J-walking Monte Carlo technique. A parallel code for simulations has been developed and optimized on SUN3500 and CRAY-T3E computers. The Lindemann criterion shows that the clusters transform from liquid to solid and then from one solid structure to another in the temperature region 60-130 K.Comment: 4 pages, 5 figures; presented at the conference on computational physics, Aachen (2001) accepted for publication in Comp.Phys.Com

A Computational Study of Thirteen-atom Ar-Kr Cluster Heat Capacities

Heat capacity curves as functions of temperature were calculated using Monte Carlo methods for the series of Ar_{13-n}Kr_n clusters (0 <= n <= 13). The clusters were modeled classically using pairwise additive Lennard-Jones potentials. J-walking (or jump-walking) was used to overcome convergence difficulties due to quasiergodicity present in the solid-liquid transition regions, as well as in the very low temperature regions where heat capacity anomalies arising from permutational isomers were observed. Substantial discrepancies between the J-walking results and the results obtained using standard Metropolis Monte Carlo methods were found. Results obtained using the atom-exchange method, another Monte Carlo variant designed for multi-component systems, were mostly similar to the J-walker results. Quench studies were also done to investigate the clusters' potential energy surfaces; in each case, the lowest energy isomer had an icosahedral-like symmetry typical of homogeneous thirteen-atom rare gas clusters, with an Ar atom being the central atom.Comment: 46 pages, 13 Figures combined in 2 .gif files, Journal of Chemical Physics, AIP ID number 508646JC

The Race for Honors

Over graduation weekend, it was pretty common to see people weighed down by massive numbers of honor cords hanging around their necks. This is a mark of respect at Gettysburg College, so students wear them proudly. I had the privilege to attend Spring Honors Day and watch many of my friends receive achievement awards. As we started winding down to the end of the ceremony, something hit me: The recipients were overwhelmingly white. [excerpt