145 research outputs found
Small Solutions to the Large Telescope Problem: A Massively Replicated MEMS Spectrograph
In traditional seeing-limited observations the spectrograph aperture scales
with telescope aperture, driving sizes and costs to enormous proportions. We
propose a new solution to the seeing-limited spectrograph problem. A massively
fiber-sliced configuration feeds a set of small diffraction-limited
spectrographs. We present a prototype, tunable, J-band, diffraction grating,
designed specifically for Astronomical applications: The grating sits at the
heart of a spectrograph, no bigger than a few inches on a side. Throughput
requirements dictate using tens-of-thousands of spectrographs on a single 10 to
30 meter telescope. A full system would cost significantly less than typical
instruments on 10m or 30m telescopes.Comment: 9 pages, 5 figures, presented at SPIE Astronomical Telescopes and
Instrumentation, 23 - 28 June 2008, Marseille, France. See
http://www.ucolick.org/~npk/MEMS for video
Electronic structure of the c(4 x 2) reconstructed Ge(001) surface
We investigate the electronic structure of the c(4 x 2) reconstructed Ge(001)
surface using band structure calculations based on density functional theory
and the generalized gradient approximation. In particular, we take into account
the details of surface reconstruction by means of well relaxed crystal
structures. The surface electronic states are identified and the local density
of states is compared to recent data from scanning tunneling spectroscopy. We
obtain almost perfect agreement between theory and experiment for both the
occupied and unoccupied states, which allows us to clarify the interpretation
of the experimental data.Comment: 4 pages, 3 figures, accepted by Chem. Phys. Let
Penerapan Lean Supply Chain pada Proses Loading Pupuk In Bag di Pelabuhan PT. Petrokimia Gresik
PT. Petrokimia Gresik merupakan salah satu produsen pupuk terbesar di Indonesia yangmempunyai jaringan supply chain lintas negara dan distribusi ke seluruh Nusantara baik pupukcurah maupun pupuk in bag. Penelitian ini dilaksanakan pada pelabuhan PT. PG yangmerupakan titik utama dari kegiatan logistik di Perusahaan ini sendiri, yakni pemuatan danpembongkaran. Dengan fokus penelitian pada proses pemuatan pupuk in bag. Permasalahanyang terjadi pada proses ini dikarenakan inefisiensi aliran Supply Chain, yang disebabkan olehadanya waste dan non value added actvity. Tujuan penelitian ini adalah mengetahui jenis wasteapa saja yang terjadi selama proses, serta saran perbaikan dengan menggunakan konsep LeanSupply Chain dan Value Stream Mapping serta mencari penyebab masalah menggunakan 5Whys dan Fishbone. Jenis pemborosan yang paling berpengaruh selama aliran proses adalahWaiting Time (20,42%), serta Non Value Added Actvity sebesar 51,9%. Dengan menggunakanfishbone dan 5Whys dapat diketahui penyebab waste terbesar diantaranya adalah lamanya trukmenunggu muatan, banyaknya crane tidak sehat, serta tidak adanya penjadwalan dan alokasimuatan. Sementara rekomendasi yang diberikan adalah penjadwalan dan pengalokasian,pengadaan lini khusus di gudang, penyediaan crane dengan kondisi kecepatan muat yang sesuai.Berdasarkan saran perbaikan diprediksi dapat mereduksi total NVA sebesar 59.8
Closed loop adaptive optics with a laser guide star for biological light microscopy
We report on the development of a widefield microscope that achieves adaptive optics correction through the use of a wavefront sensor observing an artificial laser guide star induced within the sample. By generating this guide star at arbitrary positions and depths within the sample we allow the delivery of high-resolution images. This approach delivers much faster AO correction than image optimization techniques, and allows the use of AO with fluorescent imaging modalities without generating excessive photo-toxic damage in the sample, or inducing significant photo-bleaching in the flurophore molecules
The Ge(001) (2 × 1) reconstruction: asymmetric dimers and multilayer relaxation observed by grazing incidence X-ray diffraction
Grazing incidence X-ray diffraction has been used to analyze in detail the atomic structure of the (2 × 1) reconstruction of the Ge(001) surface involving far reaching subsurface relaxations. Two kinds of disorder models, a statistical and a dynamical were taken into account for the data analysis, both indicating substantial disorder along the surface normal. This can only be correlated to asymmetric dimers.
Considering a statistical disorder model assuming randomly oriented dimers the analysis of 13 symmetrically independent in-plane fractional order reflections and of four fractional order reciprocal lattice rods up to the maximum attainable momentum transfer qz = 3c* (c* = 1.77 × 10−1 Å−1) indicates the formation of asymmetric dimers characterized by R>D = 2.46(5) Å as compared to the bulk bonding length of R = 2.45 Å. The dimer height of Δ Z = 0.74(15) Å corresponds to a dimer buckling angle of 17(4)°. The data refinement using anisotropic thermal parameters leads to a bonding length of RD = 2.44(4) Å and to a large anisotropy of the root mean-square vibration amplitudes of the dimer atoms (u112) 1/2 = 0.25 Å, (u222)1/2 = 0.14 Å, (u332)1/2 = 0.50 Å). We have evidence for lateral and vertical disp tenth layer below the surface
Realtime wavefront sensing in a SPIM microscope, and active aberration tracking
Adaptive optics (AO) can potentially allow high resolution imaging deep inside living tissue, mitigating against the loss of resolution due to aberrations caused by overlying tissue. Closed-loop AO correction is particularly attractive for moving tissue and spatially varying aberrations, but this requires direct wavefront sensing, which in turn requires suitable "guide stars" for use as wavefront references. We present a novel method for generating an orthogonally illuminated guide star suitable for direct wavefront sensing in a wide range of fluorescent biological structures, along with results demonstrating its use for measuring time-varying aberrations, in vivo
Electron correlation in the Si(100) surface
Motivated by the controversy between quantum chemists and solid-state
physicists, and by recent experimental results, spin-polarized
density-functional (DFT) calculations are used to probe electron correlation in
the Si(100) reconstructed surface. The ground state displays antiferromagnetic
spin polarization for low dimer inclinations indicating, not magnetic order,
but the importance of Mott-like correlations among dangling bonds. The lowest
energy corresponds to a higher dimer inclination with no spin. DFT energies,
however, should be taken with caution here. Our results together with
quantum-chemical findings suggest dimers with highly correlated electrons that
tend to buckle due to interactions with other dimers.Comment: 5 pages, 1 eps figure, 1 table; RevTeX v3.1. To appear in Surface
Science (proceedings of the European Conference On Surface Science, ECOSS-19,
Madrid, Sept. 5-8, 2000
Ultrathin endoscopes based on multicore fibers and adaptive optics: a status review and perspectives
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