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Imaging the epoch of reionization: Limitations from foreground confusion and imaging Algorithms
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)Tomography of redshifted 21 cm transition from neutral hydrogen using Fourier synthesis telescopes is a promising tool to study the Epoch of Reionization (EoR). Limiting the confusion from Galactic and extragalactic foregrounds is critical to the success of these telescopes. The instrumental response or the point-spread function (PSF) of such telescopes is inherently three dimensional with frequency mapping to the line-of-sight (LOS) distance. EoR signals will necessarily have to be detected in data where continuum confusion persists; therefore, it is important that the PSF has acceptable frequency structure so that the residual foreground does not confuse the EoR signature. This paper aims to understand the three-dimensional PSF and foreground contamination in the same framework. We develop a formalism to estimate the foreground contamination along frequency, or equivalently LOS dimension, and establish a relationship between foreground contamination in the image plane and visibility weights on the Fourier plane. We identify two dominant sources of LOS foreground contamination—"PSF contamination" and "gridding contamination." We show that PSF contamination is localized in LOS wavenumber space, beyond which there potentially exists an "EoR window" with negligible foreground contamination where we may focus our efforts to detect EoR. PSF contamination in this window may be substantially reduced by judicious choice of a frequency window function. Gridding and imaging algorithms create additional gridding contamination and we propose a new imaging algorithm using the Chirp Z Transform that significantly reduces this contamination. Finally, we demonstrate the analytical relationships and the merit of the new imaging algorithm for the case of imaging with the Murchison Widefield Array
Electrochemical and Nonlinear Optical studies of new D–A type π-Conjugated polymers carrying 3,4-Benzyloxythiophene, oxadiazole, and 3,4-Alkoxythiophene systems
No full textRestricted Access.We investigated the nonlinear optical (NLO) properties of two newly synthesized conjugated polymers P1 and P2 carrying 1,3,4-oxadiazole, 3,4-dibenzyloxythiophene, and 3,4-dialkoxythiophene moieties along the main chain, as potential NLO active materials. Their structures have been well characterized. The nonlinear measurements were performed by Z-scan using 532 nm, 7 ns laser pulses. Calculated values of figure of merit and β follow the criteria for good NLO materials. These results suggest that polymers are promising materials for applications in photonics
Timing and broad-band spectroscopy of 1A 1118−61 with Suzaku
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)We present a timing and broad-band pulse-phase-resolved spectral analysis of the transient Be X-ray binary pulsar 1A 1118-61 observed during its outburst in 2009 January using Suzaku observations. The Suzaku observations were made twice, once at the peak of the outburst, and the other 13 d later at its declining phase. Pulse profiles from both observations exhibit strong energy dependence with several peaks at low energies and a single peak above ˜10 keV. A weak, narrow peak is detected at the main dip of the pulse profiles from both observations in the energy bands below 3 keV, indicating the presence of a phase-dependent soft excess in the source continuum. The broad-band energy spectrum of the pulsar could be fitted well with a partial covering cut-off power-law model and a narrow iron fluorescence line. We also detect a broad cyclotron feature at ˜50 keV from both observations which is a feature common for accretion-powered pulsars with high magnetic field strength. The pulse-phase-resolved spectral analysis shows an increase in the absorption column density of the partial covering component, as well as variation in the covering fraction at the dips of the pulse profiles, which naturally explains energy dependence of the same. The cyclotron line parameters also show significant variation with pulse phase with an ˜10 keV variation in the cyclotron line energy and a variation in depth by a factor of 3. This can be explained either as the effect of different viewing angles of the dipole field at different pulse phases, or due to a more complex underlying magnetic field geometry
T-shaped non-symmetrical twin liquid crystalline compounds
No full textRestricted Access.New unconventional T-shaped non-symmetrical dimeric liquid crystalline compounds have been synthesised and their thermotropic properties studied on the hot-stage of a polarising microscope. These compounds consist of an azo-ester mesogenic unit with a range of terminal substituents (–CH3, –OCH3, –NO2 or –Cl) at one end, interconnected by a flexible spacer (n = 4, 5 or 10) via ether and ester linking units to a biphenyl moiety at the lateral hydroxyl position of the azo-ester. All the compounds were characterised using a combination of elemental analysis and standard spectroscopic methods. The compounds were found to exhibit enantiotropic nematic and smectic mesophases. The effect of different terminal substituents on mesomorphism is discussed
Structure-property correlation of a hockey stick-shaped compound exhibiting N-SmA-SmCa phase transitions
No full textRestricted Access.We report the measurement of birefringence, dielectric constant, splay, bend elastic constants and rotational viscosity in the nematic phase (N) of a hockey stick-shaped compound exhibiting smectic-A (SmA) and anticlinic smectic-C (SmCa) phase transitions at lower temperature. It is found that the physical properties are significantly different than calamitics and partially similar to the bent-core liquid crystals. It exhibits the positive dielectric and the negative elastic anisotropy. The optical and the thermal measurements show that all the transitions are first order. The rotational viscosity is comparatively higher than calamitic liquid crystals and exhibits weak pre-transitional divergence. The temperature dependent static dielectric measurements indicate antiparallel correlation of both the longitudinal and the transverse components of dipole moments in both the smectic phases. The parallel component of the dielectric constant exhibits a single Debye type relaxation in all the phases. The activation energy in the N, SmA and SmCa phases are comparatively larger than that of the respective phases of calamitic liquid crystals and can partially be attributed to the higher rotational viscosity of the hockey stick-shaped compound
Method and device to optimize power consumption in liquid crystal display
No full textRestricted AccessPower consumption in liquid crystal displays is analyzed by including frequent polarity reversals and duty cycle control. A multi-step voltage profile is proposed to reduce the power consumption in multiplexed and non-multiplexed displays. The present invention relates to a method to optimize power consumption in Liquid Crystal Display, wherein said method comprises steps of: applying multi-step waveform for selecting pre-determined address lines, maintaining ratio of step-width (Ts) and pulse width (T) between 0.02 to 0.25, and making final step duration (Tf) greater than or equal to twice the step width (Ts) to optimize power supply of the Liquid Crystal Display and apply a correction voltage if the distortion is significant and modifying the step sizes to reduce the supply voltage of the driver
Gray scales by cross pairing of select and data voltages in multi-line addressed liquid crystal displays
No full textRestricted Access.The idea of cross pairing of select and data voltages in line-by-line addressing is extended for multi-line addressing; to display a large number of gray scales with a small number of time intervals as compared to successive approximation technique. The method is illustrated by selecting two rows at a time with waveforms based on Hadamard matrix
Intensity Modulation of Light Sources for Gray Scales in Projection Displays
No full textRestricted Access.Intensity modulation of light source(s) is proposed to achieve a drastic (logarithmic) reduction in number of time intervals that are necessary to display grayscales in DMD and other projection displays. Methods to reduce power consumption of light sources in projection system are also proposed
