We propose a new approach to direct imaging of extrasolar planets: one-sided phase apodization. It is based on a discovery that an antisymmetric spatial phase modulation pattern imposed over a pupil or a relay plane causes diffracted starlight suppression sufficient for imaging of extrasolar planets. Calculations with specific square-pupil (side D) phase functions, such asdemonstrate annulling in at least one quadrant of the diffraction plane to the contrast level of better than 10 À12 , with an inner working angle down to 3:5k=D (with a ¼ 3 and ¼ 10 À3 ). Furthermore, our computer experiments show that phase apodization remains effective throughout a broad spectrum (60% of the central wavelength) covering the entire visible-light range. Phase-only modulation has the additional appeal of potential implementation via active segmented or deformable mirrors, thereby combining compensation of random phase aberrations and diffraction halo removal in a single optical element.
Rote memorization of vocabulary has long been a common way for Chinese students to learn lexical items. Cultural, educational background and traditional teaching practice in China are identified to be the factors that contribute to many students' heavy reliance on memorization as their sole approach to vocabulary learning. In addition to rote memorization which has been proved useful and effective in the Chinese ELT context, this paper suggests that students be presented with vocabulary learning strategies and be taught how to build vocabulary through other useful learning methods.
Poor axial precision caused, in part, by large depth of focus (tau) has been a vexing problem in extraction of particle position from digital in-line holograms. A simple method is proposed to combat this depth-of-focus difficulty. The method is based on decoupling of size and position information. With d, Delta, and lambda being particle diameter, CCD pixel size, and the wavelength, respectively, our main theoretical result is the reduction of tau from tau - d2/lambda to tau - Delta2/lambda for particles of known size. This result is confirmed in laboratory experiments with holograms of calibrated glass spheres.
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