411 research outputs found
Depth-Resolved Wavefront Aberrations using a Coherence-Gated Shack-Hartmann Wavefront Sensor
No full textIn the present paper we investigate the possibility of narrowing the depth range of a physical Shack - Hartmann (SH) wavefront sensor (WFS) by using coherence gating. For the coherence gating, two low coherence interferometry (LCI) methods are evaluated and proof of principle configurations demonstrated: (i) a time domain LCI method based on phase shifting interferometry and (ii) a spectral domain LCI method, based on tuning a narrow band optical source. The two configurations are used to demonstrate each, the possibility of constructing a coherence gated (CG) SH/WFS. It is shown that these configurations produce spot patterns similar to those provided by a conventional SH/WFS. The two proof of principle configurations are also used to illustrate elimination of stray reflections in the interface optics which normally disturb the operation of conventional SH/WFSs. The speed and noise performance of the two CG-SH/WFS implementations are discussed
wreck crew shack
No full textwreck...I was lookin' after him there in a wreck crew shack, they call it, but 'twas...'twas a nice comfortable place just the same.YesDNE-cit J. D. A. WIDDOWSONUsed IUsed IUsed Iwrecking, wrecked, wreck, wreck-crew, shipwrecks, WreckSource listed as T 43/4-6
Hartmann-Shack technique and refraction across the horizontal visual field
No full textWe compared refractions across the horizontal visual field, based on different analyses of wave aberration obtained with a Hartmann-Shack instrument. The wave aberrations had been determined for 6-mm-diameter pupils up to at least the sixth Zernike order in five normal subjects [J. Opt. Soc. Am. A 19, 2180 (2002)]. The polynomials were converted into refractions based on 6-mm pupils and second-order Zernike aberrations (6 mm/2nd order), 3-mm pupils and second-order aberrations (3 mm/2nd order), 1-mm pupils and second-order aberrations (1 mm/2nd order), and 6-mm pupils with both second- and fourth-order aberrations (6 mm/4th order). The 3-mm/2nd-order and 6-mm/ 2nd-order refractions differed by as much as 0.9 D in mean sphere on axis, but the differences reduced markedly toward the edges of the visual field. The cylindrical differences between these two analyses were small at the center of the visual field
Turbulence characterisation for Astronomical Observatories
No full textAtmospheric turbulence has two effects in astronomy; (i) the broadening of the point spread function due to phase fluctuations limiting the resolution of imaging and (ii) producing intensity fluctuations known as scintillation. Adaptive Optics (AO) can be installed on telescopes to correct for the effect of phase, and with the push to large telescopes more complex AO systems such as Multi Conjugate AO (MCAO) and Multi Object AO (MOAO) are desired. Operation of these systems requires a detailed profile of the turbulent atmosphere in real time.
In this thesis we consider two turbulence profilers, SLOpe Detection And Ranging (SLODAR) and SCIntillation Detection and Ranging (SCIDAR), two cross beam profilers that retrieve data using covariance of phase variations (SLODAR) and intensity variations (SCIDAR). We present a modification of SLODAR to allow an estimate for non resolved turbulence to be made by considering scintillation in the subapertures of a Shack Hartmann wavefront sensor. A new SCIDAR (Stereo--SCIDAR) is described, allowing dynamic re--conjugation to improve altitude resolution.
Practical considerations for the implementation of a SLODAR instrument are considered, including a discussion of potential false measurements of non Kolmogorov power spectra in the ground and surface layers of turbulence. Data is presented from SLODAR observing campaigns on La Palma, and at Paranal. Evidence is presented for orographic effects on measured turbulence, including those due to man made structures
Shack-Hartmann Thermal Lensing Characterization Of Mid-Ir Materials
No full textWe quantify the thermo-optic response of chalcogenide glasses to 2μm laser irradiation using Shack-Hartmann thermal lensing metrology. Induced dioptric powers as strong as 0.21m-1 are observed for CW pump irradiances as low as 38.2 W/cm2. © OSA 2013
Laser guide stars and turbulence profiling for extremely large telescopes
No full textThe next generation of ground based telescopes, the so-called extremely large telescopes, will offer a significant leap in sensitivity and resolution compared to current telescopes. They also present a range of technical challenges. This thesis presents work on two important problems in implementing laser guide star adaptive optics on extremely large telescopes: focal anisoplanatism and turbulence profiling. The SPLASH (Sky-Projected Laser Array Shack-Hartmann) laser guide star wavefront sensing technique is described. The technique is shown to offer reduced focal anisoplanatism compared to a conventional laser guide star for large telescopes. The technique may also offer advantages for larger apertures, including extremely large telescopes, but simulations were limited to 8 metre apertures by currently available computing capabilities. A calibration method is presented for the SLODAR (Slope Detection and Ranging) turbulence profiling technique, along with an analysis of the effects of scintillation on SLODAR when the technique is applied on a small (~ 40 cm diameter)' telescope. A new variation on the SLODAR technique, SLOTDAR (Slope Detection and Ranging through a slot), is introduced, in which the spatial sampling can be optimised based on the brightness of the available reference stars
Design of wide-field imaging shack Hartmann testbed
No full textStandard adaptive optics systems measure the aberrations in the wavefronts of a beacon guide star caused by atmospheric turbulence, which limits the corrected field of view to the isoplanatic patch, the solid angle over which the optical aberration is roughly constant. For imaging systems that require a corrected field of view larger than the isoplanatic angle, a three-dimensional estimate of the aberration is required. We are developing a wide-field imaging Shack-Hartmann wavefront sensor (WFS) that will characterize turbulence over a large field of view tens of times the size of the isoplanatic angle. The technique will find application in horizontal and downward looking remote sensing scenarios where high resolution imaging through extended atmospheric turbulence is required. The laboratory prototype system consists of a scene generator, turbulence simulator, a Shack Hartman WFS arm, and an imaging arm. The system has a high intrinsic Strehl ratio, is telecentric, and diffraction limited. We present preliminary data and analysis from the system.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
The impacts of international non governmental organisation networks on housing policies : a study of Kenya
No full textIncludes abstract.Includes bibliographical references (leaves 70-74).An assessment of the influences and impacts of development NGOs reveal that in spite of their diverse and resourceful nature, NGOs working in the field of advocacy tend to predominantly have incrementalist influences. These influences, being predominantly short term in nature, do have long term implications for public policy in developing countries. Well resourced NGO networks , historically rooted in the missionary movement of the post Second World War period have succeeded in firmly establishing themselves in the body politic of poor Africa countries
An FPGA Architecture for Extracting Real-Time Zernike Coefficients from Measured Phase Gradients
No full textZernike modes are commonly used in adaptive optics systems to represent optical wavefronts. However, real-time calculation of Zernike modes is time consuming due to two factors: the large factorial components in the radial polynomials used to define them and the large inverse matrix calculation needed for the linear fit. This paper presents an efficient parallel method for calculating Zernike coefficients from phase gradients produced by a Shack-Hartman sensor and its real-time implementation using an FPGA by pre-calculation and storage of subsections of the large inverse matrix. The architecture exploits symmetries within the Zernike modes to achieve a significant reduction in memory requirements and a speed-up of 2.9 when compared to published results utilising a 2D-FFT method for a grid size of 8×8. Analysis of processor element internal word length requirements show that 24-bit precision in precalculated values of the Zernike mode partial derivatives ensures less than 0.5% error per Zernike coefficient and an overall error of <1%. The design has been synthesized on a Xilinx Spartan-6 XC6SLX45 FPGA. The resource utilisation on this device is <3% of slice registers, <15% of slice LUTs, and approximately 48% of available DSP blocks independent of the Shack-Hartmann grid size. Block RAM usage is <16% for Shack-Hartmann grid sizes up to 32×32
Hartmann-Shack technique and refraction across the horizontal visual field
No full textWe compared refractions across the horizontal visual field, based on different analyses of wave aberration obtained with a Hartmann-Shack instrument. The wave aberrations had been determined for 6-mm-diameter pupils up to at least the sixth Zernike order in five normal subjects [J. Opt. Soc. Am. A 19, 2180 (2002)]. The polynomials were converted into refractions based on 6-mm pupils and second-order Zernike aberrations (6 mm/2nd order), 3-mm pupils and second-order aberrations (3 mm/2nd order), 1-mm pupils and second-order aberrations (1 mm/2nd order), and 6-mm pupils with both second- and fourth-order aberrations (6 mm/4th order). The 3-mm/2nd-order and 6-mm/ 2nd-order refractions differed by as much as 0.9 D in mean sphere on axis, but the differences reduced markedly toward the edges of the visual field. The cylindrical differences between these two analyses were small at the center of the visual field
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