726 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
Dick Naumann's Shack
No full textPhotograph - A log building with a fence. A note on the back reads: Dick Naumann's shack, on Athabasca River 220 km north of Athabasca at Stony Rapid
shack in, down (trawls
No full textshack[S]2[S] v... in the tub hooks (placed) in spiral fashion one man hauls, the other shacksDNE-cit WKMAY. 7 1979Used IUsed I2Used IShacking gear, shack in, ~ down.The source is listed as P 113-79 in DNE and only part of the cited quotation on card is in DNE
Deep Learning Wavefront Sensing: Via Raw Shack-Hartmann Images
No full textThe Delft Center for Systems and Control (DCSC) 'Smart Optics' aim to achieve higher resolution imaging through Adaptive Optics (AO). Adaptive optics is a modern technique for detecting and correcting real-time wavefront aberrations and is widely used in biomedical imaging and astronomical imaging. Wavefront sensing lies at the core of Adaptive Optics and is known to pose some challenges. Measurement of the wavefront cannot be done directly and has to be estimated through an intensity distribution on a detector. One approach to wavefront sensing is by using a Shack-Hartmann (SH) sensor. A Shack-Hartmann sensor (a pupil-plane sensor) subdivides the wavefront into N spatial areas using sub-apertures. The individual slopes across all sub-apertures are integrated to reconstruct the wavefront. The major advantage of using a Shack-Hartmann sensor is its fast operation speed, caused by the linear relationship between local slopes and original wavefront. This enables real-time wavefront reconstruction. The Shack-Hartmann sensor however, has some limitations. Its ability to reconstruct higher-order aberrations is restricted by the amount of lenses within the micro-lens array. Furthermore, a centroiding algorithm is used to compute the local slopes. Going from spots to centroids decreases the amount of informative pixels and greatly limits its wavefront reconstruction potential. Moreover, these centroiding algorithms often add a measure of uncertainty since spots can have irregular shapes or cross-over/overlap. In this Master Thesis a novel approach to phase reconstruction from the raw SH measurement is proposed. Here, we show that Deep Learning techniques in combination with a micro-lens array can surpass traditional SH phase reconstruction methods and alleviate their current limitations. The proposed method uses the entire Shack-Hartmann Pattern (HP) as input to a neural network, supplying the network with more information than existing Deep Learning SHWR methods, which still rely on centroids. Using this approach, we can combine the accuracy of sensor-less techniques with the speed of a Shack-Hartmann sensor. Three different neural network architectures are considered in this thesis. Two of these neural networks (Alex-Net and Xception) are adapted to output a series of Zernike coefficients. Using these estimated Zernike coefficients, a wavefront can be reconstructed. The remaining neural network, U-Net, performs a direct pixel-wise estimation of the phase-map. The input Shack-Hartmann patterns are created using different micro-lens array (MLA) geometries, consisting of 25-, 256- or 900 lenses. The networks are evaluated on their ability to reconstruct a combination of 32- or 100- Zernike coefficients.Mechanical Engineering | Systems and Contro
Shack-Hartmann reflective micro profilometer
No full textWe present a quantitative phase imaging microscope based on a Shack-Hartmann sensor, that directly reconstructs the optical path difference (OPD) in reflective mode. Comparing with the holographic or interferometric methods, the SH technique needs no reference beam in the setup, which simplifies the system. With a preregistered reference, the OPD image can be reconstructed from a single shot. Also, the method has a rather relaxed requirement on the illumination coherence, thus a cheap light source such as a LED is feasible in the setup. In our previous research, we have successfully verified that a conventional transmissive microscope can be transformed into an optical path difference microscope by using a Shack-Hartmann wavefront sensor under incoherent illumination. The key condition is that the numerical aperture of illumination should be smaller than the numerical aperture of imaging lens. This approach is also applicable to characterization of reflective and slightly scattering surfaces.Team Raf Van de Pla
Aberrations and pupil location under corneal topography and Hartmann-Shack illumination conditions
No full textPURPOSE. This study was conducted to determine the magnitude of pupil center shift between the illumination conditions provided by corneal topography measurement (photopic illuminance) and by Hartmann-Shack aberrometry (mesopic illuminance) and to investigate the importance of this shift when calculating corneal aberrations and for the success of wavefront-guided surgical procedures. \ud
\ud
METHODS. Sixty-two subjects with emmetropia underwent corneal topography and Hartmann-Shack aberrometry. Corneal limbus and pupil edges were detected, and the differences between their respective centers were determined for both procedures. Corneal aberrations were calculated using the pupil centers for corneal topography and for Hartmann-Shack aberrometry. Bland-Altmann plots and paired t-tests were used to analyze the differences between corneal aberrations referenced to the two pupil centers. \ud
\ud
RESULTS. The mean magnitude (modulus) of the displacement of the pupil with the change of the illumination conditions was 0.21 ± 0.11 mm. The effect of this pupillary shift was manifest for coma corneal aberrations for 5-mm pupils, but the two sets of aberrations calculated with the two pupil positions were not significantly different. Sixty-eight percent of the population had differences in coma smaller than 0.05 µm, and only 4% had differences larger than 0.1 µm. Pupil displacement was not large enough to significantly affect other higher-order Zernike modes. \ud
\ud
CONCLUSIONS. Estimated corneal aberrations changed slightly between photopic and mesopic illumination conditions given by corneal topography and Hartmann-Shack aberrometry. However, this systematic pupil shift, according to the published tolerances ranges, is enough to deteriorate the optical quality below the theoretically predicted diffraction limit of wavefront-guided corneal surgery. \ud
\u
William Paul Young's World view in The Shack Novel (2007) : A Genetic Structuralism Approach
No full textThis study disscuses the world view that implicit in The Shack novel by William Paul Young, which is analyzed by using the theory of genetic structuralism. The purpose of this research is to identify the novel structure and to find out about the world view from the author of The Shack novel. This research belongs to qualitative descriptive research. The premier data source of this research is William Paul Young's The Shack (2007), while secondary data sources are taken from books, the Internet, articles and others. The result of this research shows the world view of the author of The Shack novel that is about unity and diversity in human life. The other world view of the author is about how to live with trust, love and forgiveness for better life
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
Increased susceptibility of ras-transformed cells to phenylacetate is associated with inhibition of p21ras isoprenylation and phenotypic reversion
No full textAlterations in the expression of ras oncogenes are characteristic of a wide variety of human neoplasms. Accumulating evidence has linked elevated ras expression with disease progression and with failure of tumors to respond to conventional therapies, including radiotherapy and certain chemotherapies. These observations led us to investigate the response of ras-transformed cells to the differentiation-inducer phenylacetate (PA). Using gene transfer models, we show that PA caused cytostasis in ras-transformed mesenchymal cells, associated with increased expression of 2',5'-oligoadenylate synthetase, an enzyme implicated in negative growth control. PA also induced phenotypic reversion characterized by loss of anchorage-independent growth, reduced invasiveness and increased expression of collagen α type I, a marker of cell differentiation. The anti-tumor activity of PA was observed in cases involving either Ha- or Ki-ras and was independent of the mode of oncogene activation. Interestingly, in contrast to their relative resistance to radiation and doxorubicin, ras-transformed cells were significantly more sensitive to PA than their parental cells. The profound changes in tumor cell and molecular biology were associated with reduced isoprenylation of the ras-encoded p21. Our results indicate that PA can suppress the growth of ras-transformed cells, resistant otherwise to free-radical based therapies, through interference with p21(ras) isoprenylation, critical to signal transduction and maintenance of the malignant phenotype
Application of Wavefront Sensorless Methods to Shack-Hartmann Patterns for Wavefront Reconstruction: A Two-step Method for Single-Frame Phase Retrieval
No full textCounter-acting image quality degradation caused by phase aberrations through physical correction requires the phase field to be known. As imaging hardware captures real-valued intensity, defined as the wave amplitude squared, obtaining this lost phase information is known as the phase retrieval problem and is a non-linear and non-convex optimisation problem. Literature treats this problem from two points of view: reconstruction using indirect phase sampling, and reconstruction using Fourier amplitude sampling. The former employs wavefront sensors such as the Shack-Hartmann sensor, which encodes phase gradient information in the form of the displacement of imaged spots. These methods, while fast, discard information such as interference and are limited to low-order reconstruction. The latter, also called wavefront sensorless phase retrieval, uses the full point-spread function (PSF) to obtain high-accuracy reconstruction at the cost of computational speed, number of intensity images required, or limited aberration magnitude. These two points of view have remained largely separated, but can be made compatible through a modelling technique called Shack-Hartmann diversity. This thesis explores the merging of wavefront sensorless methods with Shack-Hartmann intensity patterns to leverage more information from a single captured image frame.Firstly, a low-order modal reconstruction technique is presented applied to phase gradient fields obtained from Fourier demodulation of a Shack-Hartmann intensity pattern. A method of minimising the amount of redundant data-points used for reconstruction is illustrated through the removal of Fourier-interpolated data to speed up performance.Secondly, a novel extension of the Fourier demodulation technique to hexagonal Shack-Hartmann arrays is presented, allowing phase gradient extraction and modal reconstruction of hexagonal array intensity images using Fourier demodulation.Thirdly, Shack-Hartmann diversity is used to extend intensity-based modal phase retrieval using Taylor approximation of the intensity function to Shack-Hartmann intensity patterns. This bridges the gap between wavefront sensorless methods and Shack-Hartmann intensity patterns.Lastly, a novel hybrid method is presented for high-accuracy phase reconstruction based on applying the above intensity-based method to a single Shack-Hartmann intensity pattern with low-order pre-conditioning obtained from Fourier demodulation. The method is demonstrated to obtain highly accurate reconstruction on low-order aberrations, and better reconstruction accuracy on small-magnitude high-order aberrations with dominating large-magnitude low-order terms than traditional methods. Potential use cases are discussed, such as open-loop turbulence reconstruction for use in turbulence modelling.Mechanical Engineering | Systems and Contro
- …
