Shack–Hartmann wavefront sensor

Abstract: A Hartmann-Shack wave-front sensor is used to measure the wave aberrations of the human eye by sensing the wave front emerging from the eye produced by the retinal reflection of a focused light spot on the fovea. Since the test involves the measurements of the local slopes of the wave front, the actual wave front is reconstructed by the use of wave-front estimation with Zernike polynomials. From the estimated Zernike coefficients of the tested wave front the aberrations of the eye are evaluated. It is shown that with this method, using a Hartmann-Shack wave-front sensor, one can obtain a fast, precise, and objective measurement of the aberrations of the eye.

Abstract: developed out of a need to solve a problem. The problem was posed, in the late 1960s, to the Optical Sciences Center (OSC) at the University of Arizona by the US Air Force. They wanted to improve the images of satellites taken from earth. The earth's atmosphere limits the image quality and exposure time of stars and satellites taken with telescopes over 5 inches in diameter at low altitudes and 10 to 12 inches in diameter at high altitudes. Dr. Aden Mienel was director of the OSC at that time. He came up with the idea of enhancing images of satellites by measuring the Optical Transfer Function (OTF) of the atmosphere and dividing the OTF of the image by the OTF of the atmosphere. The trick was to measure the OTF of the atmosphere at the same time the image was taken and to control the exposure time so as to capture a snapshot of the atmospheric aberrations rather than to average over time. The measured wavefront error in the atmosphere should not change more than ␭/10 over the exposure time. The exposure time for a low earth orbit satellite imaged from a mountaintop was determined to be about 1/60 second. Mienel was an astronomer and had used the standard Hartmann test (Fig 1), where large wooden or cardboard panels were placed over the aperture of a large telescope. The panels had an array of holes that would allow pencils of rays from stars to be traced through the telescope system. A photographic plate was placed inside and outside of focus, with a sufficient separation, so the pencil of rays would be separated from each other. Each hole in the panel would produce its own blurry image of the star. By taking two images a known distance apart and measuring the centroid of the images, one can trace the rays through the focal plane. Hartmann used these ray traces to calculate figures of merit for large telescopes. The data can also be used to make ray intercept curves (H'-tan U'). When Mienel could not cover the aperture while taking an image of the satellite, he came up with the idea of inserting a beam splitter in collimated space behind the eyepiece and placing a plate with holes in it at the image of the pupil. Each hole would pass a pencil of rays to a vidicon tube (this was before …

Abstract: This paper discusses the use of Shack–Hartmann wavefront sensors to determine the vertical distribution of atmospheric optical turbulence above large telescopes. It is demonstrated that the turbulence altitude profile can be recovered reliably from time-averaged spatial cross-correlations of the local wavefront slopes for Shack–Hartmann observations of binary stars. The method, which is referred to as SLODAR, is analogous to the well known SCIDAR scintillation profiling technique, and a calibration against contemporaneous SCIDAR observations is shown. Hardware requirements are simplified relative to the scintillation method, and the number of suitable target objects is larger. The implementation of a Shack–Hartmann based turbulence monitor for use at the William Herschel Telescope is described. The system will be used to optimize adaptive optical observations at the telescope and to characterize anisoplanatic variations of the corrected point spread function.

Abstract: The design of a wavefront sensor may be determined by the lenslet array and camera selection. There are numerous different applications for these sensors, requiring widely differing dynamic range and accuracy. Performance metrics are needed to evaluate candidate designs and to compare results. We have developed a standard methodology for measuring the repeatability, accuracy and dynamic range of different wavefront sensor designs, and have experimentally applied these metrics to a number of different sensors

Abstract: The imaging depth of two-photon excitation fluorescence microscopy is partly limited by the inhomogeneity of the refractive index in biological specimens. This inhomogeneity results in a distortion of the wavefront of the excitation light. This wavefront distortion results in image resolution degradation and lower signal level. Using an adaptive optics system consisting of a Shack-Hartmann wavefront sensor and a deformable mirror, wavefront distortion can be measured and corrected. With adaptive optics compensation, we demonstrate that the resolution and signal level can be better preserved at greater imaging depth in a variety of ex-vivo tissue specimens including mouse tongue muscle, heart muscle, and brain. However, for these highly scattering tissues, we find signal degradation due to scattering to be a more dominant factor than aberration.