Design of A ShackHartmann Sensor, Vrsn2 Using the Shack Hartmann sensor allows one to find the frame by frame wavefront tilt at the telescope aperture. This second design inserts beam splitters into the optical path before and after the sensor’s lenslet array to form a parallel optical path whose blur spot is superimposed on the focal plane along with the Hartmann spots. The design allows one to use a single focal plane and avoid camera timing skew if two cameras were otherwise used. Since the measurements are simultaneous, one can double check the tilt (Zernicke tilt) derived from wavefront reconstruction with the tilt seen from a telescope’s usual star image blur spot (centroid tilt).
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Design of a Shack Hartmann Sensor, Vrsn2, Tilt Measurements Describes the different tilt measurements, G-tilt, Z-tilt, and C-tilt, which do not necessarily agree. Just as there is more than one definition of tilt for a wavefront, there is more than one way of defining the center of an irradiance pattern as illustrated by the two most common types of position sensors, quad cells and centroiding devices. Interestingly, z-tilt places the point of highest intensity in the far field closer to x = 0.
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Studies of Tilt Jitter and Tilt Anisoplanatism The telescopic view of a guide star and an imaging object of interest is a view through differing air columns of differing atmospheric turbulence. Viewing through differing air columns is called angular or tilt anisoplanatism and is a performance degrading effect. This paper addresses the theoretical and measured tilt jitter bandwidth, which is proportional to the ratio of wind velocity to aperture diameter, and the effectiveness or correlation between guide star and a separated imaging object. For the 0.5 meter telescope used in the study, 90% of tilt occurs within 1.6 Hz. Interestingly, tilt anisoplanatism is a weak function of object/guide star orientation.
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Shack Hartmann Subaperture Cross Coupling Study This paper investigates the possibility of cross coupling between Shack Hartmann sub apertures, particularly in the presence of scintillation. Weak coupling of differing strengths does occur, particularly when scintillation gives diminished light intensity in a particular sub aperture, while simultaneously giving increased light intensity in a neighboring sub aperture. Professional observatories AO systems do not have this problem because shorter wavelength visible light is used for its AO wavefront sensor, but sub aperture spacing is larger because science is done at longer infrared wavelengths, for which the Fried coherence parameter scales as wavelength to the 6/5 th power.
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