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elevenaugust

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 Posté le: Jeu 28 Juin - 17:29 (2012)    Sujet du message: Aberrations Définitions:Elles peuvent se diviser en cinq catégories:Aberration de Coma Aberration sphériqueAberration chromatiqueDistorsions (en barillet et en coussinet)Aberrations de mise au point
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Posté le: Jeu 28 Juin - 17:29 (2012)    Sujet du message: Publicité

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elevenaugust

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 Posté le: Dim 1 Juil - 19:30 (2012)    Sujet du message: Aberrations {traduction en cours}Coma aberration Coma aberration which means "comet" in latin, is similar to spherical aberration; it applies to rays entering the lens at an angle. You probably created this aberration being kid by tilting a lens under the sunlight. At the beginning the projected image of the sun is circular but as you tilt the lens with respect to the sun direction the resulting image takes an elongated shape, like a comet. This coma aberration is dependent upon lens shape. Rays incoming from the periphery of the lens focus closer to the axis and produce a larger blurry spot than the paraxial rays. As coma is proportional to the distance to the central axis, more the rays are away from the center, more the focal point changes of position and get blurry images, mainly off-axis. For a newtonian the length of the coma is expressed as L = (3/16)(D/F)2a, where D is the primary mirror diameter, F the focal length and a the distance to the axis. For the Palomar telescope of 5.08 m used in newtonian configuration, the focal length is 16.3m. The relation tell us that L = a/55. That means that at 55" for the axis the coma is already 1" long ! To stay below that resolution, the useful field is only 9mm wide... This effect is the most obvious on fast scopes like dobsonians or poor quality astrographs. SCT will all their spherical mirrors display also an important coma larger than an optimized design with one or two aspherical or aplanetic mirrors (Ritchey-Chrétien, etc). Commercial designs could perform significantly better scopes but this is not often the case for financial reasons. Coma can be corrected by using corrective lenses placed symmetrically around the axis. Ross or Wynne corrector using two to four lenses are built to correct this aberration as well as the others. For visual observations, this effect can be suppressed using for example the Tele Vue Paracorr suited to newtonian scopes.
elevenaugust

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 Posté le: Dim 1 Juil - 19:31 (2012)    Sujet du message: Aberrations {traduction en cours}Spherical aberration Spherical aberration is an optical effect observed in an optical device (lens, mirror, etc.) that occurs due to the increased refraction of light rays when they strike a lens or a reflection of light rays when they strike a mirror near its edge, in comparison with those that strike nearer the centre. It signifies a deviation of the device from the norm, i.e., it results in an imperfection of the produced image. A sphere lens has an aplanatic point (i.e., no spherical aberration) only at a radius that equals the radius of the sphere divided by the index of refraction of the lens material. A typical value of refractive index for crown glass is 1.5, (See list) which indicates that only about 43% of the area (67% of diameter) of a spherical lens is useful. It is often considered to be an imperfection of telescopes and other instruments which makes their focusing less than ideal due to the spherical shape of lenses and mirrors. This is an important effect, because spherical shapes are much easier to produce than aspherical ones. In many cases, it is cheaper to use multiple spherical elements to compensate for spherical aberration than it is to use a single aspheric lens. "Positive" spherical aberration means peripheral rays are bent too much. "Negative" spherical aberration means peripheral rays are not bent enough. The effect is proportional to the fourth power of the diameter and inversely proportional to the third power of the focal length, so it is much more pronounced at short focal ratios, i.e., "fast" lenses. For small telescopes using spherical mirrors with focal ratios shorter than f/10, light from a distant point source (such as a star) is not all focused at the same point. Particularly, light striking the inner part of the mirror focuses farther from the mirror than light striking the outer part. As a result the image cannot be focused as sharply as if the aberration were not present. Because of spherical aberration, telescopes shorter than f/10 are usually made with non-spherical mirrors or with correcting lenses. In lens systems, the effect can be minimized using special combinations of convex and concave lenses, as well as using aspheric lenses. For simple designs one can sometimes calculate parameters that minimize spherical aberration. For example, in a design consisting of a single lens with spherical surfaces and a given object distance o, image distance i, and refractive index n, one can minimize spherical aberration by adjusting the radii of curvature and of the front and back surfaces of the lens such that .

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