An optical aberration is a defect in the image due to the optical system itself. The resulting image is not consistent about observed. It is possible to classify the aberrations in two groups:
► Chromatical aberrations, which are responsible for colorimetric order errors in the images.
► Geometric aberrations that cause deformation in the image.
Chromatism ] The chromatic aberration appears when the colors do not converge to the same point. It is manifested by the presence of red and/or blue piping on the edge of each object observed. This aberration is often present, but to different degrees depending on the optical design. This optical defect is typical of telescopes. It is explained by the fact that a lens does not converge at the same point each wavelength of light from a star. Focal slightly reveals different according to each color. Combinations of 2, 3 or 4 lenses can mitigate chromaticism (achromatic doublet and triplet apochromatic). It is important to take into account when buying, because there is no solution to correct chromatic aberration when present.
Spherical Aberration ] Fault because the rays from the lens edge does not converge to the same level as the rays from the center. spherical aberration is difficult to be detected because it impasted star’s image observed while retaining its circular shape. or it affects the contrast and resolution of the instrument. it is present in newtonian telescopes.
Curvature of Field ] the curvature of field is related to the combination of an optical instrument. This defect is present when the image is not formed in a plane, but on a slightly spherical surface. This translates into the stars of increasingly blurred as one approaches the edges of the images. By acting on the focus, we get to make the star net field edge, but losing the sharpness in the center. The solution is to use a field flattener (field flattener) to eliminate this defect. In the case of a newtonian, where the dominant aberration is coma, the field correction dedicated to these instruments compensate both coma and field curvature.
Coma ] Aberration that distorts objects located in edge of field. It occurs when the light rays that form the image at a point more convergent. A star is thus affected most punctual but stretched comet-shaped (hence the name coma). This defect is marked in optical instruments are not aligned. In this case, the coma can be reduced by a better adjustment (collimation). It is also very present with the said instruments or wide open fields (newtonian). In this case you can use a field corrector to mitigate it. Certain telescope photographic vocation, such as Ritchey-Chrétien, are naturally corrected for coma. They said aplanatic.
Astigmatism ] Fault which, at high magnification, gives the star a cross shape. It is often related to how the main mirror is supported inside the tube. Too tight straps or dorsal point incorrectly set the twist so that the image is affected. In these cases, reduce the stress on the optical to overcome. Sometimes it comes from a lack of lens grinding or, as with coma, poor alignment of the optics.
Clover or trefoil ] It is when a star diffraction spot is shaped like a clover.It is due to poor support of the main mirror or main lens. Again, it can be corrected by reducing the stress on the optics. But if the system is poorly designed, it will not be enough.
Distortion ] As the name suggests, it is a distortion of the image. uncommon with telescopes, it is quite common hand with binoculars and camera objectives. When buying an instrument, do not hesitate to do a test by pointing a building, structure or composition having parallel lines to be sure that there is no distortion.
Egrets ] This is a diffraction phenomenon of light caused by the branches of the spider supporting the secondary mirror of the telescope so the tube is open. A spider with four branches causes four egrets, while a spider with three branches creates six. A photograph taken with a telescope or a catadioptric telescope should therefore not show unless you have added to the front of the instrument, two son purposes for creating this. The thickness of the son depend on the fineness and length of egrets. multiple egrets sometimes visible on photographs taken with photographic lenses are due to the blades of their diaphragms.
Vignette ] This aberration gradually darkens the edges of the image, especially the corners. It may be due to a defect in the lens or a filter. Over the field is wide and the smaller and more focus. Vignetting can be either due to a lack of optical light homogeneity (your goal will let more light in the center) or to the edges of the tube that enters the image field.