Editing Optical telescope (section)

===The five Seidel aberrations===
{{main|Optical aberration}}
; [[Spherical aberration]] : The difference in focal length between paraxial rays and marginal rays, proportional to the square of the objective diameter.
; [[Coma (optics)|Coma]] : A defect by which points appear as comet-like asymmetrical patches of light with tails, which makes measurement very imprecise. Its magnitude is usually deduced from the [[optical sine theorem]].
; [[Astigmatism (optical systems)|Astigmatism]] : The image of a point forms focal lines at the sagittal and tangental foci and in between (in the absence of coma) an elliptical shape.
; [[Petzval field curvature]]: The Petzval field curvature means that the image, instead of lying in a plane, actually lies on a curved surface, described as hollow or round. This causes problems when a flat imaging device is used e.g., a photographic plate or CCD image sensor.
; [[Distortion (optics)|Distortion]]: Either barrel or pincushion, a radial distortion that must be corrected when combining multiple images (similar to stitching multiple photos into a [[Panoramic photography|panoramic photo]]).

Optical defects are always listed in the above order, since this expresses their interdependence as first order aberrations via moves of the exit/entrance pupils. The first Seidel aberration, Spherical Aberration, is independent of the position of the exit pupil (as it is the same for axial and extra-axial pencils). The second, coma, changes as a function of pupil distance and spherical aberration, hence the well-known result that it is impossible to correct the coma in a lens free of spherical aberration by simply moving the pupil. Similar dependencies affect the remaining aberrations in the list.