Telescope

A telescope (from the Greek tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing') is an astronomical tool which gathers and focuses electromagnetic radiation. Telescopes increase the apparent angular size of ingerso objects, as well as their apparent brightness.

Imperfect Images

No telescope can form a perfect image. Even if a reflecting telescope could have a perfect mirror, or a refracting telescope could have a perfect lens, the effects of aperture diffraction could still not be escaped. In reality, perfect mirrors and perfect lenses do not exist, so image aberrations in addition to aperture diffraction must be taken into account. Image aberrations can be broken down into two main classes, monochromatic, and polychromatic. In 1857, Philipp Ludwig von Seidel (1821-1896) decomposed the first order monochromatic abberations into five constituent aberrations. They are now commonly referred to as the five Seidel Aberrations.

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The Five Seidel Aberrations

; Spherical Aberration : The difference in focal length between paraxial rays and marginal rays, proportional to the square of the aperture.

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; Coma : A most objectionable defect by which points are imaged as comet like asymmetrical patches of light with tails, which makes measurement very imprecise. It's magnitude is usually deduced from the Optical Sine Theorem.

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; Astigmatism : The image of a point forms focal lines at the sagittal and tangiental foci and in between (in the absence of coma) an eliptical shape.

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; Curvature of Field : The Petzval curvature means that the image instead of lying in a plane actually lies on a curved surface which is described as hollow or round. This causes problems when a flat imaging device is used e.g. a photographic plate or CCD image sensor.

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; Distortion : Either barrel or pincushion, causing the same image shape errors as the eponymous controls on your monitor!

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They 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 abberation, 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 is 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 remaing aberrations in the list.

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The Chromatic Aberrations

; Longitudinal Chromatic Aberration : As with spherical aberration this is the same for axial and oblique pencils.

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; Transverse Chromatic Aberration (Chromatic Aberration of Magnification)

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