aberration

In optics, aberration is an optical system's deviation from true, such as points in the image being smeared and/or shapes being changed. Lenses and mirrors inevitably cause some aberration which can be the primary limit to a telescope's use. Though some aberrations cannot be entirely eliminated, specific lens and mirror designs reduce some, as can the overall design of telescopes (e.g., a larger focal length reduces some of them). Aberrations come from a variety of causes resulting in a variety of symptoms. Some terms/classes:

• chromatic aberration.
• spherical aberration.
• coma.
• astigmatism.
• field curvature.
• distortions (such as pincushion distortion and barrel distortion).
• focal plane tilt.
• wavefront error.

The five Seidel aberrations (aka Seidel errors) are five of these that were explained in the mid-19th century by Philipp Ludwig von Seidel, which were spherical aberration, coma, astigmatism, field curvature, and distortion.

The term aberration has other uses in astronomy that are not related to optics, i.e., "perfect" optics would still be affected:

• astronomical aberration (sometimes just called aberration) is the apparent difference in position in the celestial sphere of a celestial body (e.g., star) due to the motion of the observer. Given the orbit of the Earth around the Sun, light received from a particular star is approaching us at a slightly different angle because both we and the light are moving. This factor is sufficiently significant (several arcseconds) that it must be taken into account in determining accurate and precise coordinates for a celestial object, and is especially needed in astrometry and in the determination of orbits of solar system bodies.
• relativistic aberration refers to effects due to the speed of light and relativity; among the effects is relativistic beaming.