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. They come from a variety of causes resulting in a variety of symptoms. Some classes:

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

The term aberration has other uses in astronomy that are not related to optics, i.e., better optics would not help:

• 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 need in astrometry and in the determination of orbits of solar system bodies.
• relativistic aberration further effects due to the speed of light due to relativity including relativistic beaming.