optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

(especially

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

s), the coma (), or comatic aberration, in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

or other components that results in off-axis

point source A point source is a single identifiable ''localized'' source of something. A point source has a negligible extent, distinguishing it from other source geometries. Sources are called point sources because, in mathematical modeling, these sources ...

s such as stars appearing distorted, appearing to have a tail (

coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to Nociception, respond normally to Pain, painful stimuli, light, or sound, lacks a normal Circadian rhythm, sleep-wake cycle and does not initiate ...

) like a

comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...

. Specifically, coma is defined as a variation in

magnification Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a size ratio called optical magnification. When this number is less than one, it refers to a reduction in size, so ...

over the

entrance pupil In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the optical elements in front of the stop. The corresponding image of the aperture stop as seen through the optical elements behin ...

. In

refractive In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...

diffractive Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wa ...

optical systems, especially those imaging a wide spectral range, coma can be a function of

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

, in which case it is also a form of

chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion, color aberration, color fringing, or purple fringing, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the ...

Overview

Coma is an inherent property of telescopes using parabolic mirrors. Unlike a spherical mirror, a bundle of parallel rays parallel to the optical axis will be perfectly focused to a point (the mirror is free of spherical aberration), no matter where they strike the mirror. However, this is only true if the rays are parallel to the axis of the parabola. When the incoming rays strike the mirror at an angle, individual rays are not reflected to the same point. When looking at a point that is not perfectly aligned with the optical axis, some of the incoming light from that point will strike the mirror at an angle. This causes an image that is not in the center of the field to appear as wedge-shaped. The further off-axis (or the greater the angle subtended by the point with the optical axis), the worse this effect is. This causes stars to appear to have a cometary coma, hence the name. Schemes to reduce coma without introducing spherical aberration include Schmidt, Maksutov, ACF and Ritchey–Chrétien optical systems. Correction lenses, " coma correctors" for Newtonian reflectors have been designed which reduce coma in newtonian telescopes. These work by means of a dual lens system of a plano-convex and a plano-concave lens fitted into an eyepiece adaptor which superficially resembles a Barlow lens. Coma of a single lens or a system of lenses can be minimized (and in some cases eliminated) by choosing the

curvature In mathematics, curvature is any of several strongly related concepts in geometry that intuitively measure the amount by which a curve deviates from being a straight line or by which a surface deviates from being a plane. If a curve or su ...

of the lens surfaces to match the application. Lenses in which both spherical aberration and coma are minimized at a single wavelength are called ''bestform'' or ''aplanatic'' lenses.

In human vision

Vertical coma is the most common higher-order aberration in the eyes of patients with keratoconus. Coma is also a common temporary symptom of corneal injuries or abrasions, in which case the visual defect gradually resolves as the cornea heals. Baader Rowe Coma Corrector Comparison

References

{{reflist

External links

About coma in a Newtonian telescope

Coma Aberration in Youtube
Geometrical optics Image defects de:Abbildungsfehler#Koma hu:Optikai lencsék leképzési hibái#Kóma

Overview

In human vision

See also

References

External links