ANALYSIS Forum (IPACO) Forum Index

ANALYSIS Forum (IPACO)
Dedicated to the analysis of alleged UFO photos and videos

 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

Aberrations

 
Post new topic   Reply to topic    ANALYSIS Forum (IPACO) Forum Index -> Optics in photography -> Aberrations and effects
Previous topic :: Next topic  
Author Message
elevenaugust
Administrator
Administrator

Offline

Joined: 20 Jun 2012
Posts: 67

PostPosted: 06/28/2012, 04:47 pm    Post subject: Aberrations Reply with quote

Definitions:

They can be divided into five categories:

  • Coma aberration
  • Spherical aberration
  • Chromatic aberration
  • Distortions (Barrel and Pincushion)
  • Defocus/focus aberration


Back to top
Publicité






PostPosted: 06/28/2012, 04:47 pm    Post subject: Publicité

PublicitéSupprimer les publicités ?
Back to top
elevenaugust
Administrator
Administrator

Offline

Joined: 20 Jun 2012
Posts: 67

PostPosted: 06/30/2012, 07:29 pm    Post subject: Aberrations Reply with quote

Coma aberration


 

Coma aberration which means "comet" in latin, is similar to spherical aberration; it applies to rays entering the lens at an angle. You probably created this aberration being kid by tilting a lens under the sunlight. At the beginning the projected image of the sun is circular but as you tilt the lens with respect to the sun direction the resulting image takes an elongated shape, like a comet.
  
This coma aberration is dependent upon lens shape. Rays incoming from the periphery of the lens focus closer to the axis and produce a larger blurry spot than the paraxial rays. As coma is proportional to the distance to the central axis, more the rays are away from the center, more the focal point changes of position and get blurry images, mainly off-axis.  
For a newtonian the length of the coma is expressed as L = (3/16)(D/F)2a, where D is the primary mirror diameter, F the focal length and a the distance to the axis. For the Palomar telescope of 5.08 m used in newtonian configuration, the focal length is 16.3m. The relation tell us that L = a/55. That means that at 55" for the axis the coma is already 1" long ! To stay below that resolution, the useful field is only 9mm wide...  


 
  
This effect is the most obvious on fast scopes like dobsonians or poor quality astrographs. SCT will all their spherical mirrors display also an important coma larger than an optimized design with one or two aspherical or aplanetic mirrors (Ritchey-Chrétien, etc). Commercial designs could perform significantly better scopes but this is not often the case for financial reasons.  
Coma can be corrected by using corrective lenses placed symmetrically around the axis. Ross or Wynne corrector using two to four lenses are built to correct this aberration as well as the others. For visual observations, this effect can be suppressed using for example the Tele Vue Paracorr suited to newtonian scopes.



 
 


Back to top
elevenaugust
Administrator
Administrator

Offline

Joined: 20 Jun 2012
Posts: 67

PostPosted: 06/30/2012, 07:37 pm    Post subject: Aberrations Reply with quote

Spherical aberration




 


Spherical aberration is an optical effect observed in an optical device (lens, mirror, etc.) that occurs due to the increased refraction of light rays when they strike a lens or a reflection of light rays when they strike a mirror near its edge, in comparison with those that strike nearer the centre. It signifies a deviation of the device from the norm, i.e., it results in an imperfection of the produced image.
A sphere lens has an aplanatic point (i.e., no spherical aberration) only at a radius that equals the radius of the sphere divided by the index of refraction of the lens material. A typical value of refractive index for crown glass is 1.5, (See list) which indicates that only about 43% of the area (67% of diameter) of a spherical lens is useful. It is often considered to be an imperfection of telescopes and other instruments which makes their focusing less than ideal due to the spherical shape of lenses and mirrors. This is an important effect, because spherical shapes are much easier to produce than aspherical ones. In many cases, it is cheaper to use multiple spherical elements to compensate for spherical aberration than it is to use a single aspheric lens.



 

"Positive" spherical aberration means peripheral rays are bent too much. "Negative" spherical aberration means peripheral rays are not bent enough.
The effect is proportional to the fourth power of the diameter and inversely proportional to the third power of the focal length, so it is much more pronounced at short focal ratios, i.e., "fast" lenses.
For small telescopes using spherical mirrors with focal ratios shorter than f/10, light from a distant point source (such as a star) is not all focused at the same point. Particularly, light striking the inner part of the mirror focuses farther from the mirror than light striking the outer part. As a result the image cannot be focused as sharply as if the aberration were not present. Because of spherical aberration, telescopes shorter than f/10 are usually made with non-spherical mirrors or with correcting lenses.
In lens systems, the effect can be minimized using special combinations of convex and concave lenses, as well as using aspheric lenses.
For simple designs one can sometimes calculate parameters that minimize spherical aberration. For example, in a design consisting of a single lens with spherical surfaces and a given object distance o, image distance i, and refractive index n, one can minimize spherical aberration by adjusting the radii of curvature
and
of the front and back surfaces of the lens such that
.
 

 


Back to top
Contenu Sponsorisé






PostPosted: Today at 08:14 am    Post subject: Aberrations

Back to top
Display posts from previous:   
Post new topic   Reply to topic    ANALYSIS Forum (IPACO) Forum Index -> Optics in photography -> Aberrations and effects All times are GMT + 2 Hours
Page 1 of 1

 
Jump to:  

Index | Create free forum | Free support forum | Free forums directory | Report a violation | Conditions générales d'utilisation
Powered by phpBB © 2001, 2005 phpBB Group