The following is a short description of the various common defects and aberrations possible in eyepieces. Even the most expensive eyepieces are not without their imperfections. It is up to the user to judge what is acceptable to his needs and to the specific task. Frequently these defects will be stated in vendor advertisements, word of mouth and reviews. A basic knowledge of the terms and knowing how to judge the view through an eyepiece are important.
This is a distortion in the field of view likened to what is called the “fishbowl effect”. As the scope is moved across the field of view the objects will not move in straight lines but will be bowed in their path. This distortion may not affect observation but when searching the sky may prove to be unacceptable and disorienting. Also, an eyepiece with this distortion could not be used for performing drift alignment, as the star would need to appear to be proceeding in a straight path.
This can be seen as stars that are out of focus on the periphery of the field of view while the center stars are in focus and is more exaggerated at low magnifications.
Some degree of field curvature is inherent in all lenses but the corrective combination of concave and convex lens elements, their spacing and glass quality, can compensate for this problem. This is one reason that telescopes and eyepieces are not always a good match. Also, although visually we can compensate for this problem to a certain degree, the ability to do so diminishes with age.
When the horizontal and vertical curvature of a lens system is not the same, a defect called astigmatism can occur. This is corrected in eyepieces by using better quality lenses with the correct curvature, spacing and refractive indices.
This defect is observable as stars looking like crosses on the periphery of the field of view while the center stars are in focus. This problem is mostly seen in less expensive eyepieces and to an even greater degree if having a larger Apparent Field Of View.
Angular Magnification Distortion
This problem cannot be easily noticed by the observer. The defect is that the degree of magnification differs across the field of view. A star would thus appear to move faster across the center of the field than at the periphery. An eyepiece with this type of defect and performing such procedures as star drift timing would give inaccurate results.
Spherical Aberration of the Exit Pupil
This defect reveals itself as a floating black spot or “kidney bean” making it difficult to see the entire field of view. In addition, the field will not be in correct focus due to the fact that light waves passing through the periphery of a lens are not brought into exact focus with those passing through the center. A compound corrective lens group of convex and concave lenses with the appropriate thicknesses will amend this problem.
This defect is similar to spherical aberration and is most pounced when the telescope is misaligned. This defect shows itself as a “comet tail” extending from a focused star. This defect predominates at the periphery of the field of view and is caused by the refraction differences of the light rays passing through the various lens zones. Coma aberration is greater for lenses with wider apertures. Coma of a single lens or a system of lenses can be minimized (and in some cases eliminated) by using the correct combination of lens curvatures.
Vignetting presents itself as a noticeable darkening of the field of view towards the edges. It occurs when a lens of an eyepiece is not able to field all the lights rays coming through the previous lens. The factors affecting vignetting in observational astronomy are related to the Apparent Field Of View (AFOV) and the Eye Relief of the eyepiece. The maximum field of view deliverable by the telescope must be less than that of the eyepiece. Also the eyepiece with the correct Eye Relief must be used or the user will not be able to position his eye to take advantage of the complete field of view.
This aberration can be seen mostly on the edge of the field where an object will have a false color fringe. This is due to the differential dispersion of light through a lens. It is caused by the fact that the lens refracts or bends the various wavelengths or colors of white light to a different extent thus leading to the image coming to focus separately according to wavelength, at multiple spots along the optical axis. A discrete focal point is not obtained. A halo effect can thus be seen. In order to correct for chromatic aberration the lenses have to be highly polished and more precisely spaced, using better quality glass.
Ideally, and separate from the errors of focus and projection, all light entering the eyepiece should be transmitted to the focal plane as a coherent image. When this does not happen, a variety of “stray light” artifacts appear in the image. These are reflections off interior surfaces that are combatted by painting surfaces flat black, installing baffles, etc.
This is the appearance of secondary images due to reflection of the lenses. These are normally controlled by antireflection coatings, which are very thin layers of materials with refractive indices that differ from each other and from air or glass and therefore provide intermediate steps in the refraction.