When choosing eyepieces it is also important to be able to understand what issues may be prevalent in the eyepiece design. If you are researching which eyepieces to purchase you will sometimes read about defects or aberrations in a certain eyepiece, often the reviewer will state what problem they found e.g. “there is rectilinear distortion present”, this is only useful if you know what rectilinear distortion is and how it effects the view through an eyepiece. The following is a list of the more common defects/aberrations that effect eyepieces and a short explanation of what might be seen through the eyepiece if they are present.
As this is mentioned above I will start with it. When there is rectilinear distortion present reviewers quite often write of the ‘fishbowl effect’, the picture below illustrates the issue.
The illustration on the left depicts a field of view without any rectilinear distortion, the illustration in the middle depicts a form of rectilinear distortion usually called ‘barrel distortion’ and the illustration on the right depicts a form of rectilinear distortion usually called ‘pincushion distortion’. For most of us when observing this issue has little impact, what it means is the stars are not in exact position relative to each other. However, if your observing involves lots of sweeping the sky, say as a comet hunter, as you move your scope the stars will not move across the field of view in straight lines they will bow one way or the other giving the ‘looking through a fishbowl effect’, some observers find this difficult to tolerate and some even find it nauseating.
Field curvature is not constant with every telescope, an eyepiece may present field curvature in one scope but not in another, this is because all telescopes and eyepieces have a slightly curved focal plane (in scopes it is usually convex but in eyepieces it can be either convex or concave). It is the interaction between the two focal planes that can cause field curvature to be present, if the two planes are a perfect match the concave plane of the eyepiece will exactly counteract the convex plane of the scope resulting in a perfect flat field. Our eyes can compensate for a degree of field curvature so we will not notice slight field curvatures (the ability of our eyes to accommodate for field curvature lessens with age so older observers are bother more by this than the younger observers).
Field curvature is recognised by stars at the edge of field being out of focus, slightly blurred when the central stars are in focus. If it is field curvature that is causing this you will be able to focus the stars at the edge of the field by either out-focusing (then the curvature is concave) or by in-focusing (then the curvature is convex) but of course the central stars will now be out of focus. As I stated field curvature is not purely a result of the eyepiece but as a result of the interaction of scope and eyepiece so if a reviewer states that field curvature was present this does not mean that in your set-up you will experience a similar issue.
Astigmatism is widely regarded as the most ‘annoying’ aberration to be present in an eyepiece and the one that most people find unacceptable (usually a degree of rectilinear distortion is introduced by the designer to counteract astigmatism). Astigmatism is caused by the vertical curvature of eyepiece field being different form the horizontal curvature of the field. Astigmatism is seen when the stars in the centre of the field of view are in sharp focus the stars at the edge of the field are usually like crosses or some say they look like bats at the edge of the field. Astigmatism is quite often present in inexpensive eyepieces, especially if they have a larger AFOV. And the shorter the focal ration of the scope being used the more apparent this will become.
Angular Magnification Distortion
An angular magnification distortion is where the degree of magnification differs slightly from the centre of the eyepiece to the edge. The impact of this on most observers if negligible, indeed most observers will never know if it exists in an eyepiece. The only real impact is if an observer is carrying out star drift timings, i.e. timing a star moving across the field of view in an undriven scope, then the timings will be slightly off as the speed with which the star moves across is not constant (it will move faster across the centre than at the edges).
Spherical Aberration of the Exit Pupil
A spherical aberration of the exit pupil is caused by different bits of the exit pupil having different best focus points. This is often referred to kidney-beaning in reviews. This is a particular annoying defect in an eyepiece as it results in a black spot or kidney bean floating around the view depending on where you place your eye and it will be very difficult to view the entire field of view.
Vignetting is caused when a lens of an eyepiece is not able to field all the lights rays coming through the previous lens. Vignetting presents itself as a noticeable darkening of the field of view towards the edges.
Chromatic aberration is caused because no lens no matter how well configured or made can focus all colours of light at exactly same point. It is not that common for an eyepiece to be severely effect by CA but if an eyepiece suffers from CA it will be noticed, usually, at the edge of the field of view where a bright star, planet or the moon would have a false colour fringe.
There are other terms often used in reviews to describe flaws in eyepieces but in most cases the effects of these are self explanatory e.g. light loss, light scattering, ghost images.
Choosing the correct eyepiece or selection of eyepieces is very important to maximising your observing experiences, when contemplating an eyepiece it is very important to consider the above factors. Getting informed about any potential eyepiece purchase will protect against the disappointment of spending good money on something that is unsuitable to your requirements, in this I definitely speak from bitter experience. Reading reviews, speaking to fellow observers and trying out various eyepieces will enable you to make good choices for your eyepieces.