1. Poor Sharpness on-axis. This may seem like the easiest to see, but it is also the hardest to diagnose: Is it the seeing? The mirrors? The objective lens? Miscollimation? Chromatic aberration? Light scatter? I could go on. Many factors influence this, so it is hard to pin down unless you switch to another eyepiece of the same focal length (and it has to be identical) and see a different image. If switching back and forth reveals one to be consistently poorer than the other in sharpness, then that eyepiece qualifies for having poor sharpness on axis. In my experience, this is one of the least of the aberrations in modern eyepieces. It exists, but you might see it in perhaps one eyepiece out of 200.
2. Chromatic Aberration-lateral and fringe. Yes, eyepieces, like achromatic lenses, can produce chromatic aberration in a given scope. What's seen may be from the objective, if a refractor, but eyepieces are not immune to this. On axis chromatic aberration is rare, so that would probably be from the objective. But lateral chromatic aberration can be simply having an oblique angle interact poorly with the coatings used (because their spectra of transmission varies with angle) or because of glass angle interactions. It is very hard to produce an ultrawide field in an eyepiece and NOT have any chromatic dispersion at the edge. Edge chromaticism can be a result of holding the eye at the wrong angle, too, since our eyes are not immune to chomatic effects. If it's seen at the edge, try holding the eye differently to see if it disappears. If it does, it was in the eye. if it doesn't, it's in the eyepiece. Good suppression of this leads to high-priced eyepieces, so a less-costly cure may be to restrict the field of view.
Many eyepieces have a tiny ring of aberrant color at the edge of the field. This is usually due to the oblique angle of vision at the edge of the lens and the coatings selected for the anti-reflection coatings on the lens.
3. Field Curvature-negative and positive and scope interaction
If the field of the eyepiece is curved, the edge may be sharp, but only if the focus is changed from the position that produces a sharp image in the center. Field curvature may go either way. Note that short focal length scopes have more strongly-curved fields, and even an eyepiece with no field curvature may be seen to show it, though it is not due to the eyepiece in that case. An eyepiece's field curvature may also cancel the curvature in the scope by being of opposite sign. In that case, the combination may show a flatter field than either evaluated separately. This would appear to be rarer, but it does seem to account for why some eyepieces that do have measurable field curvature get rave reviews by the users of certain scope types.
If there is a trace more curvature than you can accommodate with your eye, focusing on a star half-way to the edge may bring the entire field into focus. In that case, your eye is accommodating both the defocus at the center and at the edge.
The older the observer, the more field curvature becomes problematic, due to a loss of accommodation in the eye as we age.
4. Angular Magnification Distortion (+/-). This shows itself most easily by looking at the letters on a sign and noting whether the letters increase or decrease in size as they near the edge of the field. This is a lateral aberration, and shows up most at the edge of the field. Eyepieces used for astronomical use typically have very little. It could be evaluated at night with a close double star--see whether the separation appears to narrow or widen at the edge of the field. Of course, other aberrations may swamp your ability to do this test. Note that angular magnification distortion (change in magnification with position in field) and rectilinear distortion (linear distortions) cannot be simultaneously corrected except in very narrow field eyepieces (30 degrees or so), so the designer always has to choose which to correct or what percentage of each to leave in the design.
5. Rectilinear Distortion a) pincushion b) barrel. This causes linear changes in the images as they approach the edge. Pincushion, as a line moves across the field, looks like this )|(
while barrel distortion (the opposite sign) looks like this (|). This is a common distortion due to the likelihood of angular magnification distortion being corrected in the eyepiece. Lucky for us, a small percentage of pincushion distortion is invisible to the eye, so an eyepiece that is better corrected for angular magnification distortion than rectilinear distortion usually appears better to the eye.
Eyepieces used in the daytime seem to need to have RD corrected, while eyepieces used at night on the stars need to be corrected for AMD. This is a generalization, and circumstances may dictate differences in personal preference.
6. Spherical aberration. Caused by having different distances from the center of the lens coming to focus at different places, this results in stars that have more energy in the diffraction rings and different intra- and extra-focal appearances. It results in the blurring of images and a diminution of image quality. It is the most prevalent problem with inexpensive reflective optics. It is hard to identify without learning how to star test, but its effects are visible everywhere in the field of view and affect high powers dramatically. If you have a scope that never seems to perform at higher powers, even when those around you have great high-power images in their scopes, this is the most likely culprit if your optics are cooled and collimated. It isn't common in eyepieces in any amount that would affect image quality.
7. Transmission anomalies by Frequency: coloration (tint) and overall transmission. The truth is that not all visible wavelengths are transmitted through eyepieces with equal percentages. Our eyes are most sensitive at night to the blue around 500nm. If the eyepiece's transmission peaks at 500nm, we will see it as brighter than, perhaps, another that peaks elsewhere in the spectrum. And if the blue wavelengths roll off, we may see the image as more yellow (as happens with many minus-violet filters used in achromatic refractors). If longer wavelengths are accented in transmission, we may see a "warm" tint to the image, and if short wavelengths are favored, a "cool" tint to the image. The best would be a flat transmission across the visible band, but no eyepiece currently made has this. This facet of eyepieces most affects Moon viewing and planetary images. The good news: the eye adjusts quickly to see everything as normal, even if the transmission spectrum is slanted toward certain wavelengths. Differences are usually only noticed when comparing eyepieces.
As for transmission %, this is something unlikely to be seen outside of a laboratory test. IF you see a difference, the difference is huge. Generally, good anti-reflection coatings bring transmission up, so it is desirable to have every surface inside an eyepiece have the best anti-reflection coatings, what is called "Fully Multi-coated".
8. Light loss due to
a) reflection
b) absorption
c) scatter
d) internal vignetting
"A" results from incomplete or poor coatings. The poorer the coatings, the more the internal reflections. This can produce "ghost" images of planets and bright stars in the field, or out-of-focus images surrounding a bright star or planet in the field. It means less light ends up where it should--in the image.
"B" could happen with a large number of inches of glass in the image. The percentage in most eyepieces is tiny, but there are eyepieces with 12 elements that are several inches long, so this could become a more important factor with new eyepiece development, as it is in camera lenses.
"C" means that light is moved away from where it belongs because of scatter from poorly-polished glass surfaces, or improperly-applied coatings, or internal surfaces in the eyepiece. At its worst, it lightens the entire background of the field. Normally, it's easiest to see as a diffuse glow around a bright star or planet. Careful, though! This can also be due to a small amount of fog or oil on the optics. At its worst, it can result in a brightening of the edge of, or the entire field. See Point 13 below.
"D" Some eyepieces are not designed to adequately illuminate the edge of the field. In this era of fairly bright backyard skies, this type of vignetting is easily seen as a dimming of the edge of the field as a slightly darker ring surrounding the brighter center of the field.
9. Spherical Aberration of the Exit Pupil and relationship to eye relief
If not every part of the exit pupil is the same distance from the lens, positioning the head becomes very difficult. With small movements of the head, kidney-bean shaped dark areas can be seen drifting around in the outer parts of the field. This is different from the edge-of-field shading known as "blackouts" caused by moving the head too close to the eyepiece. It is found in many early ultrawidefield eyepieces, but is fairly rare in eyepieces today (not extinct, though).
10. Coma in off-axis light. 99% of the time, this is from the mirror. But some very simple eyepiece designs are not fully-corrected for this and display the problem. Most of those eyepieces aren't popular today, so you are unlikely to run into this. Coma, of course, makes stars appear like small comets as you get progressively farther off-axis.
11. Astigmatism
a) tangential and sagittal focus differences
b) tilted elements
c) wedge
d)relationship to focal ratio of scope
e)relationship to astigmatism of objective
The result: a star image elongated in a radial direction on one side of focus, and a circumferential direction on the other. The best focus is a small blur or cross. With extended images, it will mean you will not be able to achieve a sharp focus at the edge as you can in the center of the field.
"A" means the focus position for the vertical and horizontal curves are different. A sign of poor execution in an eyepiece, but commonly seen in objective elements like mirrors.
"B" This can happen if an internal retaining ring is loose of the eyepiece is sloppily assembled. It can be fixed unless the barrel is mis-machined.
"C" is a result of poor lens manufacture in the eyepiece. I've seen it in poorly-executed cementings of internal elements, but this is more of a problem with mirrors and objective lenses.
"D" is VERY common. If the design of the eyepiece cannot accommodate the oblique angles of the light rays from a short focal ratio entering the lenses, the most common problem is astigmatism at the edge (or chromatic aberration). Every eyepiece has a "Critical F/Ratio" below which it performs poorly (though it's a gradual thing, not a sudden cutoff). The problem is, the manufacturers won't tell you this.
You can look up the CFR of older designs, but many of today's eyepieces don't have that stated anywhere in the manufacturer's info.
"E" shows that astigmatism can be introduced by tilting elements of the optical system relative to one another, and points out the importance of collimation, which can insure no focal plane tilt at the final focal plane of the scope, relative to the eyepiece.
12. Wavefront aberration a) poor polish b) poor figure c) result of more surfaces in eyepiece
This can be seen on a test bench. In the field, it just means an image of poor quality that just never seems to "snap" into focus. 99% of the time, this is from the objectives in the scope, not the eyepiece, but, with today's multi-element designs, a poor figure could be the result of a poor execution of a design or simply a bad lens.
13. Light Scatter:
a. Surface scatter-roughness
b. Reflections: lens (edge and surface-polish and coatings), and barrel surfaces (one of the causes of ghosting or spiking)
c. Lateral rays (lens edge and barrel and low incidence scatter by coatings)
There are often a host of issues affecting the presence of light scatter in an eyepiece. Some can be as simple as a bright image reflecting off the cornea, back to the eyepiece, and then back to the eye. That's hard to cure without a fully-multi-coated cornea (
). I have seen eyepieces so poorly baffled that the reflection of the bottom rim of the eyepiece back into the mirror and back to the eyepiece showed up in the field of view as a round ghost! Light scatter will cause a glow around the object, or a glow in the field, or a glow near the edge of the field when a bright object is outside the field, or even a spike in from the edge when a bright object is outside the field of view. An inadequate control of this is a sign of a poorly-designed eyepiece, and it is most often found in less expensive eyepieces (though not all, of course).
14. Design Flaws
a. Field stop not in focus. Annoying when the edge of the field is a soft blur and a star passes out of the field in a vague manner. It is a sign the field stop is not at the focal plane of the eyepiece.
b. Critical f/ratio too high (inadequate off-axis ray handling). If the eyepiece doesn't work with the f/ratio (like a Kellner in an f/4.7 scope), that's a problem. If a modern designer doesn't design his eyepieces to work with scopes down to below f/4, I would view that as poor design. Any modern design should work well in all scopes.
c. Improper internal ray handling, causing vignetting or reflections. Related to scatter (noted above), it is a sign the manufacturer/designer didn't properly design the eyepiece. Sometimes it's inadvertent, like having a reflective surface on a retaining ring. But it can also be a manufacturer saving a dime by not putting in a proper baffle in the proper place or using shiny aluminum instead of a flat black surface.
d. Wrong glass refractive index used. I've not sure how often this happens, but it explains chromatic aberration in some designs and poor light transmission (when the coatings are wrong for the refractive index of the glass).
16. Thermal issues due to size, improper housing. Everything made by the hand of man has variations in tolerances. Over the years, I've seen eyepieces that had such a tight fit over the lenses that, as the aluminum barrel cooled down, the internal lenses were pinched by the aluminum. The resultant image had what looked like astigmatism, but only on one side of the field. And it went away after everything was cooled to the ambient temperature.
In theory, a big 2-4lb eyepiece could have lenses that don't conform to the design parameters until cooled.
Of course, one of the problems is that a number of the above issues could be present simultaneously. Then, evaluating what to blame for the poor images becomes hard to do. Coma's appearance can be exaggerated by field curvature and astigmatism. Chromatic issues at the edge can be exacerbated by poor coatings, poor design, lack of cooling (if a large eyepiece), poor figure, etc.
No eyepiece exists in a vacuum, however. If another eyepiece of the same focal length solves the problems seen, then the design of the second eyepiece interacts with the telescope and you, the observer, in a better way. It doesn't really matter what results in a better image quality if you see a better image quality from one eyepiece than from another. Perhaps it's aberrations in the eyepiece canceling aberrations in the scope.
But you are more likely to see good image quality from eyepieces that are better-designed and executed, and with less variation from piece to piece. No eyepiece is perfect, but some are more perfect than others.
Perhaps some of you could point out the issues that most annoy you with some particular eyepiece designs. Be as specific as you can or want. Let 'er rip!
try finding all this on Google. lol.
