113 replies to this topic

[Starman1]

Bright spot?  Where in the field?  Does it move as the star drifts across the field?

Is it stellar or diffuse?

[RockyMtnRR]

Just want to say thank you for this.   I'm coming to the conclusion that I despise one of my EPs (9mm Plossel with Squash-face-to-scope eye relief) that looks variously mediocre to awful and this is giving me some useful avenues to explore that suggest it's just that I can only see the issues through that higher mag EP.

[GaryJCarter]

When eyepieces were made by hand to match the scopes made (in the 18th century, for example), I wonder if eyepieces were ever refigured to do just that.

Or even in the 20th century....the venerable Hubble telescope comes to mind! ;-)

Edited by GaryJCarter, 27 October 2020 - 04:35 PM.

[TOMDEY]

Greetings, Don,

 

I read every word with great interest, and now know more about eyepieces! You undoubtedly have scrutinized more eyepiece makes and models, more thoroughly, under both lab and field conditions --- than anyone around. That makes your summary a goldmine that couldn't have been realized any other way. Thanks!

 

I've always liked the high-end Zeiss Binos because the eyepieces are optimized together with the objectives and prisms, all together, in one package.

 

And the peculiar, dedicated Night Vision "Eyepieces" that I got involved with for ITT, back when I worked there. That system is unusual in that the amplified image appears on an 18mm diam phosphor screen that is intimate to an optical fiber bundle, twisted 180^o to invert the image to right-side-up. And that output side is ground and polished concave to exactly match the field curvature of the "eyepiece" magnifying glass... so the user sees the entire field free of curvature. No field lens is needed because the light squirts out of those polished fiber tips at the innate "F#" characteristic of the clad fibers. That gives the user effectively infinitely accommodating eye relief with huge pupils. Peculiar and effective. Behaves like 26mm with 50^o field. Feels strange, comfortable, and fun to use.    Tom

[25585]

Maybe include EOFB (Edge of Field Brightening) as an eyepiece aberration?

 

Mike

If not an aberration, its an abomination.

[Starman1]

If not an aberration, its an abomination.

It is in the light scatter category, but, in general, I agree with you.

I even find mild vignetting more tolerable.

[ECP M42]

Greetings Don and everyone else.

I ask two things about the eyepieces and I don't know if they are in the correct place.

 

1 - is it possible to calculate and verify the focus ratio of the eyepieces?

 

2 - all else being equal and analyzing the same field of view, which aberrations are in any case better or worse, between an orthoscopic eyepiece and the other ultra wide?

 

Thanks a lot to everyone,

 

Henry

Edited by ECP M42, 01 June 2021 - 08:41 PM.

[Starman1]

Greetings Don and everyone else.
I ask two things about the eyepieces and I don't know if they are in the correct place.

1 - is it possible to calculate and verify the focus ratio of the eyepieces?

2 - all else being equal and analyzing the same field of view, which aberrations are in any case better or worse, between an orthoscopic eyepiece and the other ultra wide?

Thanks a lot to everyone,

Henry

I don't know if it us possible to calculate the critical f/ratio of an eyepiece without knowing the exact glass and then doing ray tracing. It's left to us to experiment, unfortunately.

I'm not sure what you are asking with the second question, but all eyepieces have the same kinds of aberrations, just in greater or lesser amounts. It takes more glass and/or more exotic glass to correct a wider field to the exact amount as a narrower eyepiece, if talking about the edge of the field. The typical 42 deg Abbe orthoscopic eyepiece has 4 elements. ES' 120 degree eyepiece has 12.
It is a credit to some designers that their 100 degree eyepieces have as good or better correction at the edge as the 42 degree Ortho.

[ECP M42]

Sorry for my somewhat strange and unusual questions.

 

1 - I don't know if it us possible to calculate the critical f/ratio of an eyepiece

2 - I'm not sure what you are asking with the second question

1 - Is it possible that the field diaphragm could somehow represent the opening of the eyepiece?

Or, since the eyepiece behaves like an inverted objective, where the outgoing rays are parallel and the focus is at the focal length, is it perhaps the diameter of the outer lens of the eyepiece that determines its aperture?

Surely your answer is the right one, but it doesn't answer!  

 

2 - I try to explain my question better, but with your answer you have already clarified some points. Obviously, an orthoscopic eyepiece is much cheaper, smaller, lighter and brighter than a Nagler with 100 ° or 120 ° AFOV. But that's the point: I was talking about the same visual field analyzed.

What would be the most obvious typical aberrations (if any) within 2 ° of field of view (for example), if I use a 50 ° orthoscopic which shows me 2 ° of field and a 70 ° UWA of the same price ( more or less), quality, focal length, FMC treatments, etc?

Which of the two would be preferable to observe only those 2 ° of field?

Edited by ECP M42, 01 June 2021 - 11:40 PM.

[Starman1]

Sorry for my somewhat strange and unusual questions.

1 - Is it possible that the field diaphragm could somehow represent the opening of the eyepiece?
Or, since the eyepiece behaves like an inverted objective, where the outgoing rays are parallel and the focus is at the focal length, is it perhaps the diameter of the outer lens of the eyepiece that determines its aperture?
Surely your answer is the right one, but it doesn't answer!

2 - I try to explain my question better, but with your answer you have already clarified some points. Obviously, an orthoscopic eyepiece is much cheaper, smaller, lighter and brighter than a Nagler with 100 ° or 120 ° AFOV. But that's the point: I was talking about the same visual field analyzed.
What would be the most obvious typical aberrations (if any) within 2 ° of field of view (for example), if I use a 50 ° orthoscopic which shows me 2 ° of field and a 70 ° UWA of the same price ( more or less), quality, focal length, FMC treatments, etc?
Which of the two would be preferable to observe only those 2 ° of field?

1.Eyepieces modify the size of the image passing through. The effective field stop determines the size of the true field and that is only very loosely associated with the sizes of lenses in the eyepieces. so the opening of the bottom lens may or may not have a determination of the field sice in the eyepiece. Eyepieces don't have an aperture per se, though larger lenses can yield more eye relief. There are just many ways light can pass through an eyepiece.
2. Maybe nothing, maybe a lot. It would depend on the eyepiece. You might assume that a small eyepiece with a small number of lenseswould be superior to the widefield complex eyepiece with many lenses, and you would be right some times an wrong on others. It depends on the eyepiece.
Obviously, to give you the same true field, the narrower eyepiece would have to be a lot lower power than the widefield.

Edited by Starman1, 02 June 2021 - 12:48 AM.

[ECP M42]

Do I write your language so badly?   ... because I see your difficulty in understanding my question.

(I swear if I can't now, then I'll stop). 

 

Maybe nothing, maybe a lot. It would depend on the eyepiece.

Obviously, to give you the same true field, the narrower eyepiece would have to be a lot lower power than the widefield.

I agree that it depends on the eyepiece, but my question was in general to understand which types of aberrations (typical and dependent on the optical pattern) will be most evident between the two types of eyepieces. Meaning it with the same magnification and therefore observing and analyzing only the same field area in both (example: 2 °).

In summary, is it possible that a UWA 70 ° eyepiece is better than an Orthoscopic 50 ° in comparison to the same field of view (2 °) at the same 25x magnification? 

 

Obviously, for the same price (or similar), quality, FMC, etc. 

 

Thank you Don, 

 

Henry

[Starman1]

Do I write your language so badly? ... because I see your difficulty in understanding my question.
(I swear if I can't now, then I'll stop).

I agree that it depends on the eyepiece, but my question was in general to understand which types of aberrations (typical and dependent on the optical pattern) will be most evident between the two types of eyepieces. Meaning it with the same magnification and therefore observing and analyzing only the same field area in both (example: 2 °).
In summary, is it possible that a UWA 70 ° eyepiece is better than an Orthoscopic 50 ° in comparison to the same field of view (2 °) at the same 25x magnification?

Obviously, for the same price (or similar), quality, FMC, etc.

Thank you Don,

Henry

I obviously misunderstand.
How could two eyepieces of different apparent field have the same 2 degree true field of view at the same power?
Or do you mean in the center 2 degrees of apparent field?
If that's what you mean the answer is there would be no difference in all likelihood. Some observers might argue the widefield eyepiece, because it probably has more lenses, could yield a increase in light scatter at the center of the field.
But that would greatly depend on the eyepiece, as some small, lower lens count, eyepieces aren't as well polished as some larger, higher lens count, eyepieces.

Edited by Starman1, 02 June 2021 - 10:23 PM.

[ECP M42]

How could two eyepieces of different apparent field have the same 2 degree true field of view at the same power?

But how, Don? 

If we use two 18mm eyepieces, one with 50 ° and one with 70 ° AFOV, on the same f450mm telescope (we don't care about the aperture), to 25x we will get a field of view of about 2 ° with 50 ° and 2.8 ° with 70 °.

Why is it not possible to evaluate only the 2 degree of field, which both eyepieces can provide?

 

You may have already answered (partially), but I know it is difficult to answer properly without understanding the question. I hope it is clearer now.  

[Starman1]

Yes,

If you had defined what the question was to begin with, I wouldn't have had to try to figure out what you meant.

 

You could easily evaluate 2° of True field in both eyepieces in that case.

Depending on the eyepiece, the wider one could have a lower aberration level at the 2° field line than the narrower eyepiece.

Or, it could be worse.  It would depend on the eyepieces in question.

If both are world class, there is likely to be very little difference, if any, between the 2 eyepieces at the 2° field size.

However, the 70° eyepiece would have a larger true field, and that might be a good thing.

Or, if a really bright star came into view due to the wider field, this might hurt the visibility of something faint.

 

But, you were talking about aberrations, and the variation between eyepieces is huge.

Some handle an f/4 light cone easily, some fall apart at f/8 and only work at longer f/ratios.  It just depends.

So there is really no way to exactly answer your question unless we knew the scope the eyepieces would be used in, and knew the eyepieces in question.

[ECP M42]

Thanks Don. 

I'm trying to decide whether to get a pair of 18mm 72 ° or 18mm orthoscopic 52 ° at the slightly higher cost, trusting that they are better as a sweet spot, but which I don't know.

 

https://www.cloudyni...3#entry11130436

[Starman1]

Thanks Don. 

I'm trying to decide whether to get a pair of 18mm 72 ° or 18mm orthoscopic 52 ° at the slightly higher cost, trusting that they are better as a sweet spot, but which I don't know.

 

https://www.cloudyni...3#entry11130436

Pretty much for everything except planets, the wider field would be more enjoyable to use.

But, that depends on the correction of the 72° eyepiece in your scope.

Of the choices you mention in the other thread, though, the Baaders are better at the edge and in general.

The 18mm has been sold by other companies, and it has lots of edge of field astigmatism below f/8.

If your scope is f/8 or longer, it's a reasonable choice.

At f/5-f/7, this is a better choice for a widefield:

https://www.astroman...e-field-of-view

It has been offered for years and does fine at that focal length at f/5.

Edited by Starman1, 04 June 2021 - 03:01 PM.

[ECP M42]

Pretty much for everything except planets, the wider field would be more enjoyable to use.

That it is pleasant to have a wider field of view, I am already aware of that.

What I am trying to understand is which of the two types would be the best choice for optical quality, in the vision of those 2 ° of field. Which in reality will be about 2.8 °, because the eyepieces I will almost certainly use them with refractors f330mm Ø 50mm approximately (f/6.6).

The fact that the Baaders are better at the edge and in general, it is good information. The cost is almost double the Svobony 18mm 72 °, but they attract me a lot for some factors. I still have to decide.

 

The 17mm eyepiece you recommend is definitely a great choice and thank you very much for your time, but it's too big and more than doubly expensive for my humble handy binocular project.

 

Have you ever tried these two eyepieces (18mm Baader e Svobony) in direct comparison?

Or do you know someone who has made this comparison, you may ask?

[Starman1]

The Svbony has been previously sold by Meade and a few other companies.

They've been around for years.

They work OK at long f/ratios, but not shorter ones--too much uncorrected astigmatism in the outer field.

[ECP M42]

The Svbony ... work OK at long f/ratios, but not shorter ones--too much uncorrected astigmatism in the outer field.

I already have several 68 ° eyepieces that are sharp to the edge, but curved.

If the Svobony-Meade 18mm 72° are also astigmatic, maybe it's not worth it. 

Do you think f/6.6 is too short, for these eyepieces?

 

I can mask the aperture down to f/9 during the day, but at night I would take the mask off.

[Starman1]

Your scope has an exceptionally short f/ratio that displays field curvature when the field stop of the eyepiece is wide.

I think the f/ratio is probably fine with many many eyepieces, but you should set your sights on something different,

 Like the Celestron X-Cel LX or Ultima Edge or a host of other eyepieces.

If you are confined to the pricepoint of the Svbony, I would seriously look at Plössls, which will perform quite well in the short focal length scope.

Perhaps this conversation would best be carried out in another thread.

[Baobas3360]

The purpose of this post is to help enable amateur astronomers to identify the aberrations seen in an eyepiece used in the scope. Note in advance that some of them are "Interactive Aberrations", i.e. not the fault of the eyepiece alone but the interaction between scope type or f/ratio and the eyepiece.

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!

Bravo,Starman!

[25585]

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).

 

Reading posts over the years on the above, SAEP & "blackouts" seem to get wrongly identified with one another, most often the latter is called out as the former. Of course, an eyepiece & user can be affected by both simultaneously, though whichever springs to the affected viewer's mind first is often the one both are blamed for, and reported as.

 

A related "aberration" is vignetting, when eye distance is too short, whether due to other optics & their placement being used with an eyepiece, or anatomy affecting a viewer's accomodation of an eyepiece's design.  

[Starman1]

And a lot of eyepieces have vignetting by design, figuring a small amount of vignetting is preferable to having lenses become too thin at the edge

and make the eyepiece very fragile.

[ECP M42]

... SAEP & "blackouts" seem to get wrongly identified with one another ...

I think that seeing dark areas in the eyepiece (whether in the shape of a bean or as a shading of the edge), has a "blackout" effect in any case.
The cause is different and partly also the shape of the shadows, but the effect is practically the same. And this may possibly cause the overlap in common talk of "vision blackout" problems.

 

A bit like what happens with distortion and field curvature (which are often confused), it is always better to know the differences, but there is little to do about it. Only knowledge, separates the conscious way from the unaware way. And knowledge is power!

 

 

Thanks Don, for your effort! 

 

Henry

Edited by ECP M42, 17 January 2022 - 05:01 PM.

[Jon Isaacs]

I think that seeing dark areas in the eyepiece (whether in the shape of a bean or as a shading of the edge), has a "blackout" effect in any case.
The cause is different and partly also the shape of the shadows, but the effect is practically the same. And this may possibly cause the overlap in common talk of "vision blackout" problems.

 

A bit like what happens with distortion and field curvature (which are often confused), it is always better to know the differences, but there is little to do about it. Only knowledge, separates the conscious way from the unaware way. And knowledge is power!

 

 

Thanks Don, for your effort! 

 

Henry

 

Distortion and field curvature are very different phenomenon. One involves the relationship between points/stars over the field of view, one involves the curvature of the focal plane.  This is an issue with understanding the meaning of terms. Sometimes pincushion distortion is mistakenly called field curvature because straight lines become curved s they're moved off center.

 

Blackouts are all similar but with difference causes. Most often they're do to improper positioning of the eye in relation to the exit pupil and eye relief point, one can learn proper eye positioning. Sometimes they're due to SAEP, that's a problem with the design and there's not much an observer can do about it.

 

Jon