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Contrast in Eyepieces

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#1 Starman1

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Posted 10 August 2020 - 05:19 PM

A recent thread on upgrades in eyepieces led me to reexamine something I wrote years ago about contrast in eyepieces.

One of the obvious improvements people seek when they buy a new eyepiece (but don't always receive) is improved contrast in the eyepiece.

But what is contrast in an eyepiece?

Forgive the long post--I'll break it up into two posts.  I would be interested in hearing critiques so I can improve what I wrote.

Don

 

CONTRAST IN EYEPIECES

What do we mean when we say one eyepiece produces better contrast than another? Is it just one characteristic of an eyepiece, like apparent field, or is it the result of the interaction of many parameters?
Let's discuss the term contrast first and then talk about how contrast in eyepieces is created and enhanced.

Contrast is the differentiation between light and dark areas. Better contrast entails the ability to distinguish brightness levels that are closer to identical.
In the case of telescope eyepieces, it also refers to the ability of an eyepiece to not scatter any light from brighter areas into darker areas; maintaining, and not reducing, the differences in brightness between adjoining areas.
That would mean that fainter features in nebulae and galaxies might be seen because of better contrast with a darker background, and it might mean that small lunar or planetary features might be seen because of less bleeding of bright light into darker areas, reducing contrast between features.
The resolution of an eyepiece will always be limited by the aperture of the telescope, but we don't want the eyepiece to reduce the ability to see details by reducing contrast in the image.
I come down on the side of those who point out that contrast in eyepieces is NOT just one factor, but the synergistic interaction of many factors.
We can discuss apparent field, edge of field distortion, light transmission, and the like, and each is a measurable characteristic of the eyepiece. Contrast, however, is not, in the sense in which it is used by amateur astronomers, a single measurable thing, but many. Yet, like pornography, we all know it when we see it.
So how is contrast created, and how can we improve it and why should we value improved contrast?
Let's discuss some of the factors influencing contrast and our ability to see small differences therein (and not all of the following are always thought of as "contrast" parameters):

 

1. Improved polish will reduce light scatter and increase light concentration in the correct places. This may be one of the most important factors of all. Making the lens surfaces smoother means less light is scattered by small irregularities in the surface of the lenses. This entails good polishing and QC on the part of the manufacturers. And partially explains why high-priced eyepieces are high-priced.

 

2. Improved manufacturing tolerances. The eyepiece has to be as close as possible to the optical design or negative effects can occur, like Spherical aberration or astigmatism. This factor rolls assembly quality and adherence to proper curves and coatings into one factor. It means the lenses cannot be tilted relative to one another and the glass and coating materials have to adhere closely to the optical design.

 

3. Improved choice of the correct coatings for the glass types used will improve transmission and reduce scatter. If the coatings are properly chosen for the glass types, more light will get through, and less will be reflected internally. Reduction of internal reflections is enhanced by higher transmission coatings. Proper choice of glass types will insure this applies to all visible wavelengths.

 

4. Improvement from the lack of internal reflections in the eyepiece from lens edges, spacers, filter threads, etc. The best eyepieces have NO visible reflections from any internals. Blackening lens edges, internal spacers, the barrel interior, filter threads, even the external body of the eyepiece near the eye, will result in less scattered or off-axis light making it to your eye. Though most eyepieces fail in this, ideally the brightest star in the sky could be just outside the field of view and you would never know it. [In fact, other things could give this away that have nothing to do with the eyepiece, but you get the point].

 

5. Improved wavefront accuracy AFTER passage through the lenses. Though some cancellation of errors can occur with multiple surfaces, it seems obvious that reducing surfaces, with all other factors equal, is more likely to result in less damage to the wavefront. We talk at length about the quality of the mirrors and lenses in our telescopes, but rarely talk about the optical figure of the eyepieces we view through. Yet, there are more surfaces in our eyepieces than in our telescope mirrors or lenses. Yet the quality of these lenses determines the quality of the images we see. So does their number. This is where, in theory, a smaller number of lens surfaces will be superior to a larger number. But specific examples may not verify that contention. Some multi-element eyepieces may have superior polish and better images than some low-element count eyepieces. Generalizing might not work, though, in theory, fewer elements could yield a better wavefront.

 

6. Improve the quality of the design. If the eyepiece has chromatic aberration or aberrations induced by the f/ratio of the scope, then the eyepiece will have reduced image quality. Eyepieces are designed to amplify the images created by the telescope. If the design cannot handle the wide light cone of a short f/ratio telescope without creating noticeable aberrations or does not focus all colors of light into the Airy disc of the telescope, then the design is not appropriate for all telescopes. In the history of telescopes, eyepieces have been getting better and better in this regards, with a lot of the high-end eyepieces good in scopes as short as f/4 and below. But there are still many earlier types of eyepieces being used that, while excellent in long focal ratio telescopes, cannot handle the shorter, wider, light cones of many of today's telescopes. Since no manufacturer wants to limit his market, many eyepieces suitable only for f/6 and longer are today being purchased for f/3 to f/5 scopes, where they will perform poorly. But some modern eyepieces are truly dramatic improvements over common eyepieces of yesteryear.


Edited by Starman1, 10 August 2020 - 05:22 PM.

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#2 Starman1

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Posted 10 August 2020 - 05:19 PM

7. Interaction with the shape of the observer's cornea. Bright images can bounce off the cornea and reflect from the lens below it. Certain shapes of eye lenses are more prone to this than others, as are some corneas. The interaction of these factors can deleteriously affect the image quality by increasing glow and ghosting and reducing contrast. Not a lot of research has been done here, but some eyepieces are prone to reflections and others are not. Further research needs to be done into improving the shape of the top of eyepieces to minimize the possibility of reflections.

 

8. Reduction in contrast from obstruction in the telescope. Whether in the form of a secondary or spider vanes, light scatter and a loss of contrast accompany obstruction and this can have an effect on the evaluation of an eyepiece. So can reflections and light scattered inside the telescope that enters the eyepiece after various reflections elsewhere in the optical system. Many times it's difficult to know exactly when to attribute a loss of contrast to the eyepiece and when to the scope. It points out that not all factors influencing contrast in the eyepiece's image are due to the eyepiece itself. I only mention this here to point out that evaluation of contrast in an eyepiece can be scope-specific.

 

9. Improved contrast from the size of the exit pupil and size of image. If the size of the object or detail desired to be seen happens to be large when the exit pupil is also large, you have the best of both worlds--improved brightness and improved visibility of small details. But, most of the time, seeing details requires magnification, and that means smaller exit pupils. It's why the Contrast Threshold is better in larger scopes: higher magnifications at the same exit pupil and larger exit pupils at the same magnifications. It's also the reason exit pupil has to be exactly the same size to compare two eyepieces in one scope. Contrast for an extended object is best at the largest exit pupil your eye can admit, while contrast for a point is best when the magnification is the highest that does not produce a visible size to the Airy disk (because the background appears darker, yet the star point appears the same brightness). Since the Airy disks are smaller in larger scopes, and the magnifications are higher at equal exit pupils, contrast is improved in larger scopes.

 

10. Improvements from the spectrum of transmission. If the eyepiece rolls off the blues and the eye is most sensitive to those wavelengths at night, bluish features may become harder to see. As we age, out eye's lens also yellows and we lose the blues. A younger viewer may see bluish details better than an older observer. If the spiral galaxy's arms are bluer than the core, it would seem that the spiral arms would be more visible to a younger viewer or an eyepiece that had a flatter spectrum of transmission in the blue. We don't know, and the manufacturers don't advertise, the spectrum of transmission in our eyepieces, but this could influence a comparison of "contrast" for certain features. One published example: the Pentax XW eyepieces reach a transmission level of 96% at 550nm, but are below 80% transmission at 400nm, yet this eyepiece is often considered "cool", implying higher transmission in the blues. Perhaps other eyepiece transmissions roll off even more in the deep blue.

 

11. Improved match of spectrum with object. This is obvious. If you want to see something faint, the spectrum of transmission should be high at the appropriate wavelengths. This could influence the visibility of an object in one eyepiece and not another, or at least the brightness of an object in one eyepiece versus another. We might say there is better contrast in one eyepiece because the object stands out better from the background. This is one of the ways that the word contrast is used in a vague manner when discussing eyepieces.

 

12. Improved contrast from the difference factor. If the scope is small, the resolution of the scope may not be sufficient to identify differences between two eyepieces, whereas if the scope is large the differences might become quite apparent. Part of that is resolution, but part of that is also simply the nature of the response of the eye to light. The brighter the bright objects are in the image, the larger the number of discrete steps in brightness we can see from top to bottom in the brightness scale. If the contrast in the eyepiece doesn't allow us to see those steps, then it doesn't have sufficient contrast for that scope in those circumstances. It is going to be in the largest of scopes that this is encountered most often.

 

13. Better rendition of off-axis images, How does this relate to contrast? Simple. If the design only renders good images in or near the center of the field, but scatters light, vignettes, unfocuses, or aberrates light elsewhere in the field then contrast is non-uniform across the field. Good eyepieces have uniform contrast from edge-to-edge in the field without smearing the star images, adding chromatic aberration, or otherwise damaging image quality. Contrast is normally discussed as an axial issue in eyepieces because that's where we most often view the lowest-contrast features, but it should also apply to the outer parts of the field of view. Who would want to have a picture window in a house where only the center of the viewable field was in focus or sharp and all the edges distorted what you looked at? For the ideal presentation of images, contrast needs to be uniform across the field. This is where more elements may yield better image quality than fewer elements. Abbé orthoscopics, Plössls, Kellners, monocentrics, and other simple designs typically poorly control their outer fields. That makes them good for the motorized, driven, scope for planetary images on-axis, but makes them poor for any extended object or for contrast evaluation anywhere else in the field or for the undriven, non-motorized, scope.

 

 

One thing that may be commonly mentioned as an example of improved contrast but may or may not be is the darkness of the background. Low transmission percentage can produce a darker background. The night sky is not black, but gray--higher transmission may yield a brighter background sky in the eyepiece. Also, the reduction in sensitivity when looking at a small, bright object like a bright star or planet that occurs on the retina near the point of focus makes the sky around the planet or star appear darker that the peripheral field. The brighter peripheral field does not necessarily indicate light scatter--it indicates reduced sensitivity of the retina near the object. Yet, a true improvement in contrast may yield a darker background sky in the image and may yield a darker background near a bright object. Without evaluation of faint star limits, one could not be sure which is the case.

 

I could go on, but I think it's obvious that evaluating contrast in eyepieces is a difficult thing when so many factors influence what we see. But I think the on-axis differences between two non-defective eyepieces are always going to be subtle, and observing experience will trump what small differences may be there. That doesn't mean we shouldn't do what we can to maximize what gets through or to improve contrast as much as we can given the constraints of aperture. So what is the value of improved contrast in eyepieces to the observer?
How can it help?  It can:
• improve the visibility of faint stars or faint moons of planets.
• improve the visibility of details in nebulae and galaxies
• improve the visibility of colors in stars and possibly deep-sky objects
• improve the visibility of low-contrast planetary details
• improve the visibility of low-contrast lunar features
• provide a more aesthetic experience at the eyepiece
• improve the visibility of faint objects at the limit of the scope
Who wouldn't want all of that? As #8 above points out, though, not all of the factors that influence the contrast we see in the eyepieces necessarily come from the eyepiece. Even the highest contrast eyepiece made will perform poorly if it is in a telescope that produces poor contrast. Keeping optics free of dust and dew, reducing scattered light as much as possible, blocking any peripheral light, and properly collimating the scope all help maximize the contrast of the image the eyepiece magnifies. Each of those is a conversation it its own right.


Edited by Starman1, 10 August 2020 - 05:24 PM.

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#3 Mike G.

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Posted 10 August 2020 - 05:42 PM

flock everything between your eye and the sky, wear an observing hood and buy the most expensive EP's you can afford!

 

thanks Don, well done!



#4 The Luckster

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Posted 10 August 2020 - 06:24 PM

Don, I ain't critiquing nothing here; very articulate piece...thank you.

 

From my perspective and experience in this matter of improving contrast at the eye-lens through my personal observing habits since returning to Astronomy several years ago is as follows;  warm up/cool down all gear properly before use, I use cardboard boxes around my rig to block stray light, a dark hood over my head, get night-adapted as close as I can for my conditions, attempt to observe at near to zenith as possible, I am more mindful of exit pupil and the health of my eyes, and I have learned sometimes one has to pay a little extra to get a little extra...sometimes.

 

Furthermore, to help further enhance my views at the eye-lens I use vibration pads for my light to medium/light load mounts, and I use an observation chair for both comfort and image clarity.

 

Adding the above habits over the past several years has noticeably improved contrast through the eye-lens for me, even though my observing conditions are slowly worsening.  Now all I gotta do is add some dark site observing come retirement, and that should help boost contrast at the eye-lens.

 

jason


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#5 BlueMoon

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Posted 10 August 2020 - 06:39 PM

Don, I can't critique the underlying science but I enjoyed the information you presented. I'd place the vocabulary and sentence structure at a high school level with comprehension there or 1st year college. waytogo.gif


Edited by BlueMoon, 10 August 2020 - 08:16 PM.


#6 Starman1

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Posted 10 August 2020 - 06:41 PM

flock everything between your eye and the sky, wear an observing hood and buy the most expensive EP's you can afford!

 

thanks Don, well done!

I wouldn't go that far, but certainly paying attention to local light can help with the "external factors" I mentioned.



#7 jeffmac

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Posted 10 August 2020 - 07:25 PM


Use blackout under your observing eye(s) like the baseball players do? smile.gif
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#8 Starman1

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Posted 10 August 2020 - 07:48 PM

Use blackout under your observing eye(s) like the baseball players do? smile.gif

That's a bit extreme, but it might help reduce the impact of peripheral light leakage around the eyecup.



#9 jeffmac

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Posted 10 August 2020 - 08:34 PM

That's a bit extreme, but it might help reduce the impact of peripheral light leakage around the eyecup.


Yeah, I didn't have the laughing emoji option or I would have used it. Someone will probably try it though. An amateur astronomer do anything extreme? No more glare from your face on that eye lens! And you can see that fastball coming in a little bit better!

#10 RichA

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Posted 10 August 2020 - 11:33 PM

 

5. Improved wavefront accuracy AFTER passage through the lenses. Though some cancellation of errors can occur with multiple surfaces, it seems obvious that reducing surfaces, with all other factors equal, is more likely to result in less damage to the wavefront. We talk at length about the quality of the mirrors and lenses in our telescopes, but rarely talk about the optical figure of the eyepieces we view through.

Time to build a miniature interferometer for eyepieces.



#11 GeneT

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Posted 10 August 2020 - 11:47 PM

Don, your two posts on Contrast in Eyepieces was one of the best ever in Cloudy Nights since I became a member about 12 years ago. One of my conclusions after reading both posts is that there is a reason high quality eyepieces cost more--better polish, better baffling, more exacting tolerances when grinding the glass, and many other points that you raised. As I got older and had more disposable income, I moved up to more expensive eyepieces by TeleVue, Pentax and others because I felt they outperformed less expensive offerings. TeleVue Plossls are quite a bit less expensive than TeleVue Ethoi, but their short eye relief and small fields of view caused me to move up to Delos, Naglers, and Ethos eyepieces. In your posting, there was one item that caught my eye:

 

7. Interaction with the shape of the observer's cornea. Bright images can bounce off the cornea and reflect from the lens below it. Certain shapes of eye lenses are more prone to this than others, as are some corneas. The interaction of these factors can deleteriously affect the image quality by increasing glow and ghosting and reducing contrast. Not a lot of research has been done here, but some eyepieces are prone to reflections and others are not. Further research needs to be done into improving the shape of the top of eyepieces to minimize the possibility of reflections.

 

Within a given manufacturer of eyepieces, modern machinery, and laser and computer technology allows the optics for eyepieces to be ground to extremely exacting tolerances and therefore exacting consistency in the product line. But, one of the most important lenses in the equation is the human eye. Thousands of people enjoy the hobby of astronomy. No two of us have the same characteristics in our eyeball lens. Most of us have different characteristics between our own individual eyes. I believe that is a major reason we get differing opinions in our Cloudy Nights eyepiece reviews of the same eyepiece given by different reviewers. The eyeball is also a lens. When viewing, people will often report differences in seeing detail based on which eye they use. I agree that more research needs to be done in this area. I do not know where the research will lead because eyepiece lenses have to be made to exacting tolerances and cannot be made to an individual's eye lens. 

 

Again, thanks for your excellent posting. I saved it for future reference.

Gene Townsend  


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#12 AstroVPK

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Posted 11 August 2020 - 12:13 AM

Well-written! 



#13 MartinPond

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Posted 11 August 2020 - 12:42 AM

Wow....extensive list, Don!

 

I test directly for contrast with faint-shift lettering.

You can make custom colors in most SW, even MS-"Paint" 

by setting the rgb levels (red green blue).  ,

By setting the background to something like  120-120-120

And a large font at 118-118-118 (or more deviation) ,  you have

   a faint lettering like the driver's tests..

You can fade words in over a sentence.

 

The wavefront is interesting.   I think I see something like that

  plus surface effects when blowing up the image with a monocular.

Scatter or outright glare diffuses into a wide area, but there is this 

'sandblasting' effect at a black-white edge.

White dits in the black, black dits onto the white.

 It takes a very strong white led light onto the target, since

   blowing up the image (6x in my case) dims things a lot.

There are surprising results sometimes.

Sometimes I see a small faceting/bend in a straight line

   for eyepieces with 6--10 elements. I wonder if the bends

    have contrast issues at the edges.

 

Great contrast shows as being able to see faint things, but mainly

   as seeing subtle shades in a diffuse target.

One strange thing: even small changes in one rgb component

 cause a noticeable color effect....it seems the eye is very 

 sensitive to color balance.   


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#14 Starman1

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Posted 11 August 2020 - 12:55 AM

Time to build a miniature interferometer for eyepieces.

TeleVue said they aim for 1/100 wave error produced by the eyepiece at the wavefront.

The tests in Ciel et Espace showed they achieved that at f/7 with the Delos and Ethos.

I don't think anyone achieved it at f/3.5, though they came close.

 

The poorest eyepiece they tested was about 1/40 wave, so eyepieces obviously don't injure the wavefront by much compared to the optics in the scope.


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#15 luxo II

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Posted 11 August 2020 - 04:02 AM

Interesting list, Don.

 

Though when I want maximum contrast, for much the same reason as you try to eliminate stray light entering your telescope, eyepiece - or your eye - my preference is also to block out the extraneous sky background from the field of view in the eyepiece, and there are two ways. While the head-in-a-fishbowl effect of ultrawides has its place, its not particularly helpful when looking at really faint things. An eyepiece with a smallish field stop does this - possibly even an ortho - the field stop is invariably much blacker than the sky.

 

The second is a narrowband filter (eg an OIII) which blocks the visible spectrum except for a narrow passband. While not much use on anything except specific nebulae, nevertheless it indicates how well the original eyeball can perform when fully dark adapted and the background sky is suppressed.  Which brings us to the third option...

 

Increasing magnification, to the magic x1 per mm of aperture.


Edited by luxo II, 11 August 2020 - 04:05 AM.

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#16 Mike G.

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Posted 11 August 2020 - 08:47 AM

I wouldn't go that far, but certainly paying attention to local light can help with the "external factors" I mentioned.

HI Don,  first, thank you for taking the time to write this informative post. It’s much appreciated. My comment was primarily facetious but in fact I do use a hood (when temps allow) and am a bit O/C with my flocking. Additionally I own a number of TV eyepieces. So, my comment was meant in jest, but based in my own personal experience. 
Thanks again for providing the information to those who were unaware and reminding the rest of us what matters and why. 
 

Mike G



#17 BillP

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Posted 11 August 2020 - 09:15 AM

Many who do not observe from a true dark site also often neglect completely blocking all stray light as they view...so using a hood or having an eyecup that can make a complete light seal around your eye.  Doing this really make the view pop a whole lot more than letting even a little amount of stray light coming into the eye from the environment.


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#18 kklei940

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Posted 11 August 2020 - 10:44 AM

I had no idea the amount of factors that could influence the contrast. It's a bit mind numbing. It seems even if you take care to invest in quality eyepieces, that will only get you so far. For instance, the differences in the spectrum transmission of a 5mm Radian and a DeLite might very well warrant using one over the other depending on the object your viewing. And this considering those two eyepieces have likely done a good job covering points 1-6.

 

Anyway, great post here. I really learned a lot and appreciate having this information!



#19 rexowner

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Posted 11 August 2020 - 11:11 AM

>> 4. Improvement from the lack of internal reflections in the eyepiece from lens edges, spacers, filter threads, etc.

>> The best eyepieces have NO visible reflections from any internals. 

 

Very useful piece.

 

OK, this is being really, really nit-picky, but I'm not sure this is 100% accurate.   Vantablack reflects 0.04%

of visible light, and according to this article MIT invented a material that was 10x better:

 

https://en.wikipedia...wiki/Vantablack

 

While these materials may be used in professional telescopes, I doubt they are in any of the eyepieces we

can buy.

 

Sorry, I am a literalist, and "NO" means 0% / 100%.  I might say "...best eyepieces have effectively no...."

I might have some sort of condition, but when I read something I know isn't 100% true, I find it distracting

and makes me question the whole thing.


Edited by rexowner, 11 August 2020 - 11:12 AM.


#20 Duke93

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Posted 11 August 2020 - 11:35 AM

Great article Don.  The first thing I noticed when I upgraded to my first high end eyepiece (9 Nagler) was all the dark lanes in objects like M11, NGC 7789 etc.  The dark lanes had been grayed out with cheaper eyepieces.



#21 rexowner

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Posted 11 August 2020 - 12:15 PM

>> 4. Improvement from the lack of internal reflections in the eyepiece from lens edges, spacers, filter threads, etc.

>> The best eyepieces have NO visible reflections from any internals. 

 

Very useful piece.

 

OK, this is being really, really nit-picky, but I'm not sure this is 100% accurate.   Vantablack reflects 0.04%

of visible light, and according to this article MIT invented a material that was 10x better:

 

https://en.wikipedia...wiki/Vantablack

 

While these materials may be used in professional telescopes, I doubt they are in any of the eyepieces we

can buy.

 

Sorry, I am a literalist, and "NO" means 0% / 100%.  I might say "...best eyepieces have effectively no...."

I might have some sort of condition, but when I read something I know isn't 100% true, I find it distracting

and makes me question the whole thing.

Thought about this, and actually it probably is accurate.  While there may be a few stray reflected photons in

the visible range, they are probably not perceptible to the human eye, and thus not "visible".

 

Scratch my nit-picky remark.  Made me think about it though, so thanks.


Edited by rexowner, 11 August 2020 - 12:15 PM.

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#22 Starman1

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Posted 11 August 2020 - 12:32 PM

Interesting list, Don.

 

Though when I want maximum contrast, for much the same reason as you try to eliminate stray light entering your telescope, eyepiece - or your eye - my preference is also to block out the extraneous sky background from the field of view in the eyepiece, and there are two ways. While the head-in-a-fishbowl effect of ultrawides has its place, its not particularly helpful when looking at really faint things. An eyepiece with a smallish field stop does this - possibly even an ortho - the field stop is invariably much blacker than the sky.

 

The second is a narrowband filter (eg an OIII) which blocks the visible spectrum except for a narrow passband. While not much use on anything except specific nebulae, nevertheless it indicates how well the original eyeball can perform when fully dark adapted and the background sky is suppressed.  Which brings us to the third option...

 

Increasing magnification, to the magic x1 per mm of aperture.

Good comments.  But don't forget, you can always back away from a 100° eyepiece and see a smaller field of view.


  • Mike G. likes this

#23 Starman1

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Posted 11 August 2020 - 12:39 PM

>> 4. Improvement from the lack of internal reflections in the eyepiece from lens edges, spacers, filter threads, etc.

>> The best eyepieces have NO visible reflections from any internals. 

 

Very useful piece.

 

OK, this is being really, really nit-picky, but I'm not sure this is 100% accurate.   Vantablack reflects 0.04%

of visible light, and according to this article MIT invented a material that was 10x better:

 

https://en.wikipedia...wiki/Vantablack

 

While these materials may be used in professional telescopes, I doubt they are in any of the eyepieces we

can buy.

 

Sorry, I am a literalist, and "NO" means 0% / 100%.  I might say "...best eyepieces have effectively no...."

I might have some sort of condition, but when I read something I know isn't 100% true, I find it distracting

and makes me question the whole thing.

You're absolutely right.  The best eyepieces cannot reduce internal scatter to zero.  However, properly placed baffles can work wonders--

note all the superb reviews of the Vixen HR eyepieces, where the mfr spent extra time designing baffles to reduce scattered light in the eyepieces.

I might have added that baffles can reduce internal reflections from surfaces merely flat black by blocking them.

Vixen shows that this is an important factor often overlooked.

 

But, semantically, "no visible reflections" is not equal to "no reflections".  It merely points out that the reflections are not visible to the eye.

Reducing them to zero would not be possible. 


Edited by Starman1, 11 August 2020 - 12:40 PM.


#24 rexowner

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Posted 11 August 2020 - 01:00 PM

You're absolutely right.  The best eyepieces cannot reduce internal scatter to zero.  However, properly placed baffles can work wonders--

note all the superb reviews of the Vixen HR eyepieces, where the mfr spent extra time designing baffles to reduce scattered light in the eyepieces.

I might have added that baffles can reduce internal reflections from surfaces merely flat black by blocking them.

Vixen shows that this is an important factor often overlooked.

 

But, semantically, "no visible reflections" is not equal to "no reflections".  It merely points out that the reflections are not visible to the eye.

Reducing them to zero would not be possible. 

You're right.  I kind of withdrew my comment after thinking about it, and I was glad the post made

me reflect on the subject..  I used to have a boss who read his email backwards in time - read the

most recent first, which was a handy trick.



#25 jjack's

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Posted 11 August 2020 - 04:40 PM

Hi Don, good post, but you forgot the diagonal...replace your favorite dielectric by a prism smirk.gif

Isn'it Bill P ?




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