27 replies to this topic

[AbdullahMhd]

Hello everyone,

I recently got the Sky Rover 28mm UWA, which is basically the same as the Astro-Tech 28mm UWA, and I have seen prominent chromatic aberration at the edges when viewing Venus and the Moon, I know this eyepiece is not meant for such objects but I used it for testing. In the center the views are perfect, it is just around the edges.

Can any users share their experiences with this type of eyepieces, is it normal for wide fov EP to show some chromatic aberration at the edges? Or is my EP defective?

I am using the EP with an 8” f/5.9 reflector.

Thank you.

Edited by AbdullahMhd, 19 April 2025 - 11:27 PM.

[Mike W]

Hello everyone,

I recently got the Sky Rover 28mm UWA, which is basically the same as the Astro-Tech 28mm UWA, and I have seen prominent chromatic aberration at the edges when viewing Venus and the Moon, I know this eyepiece is not meant for such objects but I used it for testing. In the center the views are perfect, it is just around the edges.

Can any users share their experiences with this type of eyepieces, is it normal for wide fov EP to show some chromatic aberration at the edges? Or is my EP defective?

I am using the EP with an 8” f/5.9 reflector.

Thank you.

Not familiar with this e/p but if you decide to return it I would get a 30mm UFF. (APM-Celestron etc.)

[NOLAMusEd]

Some eyepieces are very good about minimal CA at the edge, such as the Delos line from Tele Vue. I've noticed my ES 82's (11 and 6.7) definitely show a good bit in my reflector towards the edge. However, in practice, it's usually a non-issue for me as I don't really observe bright objects near the field stop, even with a non-tracking scope. Highly unlikely your sample is defective. I would live with it if you enjoy the deep sky views if that's what you intend to use the EP for.

[AbdullahMhd]

Some eyepieces are very good about minimal CA at the edge, such as the Delos line from Tele Vue. I've noticed my ES 82's (11 and 6.7) definitely show a good bit in my reflector towards the edge. However, in practice, it's usually a non-issue for me as I don't really observe bright objects near the field stop, even with a non-tracking scope. Highly unlikely your sample is defective. I would live with it if you enjoy the deep sky views if that's what you intend to use the EP for.

Yes that’s what I intend to use the eyepiece for, but I just want to know if this CA is inherent to the eyepiece design and not a defect in the one I got.

I haven’t tested it yet on wide field views yet but I assume the issue will be less pronounced on stars than the very bright Venus, right?

Edited by AbdullahMhd, 19 April 2025 - 11:58 PM.

[TayM57]

Many widefield EPs have LC the closer you get to the edge.

 

More important is to use the EP over several observing sessions, across a variety of objects. Then draw your conclusions on whether the LC is bothersome or not.

[eblanken]

Hi (aka AbdullahMhd),

 

Could it be that you are seeing colors that are perhaps Atmospheric Dispersion (AD) instead of CA  on the Moon (aka Luna) and Venus ?

 

A test is to know how low in the sky was the object . . . AD is worse for a lower object . . . Blue on one side and Red opposite, but CA is symmetric around the object . . . 

 

Also, welcome to the Eyepieces Forum of Cloudy Nights (CN) !

 

Best,

 

Ed

Edited by eblanken, 20 April 2025 - 12:26 AM.

[AbdullahMhd]

Hi (aka AbdullahMhd),

Could it be that you are seeing colors that are perhaps Atmospheric Dispersion (AD) instead of CA on the Moon (aka Luna) and Venus ?

Also, welcome to the Eyepieces Forum of Cloudy Nights (CN) !

Best,

Ed

I am a beginner and wouldn’t be able to tell the difference unfortunately, but if that was the case shouldn’t this affect the whole field and not just the edges? Because as I said the center is very good. Again I am a beginner and this is just an assumption.

Edited by AbdullahMhd, 20 April 2025 - 12:29 AM.

[eblanken]

Hi again, 

 

I am a beginner and wouldn’t be able to tell the difference unfortunately, but if that was the case shouldn’t this affect the whole field and not just the edges? Because as I said the center is very good. {Sorry, I missed that - Ed} Again I am a beginner and this is just an assumption {Trust your own eyes - Ed}.

 

Yes, I would move the object to the center & four edge positions at 0, 90, 180, 270 Degrees and look closer at the colors and how they are oriented: are they associated with the edges only, but not the center and do the opposite edges have opposite colorings ? Do the colors orient with the elevation angle above the horizon ?

 

Pinned at the top of this eyepiece forum, you can find Don (aka Starman1) Pensack's expert writeup of Eyepiece Aberrations:

 

Here: https://www.cloudyni...ce-aberrations/

 

I hope that helps you more than I can.

 

Best,

 

Ed

Edited by eblanken, 20 April 2025 - 12:44 AM.

[AbdullahMhd]

Hi again,


Yes, I would move the object to the center & four edge positions at 0, 90, 180, 270 Degrees and look closer at the colors and how they are oriented: are they associated with the edges only, but not the center and do the opposite edges have opposite colorings ?

Pinned at the top of this eyepiece forum, you can find Don (aka Starman1) Pensack's expert writeup of Eyepiece Aberrations:

Here: https://www.cloudyni...ce-aberrations/

I hope that helps you more than I can.

Best,

Ed

I believe it is more in line with lateral color. Thank you for elaborating

[RichA]

Many widefield EPs have LC the closer you get to the edge.

 

More important is to use the EP over several observing sessions, across a variety of objects. Then draw your conclusions on whether the LC is bothersome or not.

I think the main problem might be, someone uses a reflector or expensive apo, expecting no real CA then the eyepieces dump it into the system and the CA at the edge of these eyepieces is worse than a cheap achromat would show, even if it only manifests off-centre..  

Edited by RichA, 20 April 2025 - 12:57 AM.

[SeattleScott]

Honestly this sounds a lot like “ring of fire.” This is a known issue with several longer focal length ultrawides, like my ES 24/82, or the 31 Nagler, etc. It is inherent in the design. It could be corrected, but it would require more glass and more weight for eyepieces that are already heavy, to fix something for viewing targets that people really shouldn’t be using the eyepiece to view anyway.

[AbdullahMhd]

Honestly this sounds a lot like “ring of fire.” This is a known issue with several longer focal length ultrawides, like my ES 24/82, or the 31 Nagler, etc. It is inherent in the design. It could be corrected, but it would require more glass and more weight for eyepieces that are already heavy, to fix something for viewing targets that people really shouldn’t be using the eyepiece to view anyway.

I am not really visually bothered with the effect because like you said they are not meant to view such targets. I was more bothered by the idea of the eyepiece being defective in any way. I am very relieved to know that this is inherent to the eyepiece type. And this was my question really.

Thank you.

[eblanken]

Hi again,

 

I just re-read the thread and then the relevant paragraph from Don's expertise:

 

"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."

 

No surprise that CA would be the diagnosis . . . 

 

Best,

 

Ed

 

P.S. Sometimes I post for a wider audience than just the OP . . . to benefit others who also read the thread later . . .

[AbdullahMhd]

This is a photo I just found showing the aberration on the upper left side.

 

The photo was taken with a phone and the moon was not in the center of the eyepiece it was shifted to the upper left edge of the EP

 

Edited by AbdullahMhd, 20 April 2025 - 01:16 AM.

[eblanken]

Good example . . . I would expect that the color would move to each of the four corners if the offset from the center was positioned as I suggested in the experiment in my earlier post (#8) . . . and centering would have symmetric and least color or none . . . 

 

Cheers,

 

Ed

Edited by eblanken, 20 April 2025 - 01:23 AM.

[AbdullahMhd]

Good example . . . I would expect that the color would move to each of the four corners if the offset from the center was positioned as I suggested in the experiment in my earlier post (#8) . . . 

 

Cheers,

 

Ed

Yes that was the case. You think this is normal in such fov and fl eyepieces?

Edited by AbdullahMhd, 20 April 2025 - 01:24 AM.

[eblanken]

Yes, it is a difficult and expensive thing to completely eliminate from a design (according to Don's expertise) and camera lenses work hard to rid their designs of this type of issue . . . corners of full frame images are the test case . . . 

 

Ed

Edited by eblanken, 20 April 2025 - 01:31 AM.

[RichA]

Hi again,

 

I just re-read the thread and then the relevant paragraph from Don's expertise:

 

"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."

 

No surprise that CA would be the diagnosis . . . 

 

Best,

 

Ed

 

P.S. Sometimes I post for a wider audience than just the OP . . . to benefit others who also read the thread later . . .

  Yes, it is possible to carefully position your eye to eliminate central longitudinal chromatic aberration, you can even do it to an extent with an achromatic refractor, if your head is very steady, but chromatic aberration at off-centre positions of the eyepiece isn't that and there is no mistaking the two.  Side note;  it's also possible to reduce astigmatism of the eye by viewing through the "edge" of  they eyepiece by shifting your eye position, rendering a sharper image than seen directly through the centre.

[TayM57]

It's not just the moon or other bright objects that will show LC in wide field EPs. Stars will too, but to a lesser extent than moon/planets, and typically, you notice LC in stars much closer to the field stop than you would with the moon/planets.

 

If it is any consolidation, shorter focal length wide field EPs show LC on the moon and planets as well. The good EPs will minimize this so it is not objectionable. I've done plenty of lunar viewing with short focal length 100° EPs, and never found the LC to be bothersome even though it was there.

[AbdullahMhd]

It's not just the moon or other bright objects that will show LC in wide field EPs. Stars will too, but to a lesser extent than moon/planets, and typically, you notice LC in stars much closer to the field stop than you would with the moon/planets.

 

If it is any consolidation, shorter focal length wide field EPs show LC on the moon and planets as well. The good EPs will minimize this so it is not objectionable. I've done plenty of lunar viewing with short focal length 100° EPs, and never found the LC to be bothersome even though it was there.

Yes I wouldn't call it bothersome too, just wanted to be sure nothing is wrong with the EP, and from the replies it seems to be a normal thing.

 

Thank you.

[RichA]

It's not just the moon or other bright objects that will show LC in wide field EPs. Stars will too, but to a lesser extent than moon/planets, and typically, you notice LC in stars much closer to the field stop than you would with the moon/planets.

 

If it is any consolidation, shorter focal length wide field EPs show LC on the moon and planets as well. The good EPs will minimize this so it is not objectionable. I've done plenty of lunar viewing with short focal length 100° EPs, and never found the LC to be bothersome even though it was there.

Well, the real problem with CA isn't the fringe (though it is distracting) seen at the edges of bright objects, it's the overall diffusion of the defocused blue light reducing contrast on objects and between objects and (what should be) a dark sky background.

[eblanken]

Good thread . . . thanks to OP for asking . . . 

 

Best,

 

Ed

[SeattleScott]

Upon seeing the picture, I agree this is LC, not ring of fire. Regardless it is still common with premium wide AFOV eyepieces. I have seen it manifest more in a refractor sometimes so the scope can be an issue as well.

[T1R2]

Hello everyone,

I recently got the Sky Rover 28mm UWA, which is basically the same as the Astro-Tech 28mm UWA, and I have seen prominent chromatic aberration at the edges when viewing Venus and the Moon, I know this eyepiece is not meant for such objects but I used it for testing. In the center the views are perfect, it is just around the edges.

Can any users share their experiences with this type of eyepieces, is it normal for wide fov EP to show some chromatic aberration at the edges? Or is my EP defective?

I am using the EP with an 8” f/5.9 reflector.

Thank you.

One way to tell the difference if you think your EP is defective is...if the view in the center is fine, everything is nice and sharp, even with EP's that are not quite well corrected at the edges, then, if you had a lens in the wrong order or a missing lens you'd have more a problem than just a little color, you might even have severe softness and halos around stars and planets not not being able to focus correctly and other aberrations caused by spherical aberration, and on top of that, if its in Newt without a Paracorr you can combine that coma inherent in that design on top of the coma the mis-assembled EP is showing would be pretty bad, you'd def be able to know something is not right, and your problem would be worse than a little color at the edges. 

[AbdullahMhd]

One way to tell the difference if you think your EP is defective is...if the view in the center is fine, everything is nice and sharp, even with EP's that are not quite well corrected at the edges, then, if you had a lens in the wrong order or a missing lens you'd have more a problem than just a little color, you might even have severe softness and halos around stars and planets not not being able to focus correctly and other aberrations caused by spherical aberration, and on top of that, if its in Newt without a Paracorr you can combine that coma inherent in that design on top of the coma the mis-assembled EP is showing would be pretty bad, you'd def be able to know something is not right, and your problem would be worse than a little color at the edges.

I can with confidence tell that this is not the case. Thank you for the concise explanation. I believe it is clear now that it is just in the design.