113 replies to this topic

[Starman1]

Thanks.

One thing I've read is that many people see a small amount of pincushion (up to 7%) as undistorted, which agrees with your findings.

If the eyepiece had little RD and had, instead, AMD, the effect could be additive, making AMD appear worse.

I only have one eyepiece my eye perceives as having a completely flat field, and the rest vary but mostly appear to have a slightly concave appearance, just like the night sky when you look up and see the entire sky.

(The night sky appears flat at the zenith but seems to curve down to the horizon--call it a large, flattened bowl) --flat in the center, then slightly curved upward to meet the field stop.

 

One thing: a curved focal plane will be perceived to be flat so long as the focal plane curvature of scope and eyepiece are identical.  I suspect that the curvature may influence one's perspective,

but if the stars are sharp to the edge, how would we tell if the field is flat or curved?  Probably only by panning.  To me, that means that strong RD or strong AMD are negatives when panning the scope.

But, if neither RD nor AMD is solved for in the design, how much of each should be left in?  I don't have an answer.

 

One of the perceptions I haven't explained yet is that the field appears to be different distances from the eye in different eyepieces.

I once thought it was the apparent field, with wider apparent fields appearing nearer the eye, but then I discovered different 100° eyepieces appear to have the field at differing distances.

So, I cannot explain why different eyepieces appear to have their fields at different distances from the eye.  It could be the influence of eye relief, but I am not sure of that.

 

I am so glad not to be an eyepiece designer.  It's obvious to me some factors are often ignored, like EOFB, or the focal plane distance perception, or how to design an eyepiece to appear as if it has the least amount of distortion for the eye.

It seems a designer would first have to become an expert in human vision.

[LF78]

I have not (yet) noticed differences in apparent distances to the field, but I can speculate.
I read that depth is evaluated from 3 sources: eye accommodation (focusing) for short distances, binocular vision for medium distances, and perspective clues for large distances. (With a lot of overlap, of course.) Binocular vision is not used here. Perspective clues could apply, depending on the AFOV ("straw" effect). Did you see a simple relation between AFOV and apparent distance? I guess not, or you would have said so. Are you looking at this in daylight, so that the area around the field stop is partially lighted, giving more perspective clues?
This leaves eye accommodation as a possible factor. My understanding of eyepieces was that light rays emerged parallel, corresponding to visions at infinity. There is probably some leeway, since the eyes can accommodate, which would correspond to some narrow range of acceptable focuser positions. (How narrow would be correlated to tolerance of field curvature.) But this would make the apparent distance to the field a function of the precise focuser position in this narrow range. Since you describe the distance as something attached to the eyepiece, rather than variable, this fits even less than the previous attempted explanation.

[Starman1]

Yes, I do not know what it is.  But at the time I started noticing it, I had 8mm, 7mm, and 6mm 100° eyepieces on hand, and the fields seen

were close, medium, and close respectively.

A mystery, for sure.

[davidgmd]

I see what you're saying, though the circumferential expansion with pincushion would be significantly less that the radial enlargement.  Radial lines do diverge with distance from center.

However, that is true in the complete absence of distortion.  You can draw radial lines on any piece of graph paper to see what I mean. I can see a condition where the radial line is stretched but any circumferential expansion is solely due to distance from center.

 

Something is wrong in your analysis or, if AMD is reduced to zero, leaving RD in, you would still see an increase in double star separation at the edge of the field, and you do not.

I suppose you might see an increase if the separation happens to be on a radial line.  Possibly, the reason an increase in separation is not seen is that increasing separation from 1" to 1.1" might not be noticeable, yet that would be 10%.

Or, that people don't look at double stars at the edge of the field.

In eyepieces with no RD but with AMD, I do see a change in the separation of a double star near the edge, usually to a closer separation, yet straight lines stay straight.  The box gets smaller or larger, but sides stay parallel.

 

Pincushion and barrel distortion are the + and - of rectilinear distortion.   I guess the corresponding terms for AMD are "Globe" and "Bowl". 

 

Here is a chart for both distortions and how they increase from a small to a large apparent field:

  
Don, I think you once posted a link to a page with pictures that had varying amounts of distortion. It demonstrated that people have different perceptions of what zero distortion looks like. 

[T-rav82]

I am struggling to get perfect focus across the field in a refractor at the moment, the scope is an 8 ed f/6.25 (500mm fl). I wear glasses corrected for astigmatism.

 

I instantly noticed the problem with a 17.5mm morpheus when observing m45. Perfect focus could not be achieved at the centre and edge of field at the same time. Focusing on a star midway was ok but not ideal.

 

I have never experienced this problem in the dob (with the same eyepiece, glasses etc...) and by all accounts this is a well corrected eyepiece. Thus I initially thought it was field curvature.

 

However, I also notice that if I focus on stars at the centre, I can achieve better focus at the edges just by rolling my head and looking straight at them. Likewise, if I am not looking straight at any stars, they appear out of focus. It is akin to looking through a glob from the inside out. If I focus on a star at the centre, stars on the left are not sharp and seem to elongate towards the the field stop on the left and vice versa on the right.

 

What complicates the problem is that I have noticed a slight improvement when using a pair of glasses I have for observing that have a larger lens. I also noticed an improvement when I swapped eyepiece and used an 18.2mm delite.

Adding a dioptrix also seems to help. Still though, I seem to have to observe perfectly on axis and not move at all in order to get a decent (but not perfect) image.

 

Given these facts, I have come to the conclusion that the problem is due to The combination of the short focal length of the refractor, eyepiece design and possibly by glasses.

 

I need to further investigate the issue at higher power and by using the dioptrix on the morpheus and by also observing the moon perhaps.

 

Is this a combination of field curvature and astigmatism?

 

I have asked the same question elsewhere and several people have kindly replied. It has been suggested that a TS FLAT2 field flattener with appropriate spacers may help. I am willing to try but I am concerned, from having read the thread on here about this flattener for visual, 1) this may only help with certain eyepieces unless I want to add and remove spacers as I change eyepiece and 2), that if it is astigmatism, the flattener will not fully resolve the problem anyway.

Thank you for reading,

Daniel

[Starman1]

The primary issues with the 17.5mm Morpheus are, in order of decreasing severity: astigmatism, coma, and chromatic aberration.

It's hard to distinguish astigmatism and coma at that low a power, but I agree that if you look straight at the edge by looking through the eyepiece at an angle, the edges are sharper than if you use peripheral vision.

Still we evaluate the edges of fields by looking directly at them, so the edges appear to be pretty good, despite what is found on a test bench.

Personally, I found the edges of the 17.5mm to most aberrated of all the Morpheus focal lengths, but that may relate to my own eye's astigmatism at that exit pupil, and a less-than-perfect correction with my glasses.

Suffice it to say, the edges are not bad and the eyepiece is pretty sharp, but then, my scope's radius of curvature is over 10x as long as yours, so a lot flatter field.

 

Field curvature is not noted in the eyepiece at all, so would almost certainly be due to the 72.2° field and the 500mm scope focal length.

Especially because the radius of curvature in a 500mm refractor is close to 170mm, so a VERY strongly curved focal plane.

Alas, the older you are, the less accommodation you have, so FC is highly likely to be visible.  An even wider apparent field would show more.

 

That glasses helped and that a narrower field helped shows the cause to be mostly scope and partially your eye.

The field flattener would definitely help and as to whether you'd need different spacers, that would depend on how far apart the focal planes of your eyepieces are.

+/- 3mm on spacing will have zero visual effect.

 

I found the large eye lenses in the Morpheus demand a larger glasses lens and also single vision lenses.  Otherwise, the sides are distorted by the glasses and/or the frame of the glasses is in the view.

Edited by Starman1, 06 February 2025 - 10:23 PM.

[T-rav82]

The primary issues with the 17.5mm Morpheus are, in order of severity: astigmatism, coma, and chromatic aberration.

It's hard to distinguish astigmatism and coma at that low a power, but I agree that if you look straight at the edge by looking through the eyepiece at an angle, the edges are sharper than if you use peripheral vision.

Still we evaluate the edges of fields by looking directly at them, so the edges appear to be pretty good, despite what is found on a test bench.

Personally, I found the edges of the 17.5mm to most aberrated of all the Morpheus focal lengths, but that may relate to my own eye's astigmatism at that exit pupil, and a less-than-perfect correction with my glasses.

Suffice it to say, the edges are not bad and the eyepiece is pretty sharp, but then, my scope's radius of curvature is over 10x as long as yours, so a lot flatter field.

 

Field curvature is not noted in the eyepiece at all, so would almost certainly be due to the 72.2° field and the 500mm scope focal length.

Especially because the radius of curvature in a 500mm refractor is close to 170mm, so a VERY strongly curved focal plane.

Alas, the older you are, the less accommodation you have, so FC is highly likely to be visible.  An even wider apparent field would show more.

 

That glasses helped and that a narrower field helped shows the cause to be mostly scope and partially your eye.

The field flattener would definitely help and as to whether you'd need different spacers, that would depend on how far apart the focal planes of your eyepieces are.

+/- 3mm on spacing will have zero visual effect.

 

I found the large eye lenses in the Morpheus demand a larger glasses lens and also single vision lenses.  Otherwise, the sides are distorted by the glasses and/or the frame of the glasses is in the view.

Thanks Don, that’s confirmed my suspicions. I have just come in from doing further testing and what you said makes a lot of sense. Whilst the delite does have a narrower field of view, when comparing to the Morpheus I attempted to compare the same stars at the edge so I feel it must also be down to the design of the eyepiece, not just a difference in fov.

what you said about  our glasses is interesting, I purchased a pair with a much larger lens as the frame was getting in the way. Shame they are not as well corrected as the eyepieces though.

[Starman1]

Thanks Don, that’s confirmed my suspicions. I have just come in from doing further testing and what you said makes a lot of sense. Whilst the delite does have a narrower field of view, when comparing to the Morpheus I attempted to compare the same stars at the edge so I feel it must also be down to the design of the eyepiece, not just a difference in fov.

what you said about your glasses is interesting, I purchased a pair with a much larger lens as the frame was getting in the way. Shame they are not as well corrected as the eyepieces though.

But, at the edge of the field, a star is 31° off axis in a Delite, and 36.1° off axis in a 17.5 Morpheus.  That extra 5+° can make a difference in field flatness.

Think of the focused field of the scope as a sphere with a 170mm radius and the field of the eyepiece as a short, linear, flat surface perpendicular to the radius where it touches the circle.

Just as the Earth is flat over 3", but curved over 3000 miles, the wider the perpendicular line is from the radius, the farther the end of that line is from the circle of focus.

Which is why a wider apparent field at a given focal length (the 18.2mm Delite and 17.2mm (actual focal length) Morpheus are pretty close) will see more field curvature of the scope.

That field, by the way, gets smaller as magnification goes up and eyepiece focal lengths get shorter.  So, field curvature's visibility will diminish as the eyepiece focal length gets shorter.

[T-rav82]

But, at the edge of the field, a star is 31° off axis in a Delite, and 36.1° off axis in a 17.5 Morpheus.  That extra 5+° can make a difference in field flatness.

Think of the focused field of the scope as a sphere with a 170mm radius and the field of the eyepiece as a short, linear, flat surface perpendicular to the radius where it touches the circle.

Just as the Earth is flat over 3", but curved over 3000 miles, the wider the perpendicular line is from the radius, the farther the end of that line is from the circle of focus.

Which is why a wider apparent field at a given focal length (the 18.2mm Delite and 17.2mm (actual focal length) Morpheus are pretty close) will see more field curvature of the scope.

That field, by the way, gets smaller as magnification goes up and eyepiece focal lengths get shorter.  So, field curvature's visibility will diminish as the eyepiece focal length gets shorter.

Definitely noticed the field curvature less with shorter focal lengths.

When comparing the morpheus 17.2 and the delite 18.2, I meant I was trying to compare the same stars in each cluster (m45) away from the centre. These stars were at the edge of field in the delite, but not quite in the morpheus.

My reasoning was to compare any differences other than those caused by a difference in fov. Maybe I am wrong?

[Starman1]

Definitely noticed the field curvature less with shorter focal lengths.

When comparing the morpheus 17.2 and the delite 18.2, I meant I was trying to compare the same stars in each cluster (m45) away from the centre. These stars were at the edge of field in the delite, but not quite in the morpheus.

My reasoning was to compare any differences other than those caused by a difference in fov. Maybe I am wrong?

That was valid. I though you moved the star to the edge in each eyepiece.

[MrsM75]

Hi, I am sorry English is my third language. I have a question Sir. Thank you Sir in advance.

 

So I was using the 6mm Vixen eyepiece. Looking at Jupiter, in fact any 6mm I use, whether it SvBony Redline or Vixen or the 7mm XCell. I see the same thing, so it must be something with my eyes and not the eyepiece fault.

 

But when I look at very high magnification 6mm divided by 1900 focal Mak. Jupiter, I see Jupiter good, but I also see this circle, it not ghosting (I know what ghosting is), but it like this circle, it like my pupil, like I can see my pupil in there.

 

I don't know how to explain it, I wish I have a pic to show what I was seeing. Doesn't bother me, but what I wonder what is that.

[Starman1]

With a 6mm in your scopes, the exit pupil is smaller than 0.5mm,

Floaters in the eye will appear, and Map-Dot-Fingerprint issues with the cornea will appear.

And, Jupiter is bright enough to reflect from the cornea and cause reflections to come back to the eye.

You sound like you are getting a very bright reflection from the cornea to the eyepiece and back to the eye.

I would bet you don't see this on double stars or other objects.

[MrsM75]

With a 6mm in your scopes, the exit pupil is smaller than 0.5mm,

Floaters in the eye will appear, and Map-Dot-Fingerprint issues with the cornea will appear.

And, Jupiter is bright enough to reflect from the cornea and cause reflections to come back to the eye.

You sound like you are getting a very bright reflection from the cornea to the eyepiece and back to the eye.

I would bet you don't see this on double stars or other objects.

 

omg omg you right Sir, you sure is an expert in eyepieces. 

 

Yep, it exaclty that. And yep, I do not see it in stars or other objects, I don't even recall I see it on Neptune or Uranus.

 

So there no way to fix this, it just how it is? It actually does not bother me at all, btu I feel that my cornea is staring back at me, lol.

 

Thank you so much Sir.

 

oh Sir, I have the Celestorn Luminos 10mm, okay the eyepiece is heavy Sir, with a GSO diagonal, I am genuinely worry it will crack my Visual back focuser with my small Maks. Should I worry?

 

I read about heavy eyepieces can strain the focuser. Is strain the weight on the focuser they mean here is the visual back where you put your Diagonal in?

 

I thought focuser is the Knob where you turn. And the Visual back is the round thing that attach to the back so you put in your Diagonal and eyepiece. I'm confuse.

[Starman1]

omg omg you right Sir, you sure is an expert in eyepieces. 

 

Yep, it exactly that. And yep, I do not see it in stars or other objects, I don't even recall I see it on Neptune or Uranus.

 

So there no way to fix this, it just how it is? It actually does not bother me at all, btu I feel that my cornea is staring back at me, lol.

 

Thank you so much Sir.

 

oh Sir, I have the Celestorn Luminos 10mm, okay the eyepiece is heavy Sir, with a GSO diagonal, I am genuinely worried it will crack my Visual back focuser with my small Maks. Should I worry?

 

I read about heavy eyepieces can strain the focuser. Is strain the weight on the focuser they mean here is the visual back where you put your Diagonal in?

 

I thought focuser is the Knob where you turn. And the Visual back is the round thing that attach to the back so you put in your Diagonal and eyepiece. I'm confuse.

Different eyepieces have different abilities to reflect a planet image right back at the eye.

It's entirely possible another 6mm eyepiece would not do that.

 

Though there could be a trace of sag in your optical system, it is unlikely the 10 Luminos will crack anything.

If you are concerned, you could look at lighter eyepieces.  The 10mm Luminos is available under other labels that are a bit lighter, but whether the weight difference is significant, I don't know.

 

You are correct as to what is the visual back and what is the focuser.  I don't believe the 10mm Luminos is heavy enough to strain the focuser.

It is 12 ounces, but the other versions of the eyepiece may not be much lighter:

https://astromaniaop...ariantsId=10072