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  #21  
Old 17-09-2019, 08:33 AM
Wavytone (Nick)
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Markus if they did publish the data it would be game over for most.

Keeping it as a guessing game means healthy sales continue as we are forced to try pot luck, buy them more or less at random & sell the ones we don’t like.

As you know the Japanese market is dominated by small refractors hence the Japanese eyepieces suit these, and are also ok in SCTs and maks.

Televues are aimed at the US market which is dominated by big dobs, and SCTs in which pretty much anything is ok.

The odd ones out imho are flat-field refractors (ie have a flattener, or APOs with 4...6 elements) ... there aren’t many truly flat-field eyepieces !
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  #22  
Old 17-09-2019, 11:58 AM
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But doesn't it seem strange that Pentax had the luxury of designing *any eyepiece they liked* to match their scopes, but settled on an 'inferior design'?


It implies they either didn't know, (the eyepieces are not cheap - they could have used a different design) or that the design is not actually as problematic as people say it is.


I have to say, I'm not as experienced as some, but I bought my set based on A/B Nagler comparisons and I found the Pentax to be a shade more contrasty, though it was splitting hairs.


But field curvature has never been an issue for me. Perhaps because the eye is an imperfect instrument. Curvature in a photographic setting is terrible, but in a visual observing scenario where you're trying to keep your eye centred on a <5mm exit pupil... field curvature has to be quite extreme before it's even noticable. It's far less noticable than other abberations, IMO.


All I know is that I don't go from a 20mm to a 5mm and suddenly go 'yuck!'.



-Markus
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  #23  
Old 17-09-2019, 06:07 PM
ausastronomer (John Bambury)
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Originally Posted by Stonius View Post
But doesn't it seem strange that Pentax had the luxury of designing *any eyepiece they liked* to match their scopes, but settled on an 'inferior design'?

-Markus
Hi Markus,

It comes from the fact that despite what a couple of people on this forum will try to tell you, eyepiece designers rarely design eyepieces to work in certain telescope types. They design them to correct for certain aberrations to satisfy the design parameters and intended end use of the eyepiece.

The 7mm, 10mm, 14mm and 20mm Pentax XW's were designed for Spotting Scope use and that's the way they were marketed by Pentax for well over 15 years. On the Pentax website these 4 eyepieces were listed under "Spotting Scope Eyepieces" and all of the other focal lengths in the XW series were listed under "Astronomical Eyepieces"

Being intended for Spotting Scope use these 4 eyepieces are corrected for "Rectilinear Distortion" (Barrel and Pincussion) where you need straight lines to appear straight and not bent. Most Astronomical eyepieces are corrected for "Angular Magnification Distortion" and not for rectilinear distortion. Despite modern computer programs, glass types and advanced eyepiece design, it is not possible to correct for both at the same time. I spoke at length with Al Nagler on this very topic about 12 or more years ago. He advised that his eyepieces were generally corrected for "Angular Magnification Distortion" and not rectilinear distortion, with the Nagler T4's being the exception, as these were corrected for rectilinear distortion and suitable for daytime use in refractors.

Cheers
John B
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  #24  
Old Yesterday, 07:05 AM
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Originally Posted by ausastronomer View Post
Hi Markus,

It comes from the fact that despite what a couple of people on this forum will try to tell you, eyepiece designers rarely design eyepieces to work in certain telescope types. They design them to correct for certain aberrations to satisfy the design parameters and intended end use of the eyepiece.

The 7mm, 10mm, 14mm and 20mm Pentax XW's were designed for Spotting Scope use and that's the way they were marketed by Pentax for well over 15 years. On the Pentax website these 4 eyepieces were listed under "Spotting Scope Eyepieces" and all of the other focal lengths in the XW series were listed under "Astronomical Eyepieces"

Being intended for Spotting Scope use these 4 eyepieces are corrected for "Rectilinear Distortion" (Barrel and Pincussion) where you need straight lines to appear straight and not bent. Most Astronomical eyepieces are corrected for "Angular Magnification Distortion" and not for rectilinear distortion. Despite modern computer programs, glass types and advanced eyepiece design, it is not possible to correct for both at the same time. I spoke at length with Al Nagler on this very topic about 12 or more years ago. He advised that his eyepieces were generally corrected for "Angular Magnification Distortion" and not rectilinear distortion, with the Nagler T4's being the exception, as these were corrected for rectilinear distortion and suitable for daytime use in refractors.

Cheers
John B
John,
Try the T4's in the daytime-- (look at any straight line) they have substantial rectilinear distortion, in the form of pincushion. Maybe they don't have as much AMD as the apparent fields would imply, but they are not corrected for rectilinear distortion.
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  #25  
Old Yesterday, 07:07 AM
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Originally Posted by Stonius View Post
But doesn't it seem strange that Pentax had the luxury of designing *any eyepiece they liked* to match their scopes, but settled on an 'inferior design'?


It implies they either didn't know, (the eyepieces are not cheap - they could have used a different design) or that the design is not actually as problematic as people say it is.


I have to say, I'm not as experienced as some, but I bought my set based on A/B Nagler comparisons and I found the Pentax to be a shade more contrasty, though it was splitting hairs.


But field curvature has never been an issue for me. Perhaps because the eye is an imperfect instrument. Curvature in a photographic setting is terrible, but in a visual observing scenario where you're trying to keep your eye centred on a <5mm exit pupil... field curvature has to be quite extreme before it's even noticable. It's far less noticable than other abberations, IMO.


All I know is that I don't go from a 20mm to a 5mm and suddenly go 'yuck!'.



-Markus
Markus,
The ability to accommodate field curvature diminishes with age. Older observers may not be able to accommodate the FC of certain focal lengths of the XW.
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  #26  
Old Yesterday, 07:16 AM
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Its a pity all manufacturers dont publish these. *Do any other manufacturers publish this data?

It's also interesting to note, that
1) if it is indeed true that different types of telescopes have different focal plane curvature, that...
2) must be matched to the curvature of the eyepiece used, then...
3) given that the XW's were originally designed for the pentax spotting scopes,
4) half of the range must have been not fit for purpose, by design.

Which doesn't seem right to me. Given the luxury of custom designing high quality eyepieces to suit a particular telescope, why would they design half their range 'wrong' for that purpose? It doesn't make sense.

Markus
Even Pentax no longer publishes this data. My link used the Wayback Machine to find a link several years old and long deleted from the web.

As for field curvature and sign--shorter focal length eyepieces cover smaller portions of a curved focal plane in a telescope. The Earth is curved, but 1m of ground can be quite flat. So it is more important to match the field curvature of the average scope with long focal length eyepieces than it is with short focal length eyepieces because the long ones see a larger portion of the scope's focal plane, and see more curvature.

Note that the longer focal lengths of XWs also have fewer elements than the short ones, so there is a greater freedom of design in the shorter eyepieces.

I can't speak for compound catadioptric scopes, but other forms of commercial amateur scopes have positive field curvature at the focal planes, so one would normally have expected the longer focal lengths to fare well in those scopes. Note, though, that short focal length refractors have a lot of field curvature, with a radius of curvature about the same as 1/3 the focal length, whereas newtonians have considerably flatter fields, both because of the longer focal lengths, and because their radii of curvature matches the focal length. An eyepiece that appears to have a flat field in one might not appear flat in the other, even if the field curvature in the eyepiece matches the scope's sign.
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  #27  
Old Yesterday, 11:03 AM
ausastronomer (John Bambury)
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Originally Posted by Don Pensack View Post
John,
Try the T4's in the daytime-- (look at any straight line) they have substantial rectilinear distortion, in the form of pincushion. Maybe they don't have as much AMD as the apparent fields would imply, but they are not corrected for rectilinear distortion.
I have all 3 of the T4's but I've never tried them in the daytime because I don't own a refractor, other than finderscopes, binoculars and rifle scopes.

I am only repeating what Al Nagler told me in about 2007, when I asked him some questions about them. I was only drinking coffee at the time so pretty sure I interpreted him correctly.

Cheers
John B
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  #28  
Old Yesterday, 11:14 AM
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The T4s do have, overall, low distortion.
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  #29  
Old Yesterday, 02:23 PM
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Markus,
The ability to accommodate field curvature diminishes with age. Older observers may not be able to accommodate the FC of certain focal lengths of the XW.

That's interesting - I wonder why that is?
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  #30  
Old Yesterday, 03:52 PM
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That's interesting - I wonder why that is?
The lens of the eye not only yellows then turns brown as we age (UV damage), but also hardens so the muscles in the eye gradually grow less able to change the lens shape to accommodate for distance.
It starts with near vision first. At around 40 we need reading glasses. Then, 50 requires bi-focals and 60 trifocals and the near prescription strengthens.
Alas, the lenses don't "freeze" at infinity focus, but somewhere closer because we live indoors, so older individuals need a distance prescription too.

If UV damage gets severe enough,small opacities become apparent in spots in the lenses, which gradually progress toward a cloudy translucency to the lens. This is called cataracts and the mitigation of the effects requires a lens replacement--a common surgery.
Human trials start this year for a steroid-like substance which can dissolve cataracts. It worked in 98% of all mice so there is hope that lens replacement surgery may be a thing of the past in a few years.
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  #31  
Old Yesterday, 04:42 PM
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Originally Posted by Don Pensack View Post
The lens of the eye not only yellows then turns brown as we age (UV damage), but also hardens so the muscles in the eye gradually grow less able to change the lens shape to accommodate for distance.
It starts with near vision first. At around 40 we need reading glasses. Then, 50 requires bi-focals and 60 trifocals and the near prescription strengthens.
Alas, the lenses don't "freeze" at infinity focus, but somewhere closer because we live indoors, so older individuals need a distance prescription too.

If UV damage gets severe enough,small opacities become apparent in spots in the lenses, which gradually progress toward a cloudy translucency to the lens. This is called cataracts and the mitigation of the effects requires a lens replacement--a common surgery.
Human trials start this year for a steroid-like substance which can dissolve cataracts. It worked in 98% of all mice so there is hope that lens replacement surgery may be a thing of the past in a few years.

I had heard that, but I thought that was to do with the eye's ability to focus *in general*.


I thought field curvature was more about a focal plane that is curved. Achieving sharp focus for the middle makes the outside go out of focus, and vice versa. So I'm thinking that as long as your eye can achieve focus at all, curvature should be just as much of a problem at any age, shouldn't it? Unless the shape of your retina (ie; the focal plane of the eye itself) changes with age?


Cheers


Markus
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  #32  
Old Yesterday, 04:58 PM
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Originally Posted by Don Pensack View Post
If the two curves (sagittal and meridional) deviate, that indicates astigmatism. The center of the field is at the bottom and the edge is at the top. The scale is in diopters (the eye can usually accommodate 1, but 2 would be seen as curved).
Ideally, the two curves would not deviate and they would be vertical.

So by looking at the graphs it would be true to say that;


The 14mm (and perhaps 5mm) have the least astigmatism, even though it has negative field curvature.


The 30 & 40mm have the least field curvature, even though it does have some slight astigmatism.


I'll test this next time I'm out, but I don't know how well I'll go, as it's easier to compare different EP's of the same focal length than different EP's in the same range of different focal length.


Cheers mate :-)
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  #33  
Old Yesterday, 11:22 PM
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Originally Posted by Stonius View Post
I had heard that, but I thought that was to do with the eye's ability to focus *in general*.


I thought field curvature was more about a focal plane that is curved. Achieving sharp focus for the middle makes the outside go out of focus, and vice versa. So I'm thinking that as long as your eye can achieve focus at all, curvature should be just as much of a problem at any age, shouldn't it? Unless the shape of your retina (ie; the focal plane of the eye itself) changes with age?


Cheers


Markus
What, though, do you mean by "out of focus"? Even if the edge requires a slightly different focus than the center, if you can easily focus your eye on each, would you notice the field curvature? Probably not.
Even at 68, with some eyepieces that have a curved field, if I focus the eyepiece on a star halfway to the edge, I can see the entire field in focus, whereas if I focus on a star in the center, the edge appears to be out of focus. My eye still accommodates a curve of half the diopter change, but not the complete one.
A young person can easily handle a 2 diopter change and the eye will still see the field in focus.
Here is a scholarly article on the effects of aging and accommodation in the eye:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2908311/
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  #34  
Old Today, 01:00 AM
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I'm trying to understand why *field curvature* would be related to age.


I get that eyesight deteriorates with age as does the ability of the eye to focus, and I get what you're saying about focusing halfway to the edge of the field to keep the inner and outer stars within the circle of confusion.


But correct me if I'm wrong; a curved focal plane is not fixed by shifting the focal point of the eye; instead you would have to be able to change the shape of your retina to accomodate to a curved focal plane, wouldn't you? I don't think people (young or old) are able to do this are they?


Cheers.

Markus
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  #35  
Old Today, 02:12 AM
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Originally Posted by Stonius View Post
I'm trying to understand why *field curvature* would be related to age.


I get that eyesight deteriorates with age as does the ability of the eye to focus, and I get what you're saying about focusing halfway to the edge of the field to keep the inner and outer stars within the circle of confusion.


But correct me if I'm wrong; a curved focal plane is not fixed by shifting the focal point of the eye; instead you would have to be able to change the shape of your retina to accommodate to a curved focal plane, wouldn't you? I don't think people (young or old) are able to do this are they?


Cheers.

Markus
Field curvature in an eyepiece is a fixed value. But can you accommodate the focus change in your own eye necessary to focus on both the center and the edge? That is what changes with age.
When you focus on the TV across the room and then change your gaze to a book in your lap, your eye is changing focus by changing the shape of the lens in the eye.
It is this ability that changes with age.

So if you have a sightly curved focal plane and you are young, when you change your gaze from the center to the edge, your eye automatically adjusts its own focus and the image is in focus both places.
There is a limit to this, and even young eyes cannot accommodate too much focus change. Older eyes can accommodate even less, so where the younger eye may see the whole field in focus, the older eye may not.

The point is that a slightly curved focal plane in an eyepiece may not be a problem for younger observers, but may be a problem for older observers.
And, because they have more strongly curved focal planes, this issue may arise more with refractors than with reflectors.
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