Types of eyepieces?

[turboscrew]

There are plössls. kellners and such, but what is the difference in the structure and usage?
I've read that some structures fit better with certain kinds of telescope (refractor/reflector, fast/slow, ...), than others.

Also, I wonder if it really makes sense, when in some sites they say that short focal length eyepieces, like 2 - 4 mm, are used with long focal length telescopes. Isn't that the other way around? With telescope of 2500 mm focal length, 2 mm eyepiece would give 1250x. To be useful, the aperture needs to be pretty big - about 600 mm. On the other hand, 2 mm focal length eyepiece gives 600x with a 1200 mm focal length telescope. That would be max. for my 300 mm / 1200 mm telescope.

[notFritzArgelander]

See this sticky thread: https://theskysearchers.com/viewtopic.php?f=34&t=4250

[notFritzArgelander]

See this sticky thread: https://theskysearchers.com/viewtopic.php?f=34&t=4250

It may be more than you want (it's pretty complete). It does not include all the aliases that commercial vendors apply to the various types so if you have questions about specific types ask away.

[turboscrew]

What causes the shadowing, and are some structures more forgiving than others?

[Lady Fraktor]

Spherical Aberration of the Exit Pupil

https://www.handprint.com/ASTRO/ae4.html#SAEP

[Ruud]

What causes the shadowing, and are some structures more forgiving than others?

There is vignetting, blackouts and kidney bean shadows:

vignetting:


blackouts:


kidney bean shadows (SAEP):


Kidney bean shadows are the most irritating. It makes "proper" eye placement very difficult. Here's a still:

[turboscrew]

What causes the shadowing, and are some structures more forgiving than others?

There is vignetting, blackouts and kidney bean shadows:

vignetting:


blackouts:


kidney bean shadows (SAEP):


Kidney bean shadows are the most irritating. It makes "proper" eye placement very difficult. Here's a still:

Very clearly put! Thanks!

[JayTee]

Excellent graphics Ruud.

Thanks,
JT

[helicon]

Thanks for sharing the pics [mention]Ruud[/mention] !

[Ruud]

Thanks guys, I made the animations a few years back to show how kidney beaning differs from blackouts. Many people confused the two.

My animations consist of mostly circles and polygons, easy to construct with Geogebra, a freeware math tool.

Here's another animation that explains how an internal field stop causes some eyepieces show a thin coloured ring at the edge of the field of view.

You may have to click the image to start the animation. For an explanation about the how and why, click here.

[turboscrew]

More questions.
I have 300 mm /1200 mm (F/4) tube. What kind of high power eyepieces I should get?
I've got this idea, that bigger and faster scopes require more from the high power eyepieces.

I read somewhere, that Plössls are not good as high power eyepieces for fast tubes. For lower power they are good and cost-effective.
Orthoscopic are good, or are they?
Wide angle eyepieces are not very good? Usually too many lenses?
And what are "planetary" eyepieces? Would those be useful?
Then there are "ED" eyepieces. What are those, and are they good?

[notFritzArgelander]

I wonder where you read Plossls are not good enough for fast scopes? I don’t think it’s true. Plossls are better for fast scopes than Abbe orthoscopics.

If you’re really fussy about optical quality you’ll want a very well corrected design. For example I settled on TV Panoptic and Delos with my f4 Newtonians now gone. At f5 my Z12 works fine with them but so do the Plossls and Orthoscopics.

[turboscrew]

I wonder where you read Plossls are not good enough for fast scopes? I don’t think it’s true. Plossls are better for fast scopes than Abbe orthoscopics.

If you’re really fussy about optical quality you’ll want a very well corrected design. For example I settled on TV Panoptic and Delos with my f4 Newtonians now gone. At f5 my Z12 works fine with them but so do the Plossls and Orthoscopics.

Not all Plössls were said to be not-so-good for fast scopes. Just the ones with short focal length (<5mm, I think). The less powerful ones were said to be good, especially considering the price.

I'm thinking about eyepieces 5 mm - 2 mm. (OK, the 2 mm is more of a curiosity - the max magnification.)
I have a 3.6 mm, and it doesn't seem to work well. Then again, I think it's modified achromat (MA). Those were also mentioned not to be good for fast scopes.

[Lady Fraktor]

A MA is not really good in any telescope but Meade likes to give them away so you feel like you received something.
Build quality will be more of a factor with Plossl eyepieces as they are normally good to f/4.
That does not mean every focal length will work with every telescope.
This is why it is usually mentioned to try to match the eyepiece to your eye.

[notFritzArgelander]

I wonder where you read Plossls are not good enough for fast scopes? I don’t think it’s true. Plossls are better for fast scopes than Abbe orthoscopics.

If you’re really fussy about optical quality you’ll want a very well corrected design. For example I settled on TV Panoptic and Delos with my f4 Newtonians now gone. At f5 my Z12 works fine with them but so do the Plossls and Orthoscopics.

Not all Plössls were said to be not-so-good for fast scopes. Just the ones with short focal length (<5mm, I think). The less powerful ones were said to be good, especially considering the price.

I'm thinking about eyepieces 5 mm - 2 mm. (OK, the 2 mm is more of a curiosity - the max magnification.)
I have a 3.6 mm, and it doesn't seem to work well. Then again, I think it's modified achromat (MA). Those were also mentioned not to be good for fast scopes.

The statement that short focal length Plossls are no good in fast scopes is either incorrect or confused about what "good" is or both.

Plossl is a scaled design meaning you can multiply the eyepiece geometry by a number to scale the design up or down in size and the eyepiece will work the same. So the optical performance is identical for all focal lengths at all f ratios.

Now part of the scaling is that a classic Plossl has eye relief that is 0.5 times the focal length. Sho short focal length Plossles need you to get your eye up close and personal to the eye lens. This ALSO has nothing to do with the f ratio of the scope.

An MA is not a Plossl and certainly will not do as well in a fast scope.

[turboscrew]

I wonder where you read Plossls are not good enough for fast scopes? I don’t think it’s true. Plossls are better for fast scopes than Abbe orthoscopics.

If you’re really fussy about optical quality you’ll want a very well corrected design. For example I settled on TV Panoptic and Delos with my f4 Newtonians now gone. At f5 my Z12 works fine with them but so do the Plossls and Orthoscopics.

Not all Plössls were said to be not-so-good for fast scopes. Just the ones with short focal length (<5mm, I think). The less powerful ones were said to be good, especially considering the price.

I'm thinking about eyepieces 5 mm - 2 mm. (OK, the 2 mm is more of a curiosity - the max magnification.)
I have a 3.6 mm, and it doesn't seem to work well. Then again, I think it's modified achromat (MA). Those were also mentioned not to be good for fast scopes.

The statement that short focal length Plossls are no good in fast scopes is either incorrect or confused about what "good" is or both.

Plossl is a scaled design meaning you can multiply the eyepiece geometry by a number to scale the design up or down in size and the eyepiece will work the same. So the optical performance is identical for all focal lengths at all f ratios.

Now part of the scaling is that a classic Plossl has eye relief that is 0.5 times the focal length. Sho short focal length Plossles need you to get your eye up close and personal to the eye lens. This ALSO has nothing to do with the f ratio of the scope.

An MA is not a Plossl and certainly will not do as well in a fast scope.

That's exactly the kind of information I'm after. And I mentioned the MA. because I realized it doesn't work well. I just didn't know that when I bought it. I just don't want to make similar mistakes any more - especially with maybe more expensive eyepieces. That's why I'm asking.
(BTW, good to know about the Plössl scalability, Thanks.)

[notFritzArgelander]

Good grief.....

Plossl is a scaled design meaning you can multiply the eyepiece geometry by a number to scale the design up or down in size and the eyepiece will work the same. So the optical performance is identical for all focal lengths at all f ratios.

Should have read:

Plossl is a scaled design meaning you can multiply the eyepiece geometry by a number to scale the design up or down in size and the eyepiece will work the same. So the optical performance is identical for all focal lengths at the same f ratio.

I dyslexically mistyped what I didn't mean to say. Sorry for the confusion.

[turboscrew]

Good grief.....

Plossl is a scaled design meaning you can multiply the eyepiece geometry by a number to scale the design up or down in size and the eyepiece will work the same. So the optical performance is identical for all focal lengths at all f ratios.

Should have read:

Plossl is a scaled design meaning you can multiply the eyepiece geometry by a number to scale the design up or down in size and the eyepiece will work the same. So the optical performance is identical for all focal lengths at the same f ratio.

I dyslexically mistyped what I didn't mean to say. Sorry for the confusion.

Now I'm curious. How does the F-ratio affect?

[notFritzArgelander]

Good grief.....

Plossl is a scaled design meaning you can multiply the eyepiece geometry by a number to scale the design up or down in size and the eyepiece will work the same. So the optical performance is identical for all focal lengths at all f ratios.

Should have read:

Plossl is a scaled design meaning you can multiply the eyepiece geometry by a number to scale the design up or down in size and the eyepiece will work the same. So the optical performance is identical for all focal lengths at the same f ratio.

I dyslexically mistyped what I didn't mean to say. Sorry for the confusion.

Now I'm curious. How does the F-ratio affect?

A simple way to think of it is that the focus of the telescope is where the rays converge. Those converging rays make a cone of a certain angular width. The faster the scope, the larger the angle of the cone of converging rays. An eyepiece designer has to make a design that "accepts" that cone without distorting it. Thus the minimum accepted f ratio (maximum incident cone angle) is a design requirement. It also affects how fancy the math has to be to accurately do a ray tracing diagram for the design. The larger the incident cone angle happens to be, the more complex the math. It turns out that the Plossl design works well at fast f ratios and fat cone angles.

If the ep works well at the fastest f ratio it will also work well at slower ones. But the fastest f ratio is where the design can no longer handle the incoming cone of rays without distorting the image some way.

[turboscrew]

Good grief.....



Should have read:



I dyslexically mistyped what I didn't mean to say. Sorry for the confusion.

Now I'm curious. How does the F-ratio affect?

A simple way to think of it is that the focus of the telescope is where the rays converge. Those converging rays make a cone of a certain angular width. The faster the scope, the larger the angle of the cone of converging rays. An eyepiece designer has to make a design that "accepts" that cone without distorting it. Thus the minimum accepted f ratio (maximum incident cone angle) is a design requirement. It also affects how fancy the math has to be to accurately do a ray tracing diagram for the design. The larger the incident cone angle happens to be, the more complex the math. It turns out that the Plossl design works well at fast f ratios and fat cone angles.

If the ep works well at the fastest f ratio it will also work well at slower ones. But the fastest f ratio is where the design can no longer handle the incoming cone of rays without distorting the image some way.

That explains the higher requirements of eyepieces for faster scopes.
It's just a bit rough to study optics for a year or two to be able to select working eyepieces.
Also, I guess the different glass materials that are used, bring in their own features (refractive index in different wave lengths).
The (basic) geometry of the optics may not be so hard, but going through all the structures and materials used in them...
And if it was easy, nobody would pay the designers...