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How to distinguish ortho from ploessl?

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#1 Franta

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Posted 27 January 2021 - 06:44 PM

Does someone know whether it is possible to tell whether a given eyepiece is an ortho or ploessl without dismantling or X-raying the eyepiece? Consider that the eyepiece has an AFOV of 40 degree (so if it were a ploessl it wes stepped down).

 

I am thinking about possible differences in the off axis rays in spotting diagrams on fast telescopes, or an aberration which is characteristic for a particular eyepiece construction, but nothing seems to me to be conclusive. Is there a better way how to tell ortho from ploessl?


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#2 SeattleScott

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Posted 27 January 2021 - 06:46 PM

I would say Ortho based on the AFOV.

Scott
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#3 Redbetter

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Posted 27 January 2021 - 07:36 PM

Could also be a Kellner or Huygens.  


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#4 markb

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Posted 27 January 2021 - 08:56 PM

Possibly count point sources reflection (aor-glasssurfaces) and faint reflections (cemented glass-glass) to differentiate.

 

I would think the shapes and sizes of non-point reflections, compared to known Orthos and plossls might also help.

 

By FOV it sounds non-plossl.


Edited by markb, 27 January 2021 - 08:57 PM.

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#5 MartinPond

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Posted 27 January 2021 - 08:56 PM

40 degrees.....that is tough to tell!

Both ortho(abbe) and plossl would be super-flat and sharp all the way.

There is a little geometric shift near the abbe/ortho's edge that makes the

 image "float" at you.  The Plossl, at 40 degrees, is usually super-flat, no 3D effect.

Very little difference at 40 degrees.

 

  Ramsden(still common in microscopes),

 and Kellner would make vertical lines 'bend' a bit near the edge.

Huygens would look awful even before 40 degrees.  30-35 is more common.


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#6 Dan5209

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Posted 27 January 2021 - 11:25 PM

Shine a green laser through it and see how the elements are arranged?


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#7 Ernest_SPB

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Posted 28 January 2021 - 12:04 AM

In most cases eyepieces of classical schemata like Plossl, Orthoscope and other are easy gadgets to disassemble and investigate their construction. 


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#8 BillP

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Posted 28 January 2021 - 12:55 AM

If you can take the barrel off so you can see the field lens, then try to reflect a house lamp or something off the top surface so you can see the reflected image of the light bulb.  The object being reflected will look different (larger) from a standard Plossl because the exterior surfaces of a typical Plossl are flat, whereas the field lens surface of an Abbe is convex.  So seeing a reflected image off the surface of the field lens of an Abbe will appear smaller then that same reflected object off the eye or field lens of a Plossl.


Edited by BillP, 28 January 2021 - 12:57 AM.

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#9 jjack's

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Posted 28 January 2021 - 02:16 AM

Abbes and plossls could be orthos. I think he would differentiate abbes from plossls.


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#10 Franta

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Posted 28 January 2021 - 05:58 AM

Thanks all of you for the suggestions.

 

If you can take the barrel off so you can see the field lens, then try to reflect a house lamp or something off the top surface so you can see the reflected image of the light bulb.  The object being reflected will look different (larger) from a standard Plossl because the exterior surfaces of a typical Plossl are flat, whereas the field lens surface of an Abbe is convex.  So seeing a reflected image off the surface of the field lens of an Abbe will appear smaller then that same reflected object off the eye or field lens of a Plossl.

 

This non-invasive idea sounds particularly attractive. I will do this first (I am separated from my eyepieces now, so I cannot do it today).


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#11 markb

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Posted 28 January 2021 - 11:26 AM

The flat exterior eyelens surface on a typical plossl indeed for a typical plossl.

 

I'd try a couple of different tests, like an element count.

 

I mentioned this because one of the easiest ways to distinguish a circle NJ Televue prototype silvertop Celestron P from the Vixen-made one is to run the same test, with the Televue having a rear concave surface. That's how I quickly found mine, then looked for the circle NJ. I've seen posts referring to the Televue plossls as symmetrical and 'ordinary', but Al Nagler messed with the curve geometry.

 

The point source reflection test can be a little tricky, but quickly distinguishes four element Meade 4000 2, 2 plossls from 5 element pseudo-Matsuyama 4000 2, 1, 2 plossls.

 

Of course, many designs qualify as orthoscopic in optical effects, as I understand it. I assume, as apparently have others, the OP is referring to an Abbe Ortho 1, 3 design.

 

The A-O would give 4 strong reflections and 2 weak reflections (based on an A-O 3, 1 layout), and a conventional 2, 2 plossl would give 4 strong and 2 weak reflections.


Edited by markb, 28 January 2021 - 12:08 PM.

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#12 rkelley8493

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Posted 28 January 2021 - 11:34 AM

The plossls I've used tend to show some pincushion [rectilinear] distortion compared to ortho's of similar focal length. However, I'm pretty sure it has more to do with the 10° wider field of view. Not sure how that would play out if they were the same apparent field..



#13 BillP

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Posted 28 January 2021 - 01:21 PM

This non-invasive idea sounds particularly attractive. I will do this first (I am separated from my eyepieces now, so I cannot do it today).

What is the focal length of the eyepiece in question?

 

Another thing you could try is a precise measure of the eye relief.  A "standard" Plossl has eye relief of 0.68 Fe (Focal length of eyepiece) so .68xFL of the eyepiece.  An Abbe is .8 Fe.  Problem here is that if the focal length is small, then probably the margin for error can encompass both eye relief numbers.


Edited by BillP, 28 January 2021 - 01:27 PM.

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#14 Glen10

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Posted 28 January 2021 - 03:21 PM

My 2cents~ if you can gauge how thick the lens 'stack is'  orthos tend to be  'thick' relative to their lens dia. thanks to that triplet field lens.???

 

Aside~You can't always trust what's printed on the barrel.

Someone will correct me if i'm wrong.... wink.gif Vixen did some flatops marked 'Or', they were actually Plossls. 

I had one in 5mm fl for a while. I'm, not normally one to shy away from tight ER, but that ep was pushing it somewhat.

A 5mm genuine Or not a problem!



#15 BillP

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Posted 28 January 2021 - 03:53 PM

Meade has a few "Or" that were Symmetrical when I took them apart - I have a 4mm and 6mm that  are excellent planetary performers! 



#16 Franta

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Posted 28 January 2021 - 04:31 PM

What is the focal length of the eyepiece in question?

I want to check that the Baader Classic Ploessl (32mm) is not an ortho and that the Takahashi 25mm ortho is not a Ploessl. Even though I have good experience with both manufacturers and no suspicion to distrust them on eyepiece labels, I want to know how to check this just out of curiosity. The labels are probably correct in these cases, but in the future I might face another eyepiece.

 

I tend to prefer orthos to Ploessls, I find them more crisp and comfortable.

 

Sure, there is a difference in the eye relief between an ortho and ploessl, yet it is hard to tell at a longer focal length eyepiece where the eye lens is recessed deep in the eyepiece body.

 

Thanks to all of you for sparkling so many different ideas.



#17 RichA

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Posted 28 January 2021 - 06:51 PM

Abbes and plossls could be orthos. I think he would differentiate abbes from plossls.

Except most Plossls aren't really Plossls.  They are symmetrical.  Brandons are Plossls.



#18 j.gardavsky

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Posted 29 January 2021 - 05:32 AM

Except most Plossls aren't really Plossls.  They are symmetrical.  Brandons are Plossls.

And most of the Plössls, which aren't really Plössls,

are also not aplanatic symmetricals,

and they are concave out.

 

So, no Plössls at all,

JG


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#19 MartinPond

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Posted 30 January 2021 - 09:59 AM

They never got an "appelation controlee" for Plossols

like "Champagne" or "bourbon".

 

 

Plossl is just commonly used now to describe almost

  any symmetrical 2,2  

Oddly enough, I have seen concave-out final surfaces

 and convex-out final surfaces in the same product line...

 I like convex-out (fewer glare issues), but I can't designate it,

  unless I got a Brandon, which is effectively  

  an appelation controlee.  It is a brand and model that means a shape.

 

I have a Bausch&Lomb orthoscopic that is a 1,2

At 10mm fl, that is a great choice: almost identical field

  to a 1,3 or a 2,2 but more eye relief than either.

An "ortho" could be 1,2 2,2  or 1,3. 

Choking a 1,2,1 down to 45 or 50degrees would be

  far more orthoscopic, technically, than any of those.

 

More important than any of that is:  how is the view, to you?


Edited by MartinPond, 30 January 2021 - 10:01 AM.

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#20 MartinPond

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Posted 30 January 2021 - 01:19 PM

Does someone know whether it is possible to tell whether a given eyepiece is an ortho or ploessl without dismantling or X-raying the eyepiece? Consider that the eyepiece has an AFOV of 40 degree (so if it were a ploessl it wes stepped down).

 

I am thinking about possible differences in the off axis rays in spotting diagrams on fast telescopes, or an aberration which is characteristic for a particular eyepiece construction, but nothing seems to me to be conclusive. Is there a better way how to tell ortho from ploessl?

'Do the ray/spot diagrams go out to +/-20degrees?   




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