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Is a 25mm Plossl a Bad Design?

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#1 DIYDarren

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Posted 06 August 2022 - 03:47 PM

I have a bunch of different 25mm/26mm plossls, and all show a lot of distortion around the edge of the field. And these aren't even wide angle eyepieces just the standard ~55 degree Meade Plossl. 

 

Is this an artifact of the design?

Too close to the barrel diameter?

 

Am I crazy?

 

Using in an F10 and all seem equally not so good...

 

Curious if anyone else has noticed this?

 

Thanks! --darren

 

 

 

 



#2 Starman1

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Posted 06 August 2022 - 05:16 PM

Darren,

The Plössl design is actually ~50°, not 55°, but you are just noticing a couple things:

  • Plössls are not fully corrected to the edge
  • your telescopes all have serious field curvature due to the designs, and this will cause the edge of field stars to be slightly out of focus when the center is focused.
  • the combination of the two make aberrations at the edge more visible.

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#3 sevenofnine

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Posted 06 August 2022 - 09:40 PM

I noticed a big improvement when I upgraded my 25mm plossl to a 25mm Agena StarGuider/AT Paradigm ED. It's a very reasonably priced upgrade IMO waytogo.gif


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#4 faackanders2

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Posted 06 August 2022 - 10:20 PM

Good for H-alpha solar scope viewing.  Less surfaces for reflections and fairly inexpensive.  I don't go cheaper than possils, even though some like othoscopic (or mono spherical).



#5 DIYDarren

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Posted 06 August 2022 - 10:48 PM

 

Darren,

The Plössl design is actually ~50°, not 55°, but you are just noticing a couple things:

  • Plössls are not fully corrected to the edge
  • your telescopes all have serious field curvature due to the designs, and this will cause the edge of field stars to be slightly out of focus when the center is focused.
  • the combination of the two make aberrations at the edge more visible.

 

Thank you sir, I didn't realize plossls were not fully corrected at the edge. I do have some orthos and even an erfle of about the same F.L. and they do not distort at the edge so much. But this is good info, didn't realize that even these relatively inexpensive eyepieces with relatively similar fields have different correction characteristics. I will need to repeat the experiment with one of my slow refractors and see if I notice better performance. (I was using a C8 when I noticed this, I believe those are fairly curvaceous??) 

 

Thank you!!!



#6 Starman1

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Posted 07 August 2022 - 12:07 AM

Some are better corrected at the edge than others, but the 4 element design isn't going to yield a spot size off axis as small as in the center.

See the ray tracing here:

https://www.telescop...ce_raytrace.htm

The first design with a well corrected edge is the H-ortho (not a commercial design) or the Nagler.


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#7 j.gardavsky

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Posted 07 August 2022 - 08:15 AM

Some are better corrected at the edge than others, but the 4 element design isn't going to yield a spot size off axis as small as in the center.

See the ray tracing here:

https://www.telescop...ce_raytrace.htm

The first design with a well corrected edge is the H-ortho (not a commercial design) or the Nagler.

H-ortho not a commercial design?

 

JG



#8 Starman1

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Posted 07 August 2022 - 09:30 AM

The discussion in another thread indicated the H-ortho was a modified Abbe type that would be an improvement on the Abbe design, but which

wasn't a commercial design, just as in the book Telescope Optics, where improvements to the Erfle and other designs were suggested.

It is certainly possible some company has made the design, but I've seen the number of orthoscopic eyepieces dwindle with each passing year.

And the ones from the same outfit in Japan that has made them for years are a classic Abbe design.

 

Given modern glass type availability, and computer design, it is certainly possible to improve on the older designs where edge correction is concerned.

I just doubt there is any economic incentive to do so for designs like Kellner, Erfle, Plössl, Brandon, König, etc. when the market seems to be entranced

with wider apparent fields and longer eye reliefs.

 

I heard a rumor that TeleVue had designed a line of long eye relief 52° super-planetary eyepieces but decided not to make them because they thought they wouldn't sell.

If that's true, given the difference in the sales of their Plössls and Delites, I think they were wise to put the project on the shelf.

Each year, astrophotography grows stronger and visual observing shrinks.  Each year the number of new eyepieces is a small number compared to the number of older

ones going out of production.  Astronomy equipment sales boomed during the pandemic (which isn't over), but the number of eyepiece offerings went down.

My Buyers Guide from 2007 have several hundred more eyepieces on it than now.

 

I would stand in line to buy a line of 85° eyepieces with 18mm or more of eye relief, and spend Ethos prices but are there enough people like me to justify a line?

I talk to a lot of older observers who wear glasses in their daily lives, but remove them to observe.  The sale of Morpheus eyepieces tells me there is a market at $300.

Is there a market at $600?  I don't know.

 

At some point, the visual observing market will shrink precipitously due to older observers dying off.  I think we're already seeing it.  I've heard about a number of telescopes

recently purchased at estate sales.  It's hard to know for sure since the supply chain interruptions have meant a shortage in telescopes and eyepieces.

But I think that is one of the reasons why the number of new eyepiece offerings each year has been going down, and when I say that, I exclude the numbers of rebrandings

of exactly the same eyepieces that have been offered for years under other labels.

 

A number of the ray-traced designs on Vlad Sacek's pages are not offered for sale.  Not that surprising.


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#9 j.gardavsky

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Posted 07 August 2022 - 02:45 PM

The "H-Ortho" can be ragarded as a modification of the original Abbe ortho,

when we take the changed triplet form as a differentiator, and taking into account the glass materials, etc., adapted.

 

However,

the triplet "flat out" towards the field stop can be seen on the TAKAHASHI MC Abbe 12.5mm  (MADE IN JAPAN), as in my collection.

 

Best,

JG


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#10 j.gardavsky

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Posted 07 August 2022 - 02:56 PM

Back to the 25mm Plössl "Bad Design"

 

I would not call the designs of the Plössl eyepieces generally bad.

 

The Tele Vue Plössl 25mm is very good, I used to have it.

The problems arise with the budget Plössls manufatured on the price point, or with the "Plössl" eyepieces, which are no Plössls at all.

 

The classical symmetrical convex out Plössl is still available from the Carl Zeiss Jena surplus,

https://www.ebay.de/...d=&toolid=10050

as offered by bwoptik in Germany.

The field is stopped down to 40°, and this old Zeiss Plössl is really a very good performer.

 

Best,

JG


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#11 Polyphemos

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Posted 07 August 2022 - 04:08 PM

Back to the 25mm Plössl "Bad Design"

 

I would not call the designs of the Plössl eyepieces generally bad.

 

The Tele Vue Plössl 25mm is very good, I used to have it.

The problems arise with the budget Plössls manufatured on the price point, or with the "Plössl" eyepieces, which are no Plössls at all.

 

The classical symmetrical convex out Plössl is still available from the Carl Zeiss Jena surplus,

https://www.ebay.de/...d=&toolid=10050

as offered by bwoptik in Germany.

The field is stopped down to 40°, and this old Zeiss Plössl is really a very good performer.

 

Best,

JG

Bwoptik won’t ship the linked eyepiece to the USA, so that’s nicht gut.



#12 25585

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Posted 08 August 2022 - 07:19 AM

Aging can take away well functioning eyesight before other parts!  I guess my Noblexs are Ethos price category. Its worth having fewer but totally usable eyepieces than a heap of OK-back-when. Easier and sometimes cheaper to get telescopes for your eyepieces than vice versa, I have same aperture but different FL scopes e.g. 500 & 900mm 4" refractors, used together with the same eyepiece, I can go from low wide to high (less wide) magnification.

 

Extreme specialisation designs are desirable for certain uses, but more general use and user friendliness may sell better and survive longer. The eyepieces that are designed for both spotter day and astro night use, when people may wish to wear sunglasses, take afocal photos, or use Night Vision etc will be guaranteed better longer continuity. 
 

If TV dropped a few AFOV degrees, to give longer FL Delites in 1.25" fit or kept to 62* and made 2" they would sell if priced sensibly for their target market. Brand as an excuse for overpricing is passé in this new era, as ES found, people vote with their wallets more.



#13 Sarkikos

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Posted 08 August 2022 - 07:47 AM

A 25mm Plossl behaves like a better design in a slow scope or with a Barlow.  However, as Don pointed out, field curvature in the telescope can make outer field aberrations more obvious

 

Mike


Edited by Sarkikos, 08 August 2022 - 10:49 AM.


#14 j.gardavsky

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Posted 08 August 2022 - 08:07 AM

Bwoptik won’t ship the linked eyepiece to the USA, so that’s nicht gut.

The problem is, that the shipment costs to the U.S., plus customs, plus VAT, would be higher than the price of the eyepiece.

 

That's really not good,

JG


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#15 Starman1

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Posted 08 August 2022 - 08:54 AM

The problem is, that the shipment costs to the U.S., plus customs, plus VAT, would be higher than the price of the eyepiece.

 

That's really not good,

JG

No VAT in the US.  And no customs duty on a single eyepiece.

If a simple 1-item import, brokerage fee is about $36 and shipping from the EU about $65.

So importing would add about $100 to the eyepiece.

On an expensive eyepiece, the American consumer might save that much in VAT charges he doesn't have to pay,

so importing an eyepiece into the US would only be prohibitive on a very low-priced eyepiece.


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#16 Jon Isaacs

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Posted 08 August 2022 - 10:55 AM

A 25mm Plossl behaves like a better design in a slow scope or with a Barlow.  However, as Don pointed out, field curvature in the telescope can make outer field aberrations more obvious

 

Mike

 

I have always liked 25mm Plossls, the best of the lot.  A reasonable amount of eye relief, not too much.  A comfortable view with the eye lens similar in size to the body, something of the floating effect.  Not the sharpest off-axis in a fast scope but there is more to an eyepiece than edge sharpness.  

 

Jon


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#17 Polyphemos

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Posted 08 August 2022 - 11:27 AM

No VAT in the US.  And no customs duty on a single eyepiece.

If a simple 1-item import, brokerage fee is about $36 and shipping from the EU about $65.

So importing would add about $100 to the eyepiece.

On an expensive eyepiece, the American consumer might save that much in VAT charges he doesn't have to pay,

so importing an eyepiece into the US would only be prohibitive on a very low-priced eyepiece.

Fees were less than that when I purchased my AZ GTi mount from FLO a few months ago.

 

I paid £20.13 shipping via DHL, no VAT, and no brokerage fee, so about $24.35 all told for an item considerably larger and heavier than a Plössl eyepiece.  So less costly than buying stateside if you figure shipping and sales tax.  Of course England isn’t in the EU, but I doubt that makes much of a difference.

 

Anyway, das ist schade.


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#18 Sarkikos

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Posted 08 August 2022 - 11:34 AM

I have always liked 25mm Plossls, the best of the lot.  A reasonable amount of eye relief, not too much.  A comfortable view with the eye lens similar in size to the body, something of the floating effect.  Not the sharpest off-axis in a fast scope but there is more to an eyepiece than edge sharpness.  

 

Jon

I have a pair of 25 Celestron Plossls that are FC, single coat.  I received them with telescopes I've bought over the years.  I don't think I've ever used them.  I need to try them out singly and in my binoviewer.  

 

Mike


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#19 j.gardavsky

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Posted 08 August 2022 - 02:40 PM

No VAT in the US.  And no customs duty on a single eyepiece.

If a simple 1-item import, brokerage fee is about $36 and shipping from the EU about $65.

So importing would add about $100 to the eyepiece.

On an expensive eyepiece, the American consumer might save that much in VAT charges he doesn't have to pay,

so importing an eyepiece into the US would only be prohibitive on a very low-priced eyepiece.

The UPS Economy shipment would cost at the moment as less as EUR 18.65

https://www.jumingo....DxoCPyMQAvD_BwE

 

The best way would be to e-mail Mrs. Beate Langner-Voss to drop the Plössl to the UPS shop for the shipment to the U.S.

 

According to the specs, this Plössl can be used for the telescope speeds up to 5, the distortion is stated as being less than 3%.

 

Best,

JG
 


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#20 KWB

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Posted 08 August 2022 - 02:59 PM

A 25mm plossl is a very good eyepiece and the finest example I've ever used in this focal length is the TV 25mm plossl. I think in years past TV included a 20mm plossl with their TV-76 and TV-85 refractor packages. IMO they should have included the 25mm eyepiece. smile.png


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#21 25585

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Posted 09 August 2022 - 07:29 AM

Low 20s upwards FL, seems to be what all manufacturers get right, whatever the optical design. A good beginner FL for sure. A 2x Barlow is a better 2nd optic than a shorter FL eyepiece.



#22 Sarkikos

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Posted 09 August 2022 - 12:14 PM

A 25mm plossl is a very good eyepiece and the finest example I've ever used in this focal length is the TV 25mm plossl. I think in years past TV included a 20mm plossl with their TV-76 and TV-85 refractor packages. IMO they should have included the 25mm eyepiece. smile.png

I have heard that the slightly concave outer surface on TV Plossls produce a more well-corrected outer field.

 

Mike


Edited by Sarkikos, 09 August 2022 - 12:15 PM.

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#23 KWB

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Posted 09 August 2022 - 12:29 PM

Mike,

 

I've owned most of the TV plossls over the years and the standouts to me were the 20, 25 and 32.  The 25mm is the single sharpest and most well corrected 25mm plossl eyepiece I've ever owned or viewed through. I wish I had that eyepiece back. I almost took a trip to a local brick and mortar astro shop yesterday to obtain another one. They are closed on Monday. I came that close to getting another one.

 

Today I'm working on buying another telescope. grin.gif


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#24 checcocpb

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Posted 09 August 2022 - 03:32 PM


Each year, astrophotography grows stronger and visual observing shrinks.  Each year the number of new eyepieces is a small number compared to the number of older

ones going out of production.  Astronomy equipment sales boomed during the pandemic (which isn't over), but the number of eyepiece offerings went down.

I think you're right, I started observing in the 90s and I still go to the same places on the mountaintop to observe. In the start, we were 9 people observing and maybe 1 photographing, with film, now the choice of two: or I'm alone observing or there are other 3 to 5 people photographing, with digital. The number of active amateurs, for active I intend people going to dark sites with the telescope, has shrunk and most moved to digital imaging. As always Don got it right!

Cheers

PS Me too I'm enjoying the Morpheus even if I have a full set of T4-T5-T6. The Baader are so comfortable and good performing in F/10 SCT.



#25 chemisted

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Posted 09 August 2022 - 06:07 PM

Mike,

 

I've owned most of the TV plossls over the years and the standouts to me were the 20, 25 and 32.  The 25mm is the single sharpest and most well corrected 25mm plossl eyepiece I've ever owned or viewed through. I wish I had that eyepiece back. I almost took a trip to a local brick and mortar astro shop yesterday to obtain another one. They are closed on Monday. I came that close to getting another one.

 

Today I'm working on buying another telescope. grin.gif

I have owned the 20 and 32 and agree they are excellent.  I have never owned the 25 since I have a TV 22mm Panoptic (perhaps my most comfortable eyepiece), a Celestron 24mm Ultima (parfocal with the 30 and 18) and a Vixen LV 25mm (a recent acquisition as a NOS item and quite a fine eyepiece for this focal length).


Edited by chemisted, 09 August 2022 - 06:07 PM.

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