Televue 32mm Plossl - Dissapointed

[Alan White]

Just now, Tenor Viol said:

Eyepiece options have increased greatly in recent decades. When I was younger, it was Kellners and Orthoscopics... Plossls were luxury items! I remember being excited that I'd bought a 20mm Plossl. Then unaffordable things like Naglers appeared...

Yes, Plossl's were a luxury.
I still have my first Bresser eyepiece set of Kellners and Reverse Kellners.

 

[mikeDnight]

19 minutes ago, Alan White said:

Johns often shown diagram is super, shows many lens designs and helps understand them more easily.
Sorry my draw over was so rough.

 

Don't be too hard on yourself Alan. In years to come that White original could be hung on a gallery wall, with art experts discussing the deeper underlying meaning behind every stroke. 🎨

[John]

Plossls ? ....... eh, in them days we were lucky to have a bit of broken glass to peer though .....

 

 

[sputniksteve]

5 minutes ago, Alan White said:

Yes, Plossl's were a luxury.
I still have my first Bresser eyepiece set of Kellners and Reverse Kellners.

 

Reverse kellners? Do they make things look bigger?

[Barry-W-Fenner]

5 hours ago, mikeDnight said:

Don't be too hard on yourself Alan. In years to come that White original could be hung on a gallery wall, with art experts discussing the deeper underlying meaning behind every stroke. 🎨

And the conclusion of the deeper meaning will be, it was all to try and educate some plonker called Barry 😂

[Ags]

6 hours ago, sputniksteve said:

Reverse kellners? Do they make things look bigger?

Once upon a time I was on a German retailer's site and the browser's "translate page" function was switched on. I couldn't understand why the site was selling Reverse Waiter eyepieces!

[jetstream]

16 hours ago, Alan White said:

You also get a Pseudo Masuyama like the ones Mark talks about above, but these have 5 elements and are not a Plossl at all.

Is the Televue Plossl a plossl?

Some say it is a simple symmetric...

From Lord:

"I have also provided a historical perspective to the designs of the König, Brandon and TV Plössl, and reasons why, despite TeleVue’s advertising claims, the TV Plössl is only a Symmetrical, and consequently incapable of a noticeably superior performance. I have been made aware of the reputation of Plössl eyepieces in general and the TV Plössl in particular. My findings leave no doubt that the reputation is unwarranted. When it comes to the detection of low contrast detail and preservation of low contrast details in planetary images (Jupiter in this instance - a difficult object owing to its low surface contrast) there is nothing about the TV Plössl design to commend its preferred status to either the Zeiss Abbe Orthoscopic or Zeiss König I, or the Brandon Orthoscopic."

 

https://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/BRAYOBS PUBLICATIONS.html

Edited July 6, 2020 by jetstream

[John]

This is from the Tele Vue plossl patent documentation:

Slight curvature of the outer facing lens surfaces ?

Chris Lord !

[jetstream]

5 minutes ago, John said:

This is from the Tele Vue plossl patent documentation:

Slight curvature of the outer facing lens surfaces ?

Chris Lord !

Does this design show its a pair of symmetric doublets?

[John]

7 minutes ago, jetstream said:

Does this design show its a pair of symmetric doublets?

What else did you think it would be Gerry ?

Most plossls are like that. The difference with the TV is that slight inward curvature on the outward facing lens surfaces. I think that was enough at that time to get a patent.

I read somewhere that Vixen use a similar figure for their NPL. Come to think of it, Vixen used a similar symmetrical design and called it an "orthoscopic" in the past.

As we know it's not just the optical design but the quality of the glass used, the figuring, polishing, coating, element mounting, baffling etc, etc etc that makes the difference.

 

[jetstream]

Well, maybe asymmetric doublet pairs? It seems as if it might be a symmetric achromatic pair with different outer lens curves.

 

Edited July 6, 2020 by jetstream

[jetstream]

Anyway, the spot diagrams show why I like orthos. The TV "plossls" are well executed but still limited by their design, like all others.

[Louis D]

28 minutes ago, jetstream said:

Anyway, the spot diagrams show why I like orthos. The TV "plossls" are well executed but still limited by their design, like all others.

Although they don't look too good even at f/6 at the edge in my experience, I've found.  By f/12, they clean up nicely.  I can't imagine what they look like at f/4.  I'll stick with my modern widefields for my non-tracking scopes.

[Highburymark]

For ha solar, they are unquestionably the finest plossl. Substantially brighter and more detailed than Synta/GSO Plossls. Very similar views to Fujiyama orthos (on solar), but again I marginally prefer TV Plossls. I’ve never actually used any of my TV Plossls for normal night viewing

[John]

I've owned lots of Tele Vue plossls (both the earlier ones and the current range) and found them very good apart from the eye relief which is rather short with the 11mm and 8mm given their inflexible rubber eye cups and a bit too long in the case of the 32mm which needs (for me at least) the eye guard extender to make it comfortable to use. Most of my use of these was at night.

 

[NGC 1502]

My only TeleVue Plossl is the 32mm. Once I got used to it I found that eye placement wasn’t a problem. But I know it’s a problem for some and the TV extender should sort that. However for a premium price it shouldn’t be necessary to purchase an extra, it really should have a way of adjusting the eyecup - just as other long eye relief models from them - Nagler T4, Radian, Delos, DeLite.....    
 

For me TV stuff is great and I’m a fan, but I think they overthought how to do an adjustable eyeguard, a well engineered screw type would have been simpler and better.
 

Ed.

 

[markse68]

9 hours ago, John said:

Come to think of it, Vixen used a similar symmetrical design and called it an "orthoscopic" in the past.

apparently the original Plossl patent described it as an orthoscopic. I wonder if you can get less rectilinear distortion by limiting the field? The few i’ve looked at for it have a lot of rectilinear distortion. But then so do some of my Abbé orthos🤦‍♂️😂 But rectilinear distortion isn’t so important for astro I think- maybe more so for microscope eps

[John]

Orthoscopic is a set of characteristics rather than an optical design I believe. Maybe thats where Vixen were coming from ?

TMB called their Paragon  superwide eyepieces "Orthoscopic". Today I understand that the same optical layout is found in the Aero ED range and clones.

[Don Pensack]

10 hours ago, jetstream said:

Is the Televue Plossl a plossl?

Some say it is a simple symmetric...

From Lord:

"I have also provided a historical perspective to the designs of the König, Brandon and TV Plössl, and reasons why, despite TeleVue’s advertising claims, the TV Plössl is only a Symmetrical, and consequently incapable of a noticeably superior performance. I have been made aware of the reputation of Plössl eyepieces in general and the TV Plössl in particular. My findings leave no doubt that the reputation is unwarranted. When it comes to the detection of low contrast detail and preservation of low contrast details in planetary images (Jupiter in this instance - a difficult object owing to its low surface contrast) there is nothing about the TV Plössl design to commend its preferred status to either the Zeiss Abbe Orthoscopic or Zeiss König I, or the Brandon Orthoscopic."

 

https://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/BRAYOBS PUBLICATIONS.html

Let's unwrap that remark.

1) no commercial 4-element eyepiece with 2 doublets you have used in your life is of the 1860 original configuration of Georg Simon Plössl, so no one would know exactly how the eyepiece performed.  Ray traces show it is pretty good in the center 20° of field and very sharp on axis, but we simply cannot know whether it is "better" than any of the modern varieties.

2) the revised version improved by Albert König in 1938 was the influence behind the reversed version of Chester Brandon (originally called "Brandon orthoscopics", and used in Questars) and the Clavé "Plössl" of the '70s, though the designers did improve on König's design.  Both Clavé and Brandon paid little attention to correcting astigmatism in the outer field because most, if not all, scopes of the era were long focal ratios.  An f/8 was considered normal for a 16" reflector! Fast scopes for f/5-f/6 soon revealed the poor edge correction in those designs.  Those versions did all have non-symmetrical configurations, those glass types varied.

3) By the early '60s, the term Plössl in optics manuals and design handbooks had become synonymous with a 4 element symmetrical design, generally with outward-facing convex surfaces.  Performance was good, but they suffered from lateral astigmatism in faster scopes.  This eyepiece was sometimes called Symmetrical, but the descriptor "Plössl" had become a relatively generic term.  Not in the public, however, where terms such as "Kellner Type III" or "Achromatic Kellner" were often applied.  The term "Plössl" was really made popular by Clavé and TeleVue.  Today, the term is totally generic, and several different internal configurations are sold as Plössls, which has come to mean: 4 element eyepiece made from 2 doublets.  We can complain about it, but that is the way of the world, and we have to accept that that is the case.  I also suspect, as an aside, that many of them work better than Plössl's 1860 design in the modern scopes where f/8 is considered long.

4) Nagler designed his 4-element Plössl to have concave outer surfaces for greater control of astigmatism, though this was not the first version he sold.  He did get a patent on the concave design and it went into production.  The goal was to raise money so he could introduce a 90° eyepiece he had designed, though optical correction characteristics eventually reduced that to 82° (though the production actually reached 84°).  He called the 4 element eyepiece "Plössl" because that had become, more or less, what 4 element eyepieces constructed with doublet pairs had become described as.  It also enabled a little wider corrected field at the sacrifice of a bit of edge illumination.  So, a different level of performance than the eyepieces just called "symmetrical".

5) The original TeleVue Plössls had simpler coatings than the current generation, so along the way, they were improved.

6) König was granted 38 different patents for eyepiece designs so describing an eyepiece as a "König" is about as descriptive as saying "car".  There are a lot of different eyepieces that could be called Königs.  Many are really good on axis, but few of the designs work even modestly well in today's f/4-f/6 scopes.

So, if conforming to Plössl's 1860 design is what is required to be called "Plössl", then there isn't one in existence today, and you probably have never ever seen one.  But it's like calling cotton swabs "Q-Tips", or facial tissue "Kleenex".  A particular product became a generic name.  This statement, "TV Plössl is only a Symmetrical, and consequently incapable of a noticeably superior performance"  is just a statement without any factual basis, and exhibits a bias not justified by the design.

 

 

 

[jetstream]

1 hour ago, John said:

Orthoscopic is a set of characteristics rather than an optical design I believe. Maybe thats where Vixen were coming from ?

Agreed, my 12.5mm Docter UWA is an orthoscopic eyepiece.

[Don Pensack]

17 hours ago, jetstream said:

Agreed, my 12.5mm Docter UWA is an orthoscopic eyepiece.

Sorry, completely impossible.

Orthoscopy implies a lack of distortion.  The curves for rectilinear distortion and angular magnification distortion deviate above about a 30-40° field.

Distortion goes up with apparent field, so there is no such thing as an ultrawide orthoscopic eyepiece.  You can reduce rectilinear distortion to a minimum, leaving in a large amount of angular magnification distortion, as is done in the Noblex/Docter eyepiece, or you can reduce angular magnification distortion to a minimum, leaving in a lot of rectilinear distortion, as done in, say, the TeleVue Ethos.

But you cannot simultaneously reduce both to a minimum in optics.

This shows the curves for each form of distortion with apparent field on one axis:

 

 

Edited July 7, 2020 by Don Pensack

[jetstream]

8 hours ago, Don Pensack said:

Sorry, completely impossible.

Orthoscopy implies a lack of distortion.  The curves for rectilinear distortion and angular magnification distortion deviate above about a 30-40° field.

Distortion goes up with apparent field, so there is no such thing as an ultrawide orthoscopic eyepiece.  You can reduce rectilinear distortion to a minimum, leaving in a large amount of angular magnification distortion, as is done in the Noblex/Docter eyepiece, or you can reduce angular magnification distortion to a minimum, leaving in a lot of rectilinear distortion, as done in, say, the TeleVue Ethos.

But you cannot simultaneously reduce both to a minimum in optics.

This shows the curves for each form of distortion with apparent field on one axis:

 

 

The Docter shows less distortion than some of my Abbe orthos....and much less than most TV's- at least to my eyes and the TV's I've tried. It might be my eyes and the type of distortion.

Edited July 8, 2020 by jetstream

[John]

Another thread which amply illustrates how the "your mileage may vary" adage applies to eyepieces 

Perhaps we need an eye assessment service to advise which eyepieces are likely to please before purchase. Rather like being fitted for a suit !

 

[Alan White]

44 minutes ago, John said:

Another thread which amply illustrates how the "your mileage may vary" adage applies to eyepieces 

Perhaps we need an eye assessment service to advise which eyepieces are likely to please before purchase. Rather like being fitted for a suit !

 

I actually think that is a good call John
I know my eyes have changed in the last 5 years and it does now dictate EP designs I can sensibly use.

[Ags]

54 minutes ago, John said:

Another thread which amply illustrates how the "your mileage may vary" adage applies to eyepieces

The solution is simple - buy lots of eyepieces and keep the ones that work for you!