What does Plossl mean ?

[DobDobDob (Ron)]

Can anyone tell me whether Plossl is just a name or whether it has an actual technical background. Everyone's speaks of Plossl, it seems that everyone makes them, I even own a whole set of them made by Meade, but a Google did not reveal what a Plossl actual refers to. Surely there must be some official description which makes them differ from non-Plossl ?

[GeoffW1 (Geoff)]

Hi,

From Wikipedia (permissions granted)

Georg Simon Plössl (1794-1868) was an Austrian optical instrument maker. He trained initially with the Voightländer company, but later set up his own workshop in 1823. He improved the quality of achromatic microscope objective lenses, but is best known for the eponymous telescope eyepieces made following his 1860 design, and extensively used by amateur astronomers (see Eyepiece)

Cheers

GeoffW

[DobDobDob (Ron)]

Thanks very much Phil, can I ask you to consider entering this on the IIS glossary, it would make a nice entry I believe.

[ving (David)]

Georg Simon Plössl .... gah! damn you geoff! you are too quick!

[DobDobDob (Ron)]

Thanks also Geoffw and Ving

[ausastronomer (John Bambury)]

Ron,

Many of the eyepiece designs currently in use are named after their designer. Some stem from as early as the mid 1600's, some are quite recent. Until the early 1980's most telescopes were small with very slow focal ratios. Consequently, most of the old eyepiece designs worked quite well in them. In the late 1970's, early 1980's, the trend in amateur astronomy shifted to larger aperture telescopes with much faster focal ratios. This allowed big telescopes to be portable. Unfortunately the fast steep light cone created by these scopes is very unforgiving on simple early eyepiece designs, with the possible exception of the Plössl and the Orthoscopic, which do very well as fast as F4 or thereabouts.

Here is a bit of eyepiece history:-

Ramsden, Huygens and Kellner are the oldest designs. Their designers were Christian Huygens, Jesse Ramsden and Carl Kellner. The Huygens dates back to the mod 1600's. These designs are rarely used thesedays, as there are better alternatives. Kellners can make very good lunar/planetary viewing eyepieces, in a suitable telescope (slow achromatic refractors eg 4"/F12 etc, also small Maksutovs), but their eye relief is very tight.

In 1860 came the Plössl design invented by Georg Simon Plössl. This design took eyepiece performance to a new level although it's advantages and improved performance were not recognised until many years after it was designed. The Plössl is still in wide use today. It is one of the best designs and provides very sharp high contrast images on axis, in scopes as fast as F4. Since the original design their have been many "modified" Plössl designs produced. All work very well. The Brandon is a very good one. As are all the 5 Element Plössl variations. These 5 element Plössls are also known as the Masuyama design. Some of these currently produced include the Celestron Ultimas, Antares Elite and the Parks Gold series. The Takahashi LE series use a slightly different 5 element design and are outstanding performers. The drawbacks of all Plössl variants are an AFOV limited to between 50 deg and 55 deg, and reducing eye relief as the focal length of the eyepiece reduces.

In 1880 came the Orthoscopic invented by Ernst Abbé, who was an employee of Carl Zeiss. "Orthoscopic" means free of optical distortion. The Orthoscopic eyepiece offers very similar performance levels to the Plössl and like the Plössl it is one of the best designs and provides very sharp high contrast images on axis, in scopes as fast as F4. It's drawbacks are an AFOV limited to between 45 deg and 50 deg, and reducing eye relief as the focal length of the eyepiece reduces. However, the eye relief in an Orthoscopic for a given focal length is slightly better than it is in a Plössl. Orthoscopics are still my favourite lunar/planetary eyepieces. The University Optics HD orthos are an excellent eyepiece for not a lot of cost. The Zeiss orthoscopics which went out of production about 5 years ago are superb, they offer unsurpassed levels of lunar and planetary performance. But alas they are very expensive (about $US3,000 for a set of 5) and rarely offered for sale on the 2nd hand market. They are almost unprocurable.

World War I came along and this highlighted a need for "widefield" eyepieces in both telescopes and military style binoculars. From 1915 to 1940 many widefield eyepieces were developed. All were designed to work in the slow optical systems of the period and don't do so well in todays very fast F-ratio systems.

In 1923 Albert König, an employee of Carl Zeiss developed the "original" König I design which is a variant of the orthoscopic design. There were several subsequent modifications to the König. These are still manufactured today and work very well in medium/slow telescope systems. Their downside is tight eye relief as the focal length reduces and soft edge performance in faster telescope systems.

In 1925 Heinrich Erfle, another employee of Carl Zeiss, designed the Erfle I. There were also several subsequent modifications to this design. Like the Konig they are still manufactured today and work well in medium/slow telescope systems. The Erfle is probaly the most widely modified and "butchered" eyepiece design of all. There have been many variants produced, which closely resemble the original designs. The current 80 deg "Widescan" series are based on a modified Erfle design. Like the König, their downside is tight eye relief as the focal length reduces and soft edge performance in faster telescope systems.

Between the late 1920's and 1975 not a lot happened. There was some modifications to existing designs and a few new designs, none of which are currently in production. These designs include the Galoc, Bertele and the Zeiss Astroplanar.

Note that just about everything to this point has originated in Europe.

In 1975 Dr David Rank, an employee of Edmund Scientific, modified the Kellner design and came up with the RKE. RKE stands for Rank Kellner Eyepiece. It has also been termed by amateur astronomers as a "reversed kellner", which is exactly what it is. These are excellent planetary eyepieces but like their parent design,have some faults. They do not work well in fast F-ratio systems and eye relief is short.

In 1979 Al Nagler the owner and founder of Televue in the USA, designed the original Nagler. This was an Ultra Wide field (82 deg) design that offered exceptional performance in telescope systems as fast as F4, eye relief was quite good compared to existing designs. Preceeding the Nagler was the Televue Widefield and then the Televue Panoptic. The TV widefields were replaced by the Panoptics which are still manufactured by Televue to this day. These 3 eyepiece designs from Al Nagler changed the whole expectation of eyepiece performance of amateur astronomers. Since the original Nagler there have been 4 new variations T2,T4,T5 and T6. The T4's,T5's and T6's are in current production. The Nagler really raised the bar in terms of performance in fast telescope systems and FOV. Several companies have subsequently produced similar performing eyepieces with good success, albeit with a slightly narrower FOV, trading the FOV off against aberrations and eye-relief. These include Vixen, Pentax and Denkmeier. While Meade have produced some widefield designs they didn't do anything new IMO.

I hope this explains a little bit of eyepiece history for you.

CS-John B

[DobDobDob (Ron)]

WOW, what can I say?

Firstly thank you very much.
Next, this is amazing stuff.

Did you actually write this? It's an amazing summary of eyepiece history.

Why isn't this a faq or sticky or similar? If you did write this, would you mind letting me copy and reproduce it on Barry's ASIGN website, Barry gives me a fair scope of latitude to work on his site for him and I suspect he would love something on the history of eyepieces there.

I hope I am not too presumptuous, naturally if I/we used this, we would give fully accreditation to the author/s

Regardless of reproduction, thanks again for taking the time to provide it for me (and other forum members) to read

[ausastronomer (John Bambury)]

Hi Ron,

I am glad you enjoyed reading it. It was a lot of typing for a slow typist. I wrote it off the top of my head, from information read and gleaned over many years. I was bored at work today and your thread inspired me to put it all down. I felt from the tone of your thread, that other people might be interested in reading it and maybe learn something from it. I did have to look up the exact years that each eyepiece design was first proposed, as my memory isn't quite that good.

I am happy for it to be reproduced, in whatever form anyone sees fit, with appropriate acknowledgement.

Clear Skies
John Bambury

[janoskiss (Steve H)]

yes, that was a good read John. Thanks

[Cec]

Thanks Ron
For us new comers this is fantastic, please continue the EP story
Cec

[DobDobDob (Ron)]

Originally Posted by ausastronomer

I am happy for it to be reproduced, in whatever form anyone sees fit, with appropriate acknowledgement.

Clear Skies
John Bambury

Hey John, it lives: http://www.aussiepeople.com.au/ASIGN...yspeaking.aspx

when I get a little more time, I will make links to existing manufacturers websites for you also.

Thanks again, Barry was more than happy to have this on his site and once again, please let me say how good it was of you to write it and also let me reproduce it.

Sincerely yours

[Ric]

Hi John, that was a fascinating article. I enjoyed reading it as well.
Thanks for taking the time to put it down on paper as they say.

Cheers

[ausastronomer (John Bambury)]

Glad you guys enjoyed reading it.

Another interesting tidbit which I briefly alluded to and Geoff W also gave a little insight into, relates to the Plössl eyepiece.

Geoff mentioned that Plössl was an Austrian optical instrument maker who worked on designing microscope objective lenses. I mentioned that it was many years after the Plössl eyepiece was designed that it gained any form of acceptance by the astronomers of the day.

Plössl in fact designed the eyepiece as a microscope eyepiece and not a telescope eyepiece. It was many years after its invention and use in microscopy before someone bothered to "stick it in a telescope". Modern optical knowledge now tells us that what works well in a microscope also works well in a telescope. The same applies to the Abbe orthoscopics designed in 1880. They were designed for use in microscopes and found subsequent acceptance amongst the astronomical community. Of course things have moved on from there with some of the modern widefield telescope eyepieces being the size of a beer can and unsuitable for microscope use. Similarly, some of the better eyepieces currently suited to astronomy, were initially designed for use in spotting scopes. These include a lot of the Pentax, Nikon, Zeiss and Leitz eyepieces. Unfortunately, only the Pentax are readily available in Australia today, primarily for two reasons, Cost and "Nagler Fever".

CS-John B

[Kal (Andrew)]

This page also has some useful information and some nice simple pictures explaining several popular eyepiece designs.

[DobDobDob (Ron)]

Yes a very technical page

John and I have been working behind the scenes and working on a plan to rework the story, remembering that it is a Historical work as opposed to a technical reference. Early next week we will have something which will be placed on the website and I will make an announcement on this forum when the work is finished.

[OCULUS]

not back in production