Telescope Reviews: Origins of Ploessl Eyepiece

Hi, Don.

My paper attempts to trace the developments back to Ploessl himself, using all original documents. The earliest are all in German, a few in French, and the most recent in English. I have translated the German and French into English where needed. It is for readers of the article to decide whether it is convincing.

GS Ploessl himself certainly designed no eyepieces in the modern sense. He was a fabricating optician, mainly focused on building microscopes. Rather, he replaced the plano-convex singlets of the Wilson-type eye-loupe with cemented plano-convex achromats. That is, he built in essence achromatized Ramsden eyepieces. But they still had a large air-gap, as far as I can tell. A few of these eyepieces exist in Europe, but I have not had an opportunity to examine any. My information is derived from 19th century descriptions/illustrations as indicated in the paper. They were low-power microscope eyepieces.

It was Albert Koenig, apparently, the great Zeiss designer who around 1915 in seeking for a better eyepiece for military sighting instruments, removed the unneeded airgap and began experimenting with use of high-index of refraction glasses and "bending" of the lens doublets to achieve higher performance. Similar developments happened at Goerz and elsewhere.

Many eyepieces of similar performance (and various usable field sizes) were developed between about 1890, starting with the Abbe Ortho, leading through WWI, with Erfle's various designs, and on to WWII. Koenig's own designs as shown by his American & German patents, and German publications (cited in the paper) centered on binocular or gunnery eyepieces of large eyerelief (as understood then). The most advanced 2-2 eyepiece of Koenig was patented in the years around 1940 in Germany and the US. Koenig began calling the design "the orthoscopic according to Ploessl" as a way to distinguish it from "the orthoscopic according to Abbe." All this is discussed and documented from orignal sources in the paper.

Many many eyepieces of this general type (2-2) were developed and used in military equipment such as tank telescopes, dial-sights, binoculars, etc. in WWII. They can be seen clearly in WWII documents in my collections. The most advanced, such as one devised by Chester Brandon, where asymmetrical 2-2 forms similar to Koenig's most advanced type. This is the type of the Brandon still today, I'm told. Other forms were simpler, symmetrical 2-2 types.

It was after the war that the "Ploessl" as we know it for amateur astronomers took off, through the work of Brandon, Jean Texereau/Clave, and finally Al Nagler. But the details and documentation are best seen in the article itself.

In the 50s and 60s in the US, since the name Ploessl was still very unfamiliar, people such as Edmund sold WWII surplus eyepieces (derived from military equipment) under such names as Kellner Type 3, etc. But they were Ploessls of various types, and some indeed were Brandons, I'm told.

I will be very glad to see Bill's book soon.

Cheers,
Roger