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Meet Simon Ploessl

Posted: Wed Dec 16, 2020 8:08 pm
by mikemarotta
(An earlier version appeared in the December 2020 issue of Sidereal Times newsletter of the Austin Astronomical Society.)

The Ploessl ocular (“eyepiece”) is easily the most popular design in the hobby. Regardless of focal length, from 40 mm to 4 mm, they are common because they are inexpensive and they reasonably support the limits of the largest amateur instruments. Better designs are available, but at a greater cost. Thus, the Ploessl is the first choice, whether for a refractor, reflector, or catadioptric telescope. For all of its ubiquity, the biography of its inventor, Viennese optician Simon Georg (or Georg Simon) Ploessl (1794-1868) is not widely known among astronomers.

First of all, he spelled his name Plößl and that almost rhymes with the English word “vessel.” The character that looks like a capital-B ß is a double-s. The umlaut-o ö is sounded by rounding your lips to say English long-o, but instead, saying English long-a. If you did not grow up speaking German, then “Plessl” is close enough. That is because three consonants follow the vowel. Each one clips some time off the sounding. For the signature on his nameplates, Ploessl also used the ligature œ (oe), a less common flourish.

If your typewriter has no umlaut vowels (ä ö ü) or a sharp-s (ß), you can use an e and a double-s. Thus, Plößl (which is how he spelled it) is accepted as Plössl or Ploessl, but spelling the name Plossl or saying it with the o from “postal” or “hostile” is wrong.

Plößl was born in Wieden, which had been an independent villa in the Middle Ages but by the 18th century already lay within Vienna’s shadow. He was the son of a cabinetmaker. And therefore an apprentice in his father’s shop. When he was 18, he left for the optical firm Voigtländer, starting on May 9, 1812. In 1823, he moved back to his father’s home and began his own laboratory and workshop for investigations into the production of optical instruments. In 1828 he was open for sales. At first, Ploessl made microscopes, and he soon became famous for them.

His company took a new direction when he sold a microscope to Joseph Franz von Jacquin, professor of botany and chemistry at the University of Vienna. Von Jaquin introduced Ploessl to the astronomer Joseph Johann von Littrow for whom he built a telescope in 1830.

By 1850, in addition to the microscopes which were the primary production of his firm Ploessl had delivered refractors to observatories in Romania, Hungary, Greece, and Russia. In 1851 the firm delivered an 11-inch f/11.8 refractor to the Vizier (some sources say Sultan) of the Ottoman Empire.
All of his telescopes had objectives of crown glass. They followed the Fraunhofer design of a concave and convex lens pair, but did not use flint glass which was rarer in large, high-quality blanks. Consequently, the essential element in the design was the secondary lens system, a flint glass ocular that minimized chromatic aberration. Two pairs of lenses match convex and concave curves, and the pairs are separated by a gap. The concave lenses are at the extremes, the convex face each other in the center. That was Ploessl’s stellar achievement.

Three factors kept this system from becoming widely accepted. First, telescopes are durable goods. Better ones have been built since 1610, but the old ones still work. The Ploessl refractor in Athens served the university until 1940. Second, astronomy was a private pursuit for intellectuals of independent means. While Britain had its Royal Astronomer, few such public posts existed elsewhere. Ploessl built telescopes for state enterprises in Greece and Russia, but he built more for wealthy patrons in Hungary, Romania, and Italy. Only with the explosion of science in the 20th century was there any broad consumer demand for the instruments of empirical discovery. Third, as prosperous as the Ploessl firm was, it was a sole proprietorship. When Simon Ploessl was killed by a falling sheet of glass in 1868, there was no one to step into leadership. The firm continued until 1905, but there was no visionary to drive the effort.

For example, consider The Pleasures of the Telescope (1901) by Garrett P. Serviss. Serviss was famous as a popularizer of science, especially astronomy. When he was a night editor at The Sun of New York, one of his lecture tours was subsidized by Andrew Carnegie. His passing was noted in a long obituary in Popular Astronomy (August-September 1929). The Pleasures of the Telescope was first published by Putnam in 1901, and then went through several re-publications. Google Books and Hathi Trust provide the 1915 printing by D. Appleton and Company. Serviss described two kinds of eyepieces: positive (Ramsden) and negative (Huygens). He does not mention the Ploessl.

Re: Meet Simon Ploessl

Posted: Wed Dec 16, 2020 9:41 pm
by jeff.stevens
A very interesting read, Mike. Thanks for posting. I’d not read about Ploessl before, despite having used eyepieces of that design. The older I get, the more I realise I still have so much to learn.

Best wishes, Jeff.

Re: Meet Simon Ploessl

Posted: Wed Dec 16, 2020 9:58 pm
by brian livesey
Military gunsights use/used Plossls. I had a big brass one that was war surplus. Both the field and eye lens were about 30mm in diameter. This eyepiece coupled to a 8-inch reflector gave lovely views of the Perseus Double Cluster.

Re: Meet Simon Ploessl

Posted: Sat Dec 19, 2020 9:31 pm
by David Frydman
Although Plossls are now plentiful and have been used in military optics regularly, they were not so common in the 1950s and 1960s as astro eyepieces.

Orthoscopics and Kelners were the normal better eyepieces available.

I still use the orthos and Kelners more than Plossls.
As well as Erfles, which were the main wide angle eyepieces available to me with my first scopes.


Re: Meet Simon Ploessl

Posted: Sat Dec 19, 2020 10:20 pm
by brian livesey
It's good to see you back David, you are quite knowledgable on optics. Isn't the Plossl also referred to as the Symmetrical?