[Claudiu]

Am gasit un articol care dezbate variatiunile generice alte ocularelor Plossl, cu ceva indicatii legate de producatori, si in final o explicatie a denumirii "Super Plossl"..

Revista <NightTimes> decembrie 2003


Many of you have a Plossl in your collection of eyepieces; it’s one of the most common types in use by amateur astronomers. In fact, some of you may have a Plossl and not even know it!



For some reason, the Plossl ocular disappeared for almost a century before it was finally revived in the 1960’s. This has led some to a mistaken notion that it’s a relatively new design. Georg Simon Plössl, who gave his name to the eyepiece, was born in 1794 near Vienna. He worked for the Voigtlaender optical firm, but in 1823 founded his own workshop. His main interest was improving the quality of microscope objectives and eyepieces. His microscopes were similar in design to those made in Germany by Kellner (also of eyepiece fame). In addition, Plössl made special optics – he offered diamond and sapphire lenses for use in microscopes. But he also made telescopes of a so-called “dialytic” type. His largest scope was a 10&frac12;” refractor with a focal length of 11&frac12; feet, and he made several other instruments of various diameters. Pictured here is one of Plössl’s telescopes – a 2&frac14;-inch refractor of about 800mm focal length.

The pure Plossl eyepiece design has four elements arranged as two sets of paired (achromatic) lenses. This is a variation of the Kellner, which has one achromat plus a simple lens (the achromat being the eye lens in the Kellner type I). The Plossl was also called a Kellner Type III in some earlier references; in the 1950’s, Edmund Scientific sold a “Kellner” eyepiece made from war surplus lenses, but it actually consisted of two achromats in the classic Plossl configuration. The Plossl has good color correction plus is free of the ghost images that plague the Kellner. It also has a wider field of view, but in order to attain this, some pincushion distortion is allowed at the edge of the field. Thus a Plossl is not strictly “orthoscopic” in the generic sense. Premium Plossls, such as those by Tele Vue and ClavJ, provide images with less distortion than other brands through the use of better optical glasses. The Tele Vue Plossl also uses concave external lenses, rather than the usual convex elements. According to Tele Vue, “Such a lens configuration provides a significant improvement in the correction of astigmatism and coma at the edge of the field. This results in a sharper image for large field angles with a relatively small undercorrected field curvature.”
To some extent, all eyepieces could offer a wider field of view, but it would be at the cost of an annoying amount of distortion at the edge of the field. So a field stop, or diaphragm, is incorporated into the eyepiece barrel. This limits the size of the field to an area in the center with the least amount of distortion. Another way to deal with distortion is to add an additional lens element that acts as a “field flattener”. Current variations of the Plossl design with a fifth element are known by other names, such as Celestron’s “Ultima” and Orion’s “Ultrascopic”. The five-element design variant is technically designated as a “Masuyama”. When Meade first introduced the “Super Plossl”, they too incorporated a fifth element, but they have since reverted to the classic four element design. (Nonetheless, Meade still refers to these as “Super” Plossls.) Regardless of what these eyepieces may be called by their distributors, they all have an apparent field of view around 50o.

To attain the shorter effective focal lengths (higher magnification), a few include an additional field lens that acts as a built-in Barlow. This is the route taken by the Orion Ultrascopic. So there are a lot of eyepieces in different guises and under a variety of names which essentially use the basic Plossl design. I suppose one could argue that the addition of lenses means it’s no longer a “Plossl” in the strict sense.

The two original Plössl eyepieces shown here date from the late 1830’s, and were included with the 2&frac14; inch telescope. Typically, the barrels are made of lacquered brass.

[img]http://www.bpccs.com/lcas/ Articles/plossl3.jpg[/img]