Plossl

Plössl, also spelled Plossl or Ploessl, is a four-element astronomical eyepiece design widely used in amateur telescopes. Named after the Austrian optician Paul Plössl, who introduced the configuration in the mid-20th century, the design consists of four lenses arranged as two doublets in a symmetrical layout. The arrangement aims to combine simple construction with good optical performance, delivering sharp images across a sizeable field.

Optically, the Plössl is known for a relatively comfortable balance of sharpness and practicality. It typically provides about 50 degrees of apparent field of view and good edge performance for a four-element design. The design minimizes common aberrations for its class, making it a versatile choice for both planetary and deep-sky observing. Eye relief is moderate but tends to be shorter for shorter focal-length models, which can make viewing more difficult for observers who wear glasses; longer focal-length versions offer more comfortable eye relief.

Usage and variants: the Plössl remains a standard, widely produced eyepiece and is considered parfocal with many other Plössl focal lengths. Numerous manufacturers offer focal lengths ranging from a few millimeters to several tens of millimeters. Variants such as the Super Plössl feature improved coatings and construction, sometimes with a slightly wider apparent field, while retaining the general four-element, twin-doublet design. Compared with modern ultra-wide designs, the Plössl trades a narrower field for simplicity, compactness, and lower cost.

See also: eyepiece, orthoscopic, Kellner, Nagler, Ethos.