The Lake County Astronomical Society

New Versions of Old Eyepieces

Jack Kramer

Is anything really new on the eyepiece front these days? Sure - Tele Vue has the 100^o FOV Ethos series, which has garnered rave reviews from those who have tried them. And now both TMB and Explore Scientific have launched their own versions of 100^o FOV eyepieces, undercutting Tele Vue's prices. (Early reviews say the TMB and ES eyepieces are good but not comparable to Tele Vue's.) No doubt you can name a whole bunch of other new designs that provide wider fields of view and good edge-to-edge definition. But some of the more current eyepieces are really re-workings of old designs.

In an Internet posting, Chris Lord, a fellow of the Royal Astronomical Society and recognized expert on optics, explained about some revamped ocular designs. To a certain extent, this was made possible in the late 1970's by computer aided design to optimize existing eyepieces. Although the theory of optical design was well known, eyepieces heretofore were basically a matter of trial-and-error. The Edmund Scientific RKE (Reversed Kellner Eyepiece) was introduced in the late 70s and was touted as the first optimized eyepiece, although it's not known how much of it is attributable to computer aided design. It's actually a 2-1 modified Kellner very similar to the 1915 design introduced by Konig. Al Nagler is the best known innovator for having applied CAD to improve his designs introduced in about 1980, although he wasn't the first to do so. The original Nagler eyepiece was a 2-1-2-2 design using a Smyth achromatic flattener to widen the apparent field from 52^o to 82^o, but at the expense of rectilinear distortion. The original design widened the beam but made for a bulky eyepiece in anything over 13mm, and it also suffered from spherical aberration of the exit pupil - referred-to as the Kidney Bean Effect.

It was actually camera, microscope, cinematography, and industrial optical systems that drove optimization. The first computer auto-optimized eyepiece probably was the Fleischman - a 1-1-2-1 configuration using a negative field flattener. It was patented in 1977 for Bell & Howell, but there's no record as to whether it was ever actually manufactured. In 1978 a patent was registered for a 1-2-1 Orthoscopic eyepiece variation on behalf of Kogaku KK, Japan - the first commercial telescope eyepiece known to use auto-optimization. Its relatives are available as the University Optics Orthoscopic series, which is highly regarded for its image quality, but still suffers from short eye relief and comparatively narrow field of view.

The Erfle eyepiece originally goes back to a 1917 design made for the military, based on an earlier work by Zeiss in modifying the Plossl design for wider field. Numerous variants have been made since; some having fields of 70^o or greater even used glass impregnated with thorium and uranium. The reasonably priced and highly regarded Orion Ultrascopic eyepieces are also variants of the old Plossl design with an added field-flattening element. Other improvements include new types of glass that give better light transmission and less scattering, plus finer anti-reflective lens coatings.

With the current wide use of computer aided design, we've seen a steady flow of new eyepiece offerings, some of which may just be improvements on older designs. That can be a good thing, especially for amateur astronomers who want to upgrade their eyepiece collection while watching their purse strings.