Copyright P. J. Smith

But permission is given to distribute this material in unaltered form as long as it is not sold for profit.

 This design is for a Konig plus Barlow design as a system which uses modern optical glasses. Neither the 'Barlow' nor the 'Konig' section is intended to be used separately.

The efl = 8 mm.

Eyerelief is a very generous 18 mm which is huge for an 8 mm eyepiece. Spectacle users should find the finished eyepiece very comfortable to use at the total apparent field of view of about 55 degrees. While this is hardly a very wide field of view, spectacle users will be able to use the entire field which is reasonably well corrected far from the central area. The central is not quite corrected to the 1 min criterion but the limiting factor is the diffraction pattern of the entire system at F:8 and F:16.

Very short efl. eyepieces are often used in 'overmagnified' situations where the 1 min criterion is less relevant. This is a personal choice.

 The design can be compared to a Nagler in that a negative 'Barlow' section is permanently attached. It is about the simplest possible configuration for this type of design with a few less elements than a Nagler and considerably less apparent field of view. Image quality is not as good as a Nagler but will be sufficient in most situations. The focal surface is buried between the lenses as is usual for this type of eyepiece. Resolution is only fair at F:4 but becomes good from about F:8 and above.

Four radii have been held common. This compromises the performance slightly but results in much simplified construction. No curves are extremely steep so more than one can be blocked together. Many are in fact quite slack.

Spherical aberration of the exit pupil is almost nonexistent as would probably be expected for a very short efl. moderate field of view eyepiece. Even in very bright sunlight no kidney bean effect should be apparent.

Although the length of the unit is substantial, the diameter is moderate.

The whole rationale of this design is a very short, cheap, easy to make, comfortable to use eyepiece with acceptable but not outstanding performance.

Before condemning the design it would be instructive to examine specifications for similar commercial competitors. I suspect a rigorous analysis of some might dismay their owners.

Spot diagram box sides are 20 microns.

Defocus because of curvature of field amounts to about 1 dioptre although the eye will not notice much degradation at 26 degrees off centre. Older users should not be unduly troubled by this amount of defocus.


Dotted line represents 1 minute performance limitations.


Radius of curve-------Thickness------------ Glass------------ Semi-width

efl = 8 mm.

Four different lap sizes are required.

Two of the glass types are more expensive but are not uncommon. These glasses will suffer from climatic degradation unless properly cared for. This is normal for most high performance eyepieces.

Centring, especially for the steeper curves, must be done well to take full advantage of the design.

Coating would be worthwhile but reflection losses are no more than in older uncoated Erfles.