Copyright – P. J. Smith
An EMERGENCY eyepiece design.
This design is derived from the Abbe or Orthoscopic eyepiece which first appeared over 100 years ago.
The efl = 15 mm.
With modern glasses this configuration produces very impressive performance.
In this case the design is heavily compromised by :-
(a) Not using proper optical glass. Instead 'window glass' and polycarbonate plastic (Lexan) is used.
(b) Using only one radius of curvature plus plane surfaces. This is very suited to the use of window glass which already has a plane surface.
The apparent field of only 40 degrees may seem limited. Remember that many older eyepieces of similar field are much less corrected, especially at F:8 or F:6..
The design has been skewed to very easy construction and readily available materials. Centring must, however, be well done.
If optical glass is readily available this design should be passed by.
While various plastics can replace 'flint' types of glass, polycarbonate is now available widely for very strong safety windows.
The triplet section is not the easiest for an amateur to make well. Triplet configuration was deliberately chosen to place the easily damaged plastic lens between more hardy glass elements. It is not easy to obtain a perfect polish on the polybarbonate but the process of optically bonding glass to polycarbonate minimises this problem.
A few pointers on how to shape and handle polycarbonate is to be found on another page.
The boxes surrounding spot diagrams are 50 microns across. While the central definition is excellent at the designed F:8, some edge astigmatism and field curvature is noticeable. Observers with plenty of accommodation should not find the field curvature objectionable.
Dotted line represents 1 minute performance limitations.
Radius of curve-------Thickness------------ Glass------------ Semi-width
efl = 15 mm.
Window glass has optical properties very close to K5. Unfortunately it will vary slightly depending on the source but slight deviations are not too objectionable.
Homogeneity is probably more important than specific optical properties. Any window glass should be inspected for flaws. Colour, homogeneity, and accuracy of any plane surfaces utilised in the final product are very important. Remember, if you can obtain optical glass this design makes no sense.