PAGE 7
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 a very simple Konig eyepiece of extremely short focal length which uses two modern high index optical glasses. Only two different radii are used. One convex and two concave laps are needed. These will probably will be used in pairs for easier truing.
The Concave glass surfaces can be ground directly on precision steel balls obtainable from bearing suppliers. Small concave laps have been successfully made by pressing precision steel balls into a material such as copper.
An eyepiece of such small efl. will require very careful construction to obtain the best theoretical performance.
Although the apparent field of 34 degrees is very modest, for planetary use this is really no disadvantage.
Unfortunately, eyerelief is very modest although probably no worse than most eyepieces of such a short efl. Spectacle users will not be overly impressed but the moderate field is honestly marched to the short eyerelief and represents about the best visible field for spectacle users using such a short efl. eyepiece. The only way to really overcome this limitation is to march a longer focus eyepiece with a Barlow lens as is done in the design on page6.
The design uses a minimal quantity of optical glass and would be an interesting exercise and a challenge for anyone with no precision lap making facilities. Precision testing such short radius surfaces is difficult and a challenge.
The efl = approximately 5 mm.
Spot diagram box sides are 10 microns.
Curvature of field is very small. Please note that the curvature above is highly magnified by the scale used.
Defocus because of curvature of field amounts to about 1 dioptre which can be accommodated by most aged eyes. Young eyes will have no trouble.
Dotted line represents 1 minute
performance limitations.
The usual criterion of 1 min of arc at the centre is only just met in the centre of the field, but the magnification provided by such a short efl. eyepiece means the real optical limitation is the diffraction pattern size determined by the F:NO of the entire system.
Another way to interpret this is to say that, especially for an F:16 and even an F:8 system, the image may be overmagnified. This is a very personal choice the observer must make.
I would consider using a longer efl. eyepiece at F:16 and even F:8. My preference is to use the full resolution of the eye with lower power eyepieces which have inherently longer eyerelief.
Specifications.
Radius of curve-------Thickness------------
Glass------------ Semi-width
This design uses imperial steel
balls of 3/4 and 7/16 inch diameter.
efl = 5 mm.
The following design is equivalent
and uses metric steel balls of 19mm and 11mm for the convex laps.
Performance is similar but efl is minutely smaller. The metric steel balls may be easier to obtain.
Radius of curve-------Thickness------------
Glass------------ Semi-width
The glass types are more expensive but are not uncommon and only minuscule quantities are used. These glasses will suffer from climatic degradation unless cared for properly but this is normal for high performance eyepieces.
If made well and coated, the loss of contrast resulting from only 4 air / glass surfaces may be less than some more complex commercial designs. Even uncoated, loss of light will not be a major problem.
Centring, especially for the steeper curves, must be done well to take full advantage of the design.
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