Page 20


70 mm F:10 Eyepiece.

With Extreme 80 degree Field

This is specially corrected for Spherical aberration of the exit pupil

for use in a situation where the exit pupil is very large.

Copyright – P. J. Smith

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

Special Considerations.

This is an extension of the design on p.19. where field has been expanded to a sensible maximum at F:10.

Readers must read p.19 and probably also p.18 to place it in perspective.

Spherical aberration of the exit pupil is small – but it is a little larger than the p.19 design..  Compare with p.18.

Note more radii are common than in both other designs.  Only three different radii are used. Other changes are slightly thinner elements to save expensive optical glass.

This would be a very impressive eyepiece for a huge refractor of say F:12 or more. 

A 6 mm diameter pupil has been superimposed.

Compare Spherical Aberration of the Exit Pupil with a 6 mm pupil.



I consider F:10 the limit for acceptable performance.  At F:16 it is truly excellent.

The following analysis will allow you to make up your own mind.

There is obviously some slight field curvature.  The curvature could have been reduced but other aberrations increase. 

Field curvature must be evaluated with respect to the efl. of the eyepiece AND tolerance to defocus of the eye.  This is complicated by the fact that a young eye has huge tolerance.  Often 1 dioptre of defocus is allowed but older users may like this to be reduced to 0.5 or even less.

A tolerance of 1 dioptre allows about 4 mm of defocus at 70 mm. efl.

The following graph shows less defocus than this although astigmatism at the extreme edge is noticeable..

Definition on axis is excellent but some astigmatism is evident nearer the edge of the field.  This could have been improved greatly but at the expense of Spherical Aberration of the Exit Pupil.

Now let’s look at resolution in terms of what is satisfactory to the eye.

The usual criteria are better than 1 sec spots on axis and no worse than 5 times this at the edge of the field. Some eyepieces are significantly worse than this.  The following graphs place this in perspective.

Note that any comparison between this design and the Page 18 and p.19 eyepieces must consider different field coverage.



 The dotted line represents a resolution of 1 minute for the eye.


 The Back Focal length is 36 mm and the working distance is 36 mm.  It covers an image of approximately100 mm.


It is interesting that only three different radii are used.

The above semi diameters allow extra for mounting.  Optical diameters should be 50 mm for the field lens and 39.5 mm for the eye lens.  It is important the field lens is stopped down to 50 mm as any light entering further from the axis contributes to significantly larger Spherical Aberration of the exit pupil.  If a user finds this objectionable (unlikely) a small reduction of the active field lens diameter would be in order.

Ghosting and Coatings.

This eyepiece has not been rigorously checked for ghost images.  Surface coating will help to reduce any potential problems and is almost mandatory in this three group, high index unit.