Page 19


60 mm F:6 Eyepiece.

With Very Wide Field of 70 deg.

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 eyepiece has been promised for some time.  It is meant to work on a very large telescope with a slightly oversized exit pupil and is a development of the design on Page 18.  If you have not read p.18 it will help to place it in perspective.

I am reasonably pleased with this design as there are many common radii so construction will be easier. This has compromised performance slightly, but, considering the simplification, seems warranted.

This eyepiece is very ambitious but not beyond a dedicated worker.  Glass costs will be significant but the end result should impressive and probably unique.  It is not an off the shelf component.

Few very large eyepieces have the capability to work well at F:6  

Spherical aberration of the exit pupil is very small.  Compare with p.18. 

A 6 mm diameter pupil has been superimposed.

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

There is no reason the design cannot be scaled slightly but I would suggest the 50 – 70 mm efl  range.

Glasses are high index types.  Close alternatives are available from other manufacturers but if a conversion is attempted be aware that optimisation has to comply with good images in the normal sense as well as a neat exit pupil.

Glass types are Schott’s  SK4 and SF6.  All are preferred types so will be readily available.  SK4 and SF6 are a little more prone to climatic attack than normal so the finished needs careful storage. 


I consider F:6 the limit for acceptable performance.  At F:8 or more it is excellent.

If it not for extra surfaces causing reflections, this would probably be a more generally useful eyepiece than the p.18 design.

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 astigmatism will 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 a little more than 3 mm of defocus at 60 mm. efl.

The following graph shows less defocus than this.

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 a comparison between this design and the eyepiece on Page 18 must consider different field coverage.

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


 The Back Focal length is 35 mm and the working distance is 25 mm.


The above semi diameters allow extra for mounting.  Optical diameters should be 32 mm for the eye lens and 38 mm for the others.  It is important the field lens is stopped down to 38 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.