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.
This eyepiece has been promised for some time. It is meant to work on a very large telescope with a slightly oversized exit pupil.
In other words, the telescope could use slightly higher magnification for optimum light transfer to the eye.
A large exit pupil, especially with a central obscuration, spherical aberration of the exit pupil is a very important parameter. This has ruled out many other designs as less than perfect in this respect.
Below, the design is compared with a Typical Symmetrical. Eyepiece. It should be noted that the symmetrical eyepiece used here as an example has excellent definition when used in smaller sizes (or at very high magnification). Note, however, the much reduced Spherical Aberration of the exit pupil. A typical 6 mm exit pupil is shown for comparison.
Compare Spherical Aberration of the Exit Pupil with a typical Symmetrical Eyepiece.
If you have never seriously considered this defect it may be very difficult to really understand the implications of the above diagrams. It does not degrade sharpness but causes a most annoying uneven vignetting pattern often called the Kidney Bean effect.
I will not expand further on this here. Hopefully, when time permits, a detailed chapter will be prepared on this topic.
There is no reason the design cannot be scaled slightly but I would suggest the 50 – 70 mm efl range.
All glass used is of high index type. 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. Both are preferred types so will be readily available from Schott. These are a little more prone to climatic attack than normal so the finished item will require careful storage. The SF6 is an internal surface so is protected somewhat. Be aware that SF6 has been known to stain during polishing.
The restriction of nearly zero Spherical Aberration of the Exit Pupil has made common radii impossible in this design.
Some may consider performance marginal at F:10 but for such a simple eyepiece the performance is surprising. Certainly at F:16 the performance is excellent even 30 degrees from the axis. The following analysis will allow you to make up your own mind.
There is obviously some evidence of field curvature which must be more critically examined.
Field curvature must be evaluated with respect to the efl. of the eyepiece AND the 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 with an efl. of 60 mm. 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.
Performance at F:16 is well under the 1 minute limit (dotted) near the centre and close to 5 x this limit at 30 degrees from centre. At F:8 the central definition is quite acceptable but edge definition could be better.
A 60 degree field at F:10 is not unreasonable.
Uninitiated viewers – especially if they fail to critically focus the eyepiece, will appreciate the lack of significant exit pupil aberration far more than a slight loss of definition near the edge.
The Back Focal Length is 46 mm. In use, eyerelief will always be larger.
This is far too much to be comfortable and most will find it very difficult to position the eye accurately.
An adjustable eye cone as shown above will solve the excessive eyerelief problem.
I would suggest about 20 mm as a comfortable amount when catering for spectacles with a 60 deg Apparent Field of View.
Adjusting somewhere between 25 to 15 mm should keep everyone happy.
The Back Focal length is 46 mm and the working distance is 41.9 mm.
Ghosting and Coatings.
This eyepiece has not been rigorously checked for ghost images. Surface coating will help to reduce any potential problems and is worth while if possible.