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Simple 60 mm F:10 Eyepiece.
With Medium to Wide Field of 60 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.
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.
Performance.
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.
Eye Relief.
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.
Prescription.
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.
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