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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.
Performance.
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.
Prescription.
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.
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