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.
Performance.
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.
Prescription.
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.
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