Evolution of the Astronomical Eyepiece - Brayebrook Observatory

THE EVOLUTION OF
EYEPIECE DESIGN
EVOLUTION of the ASTRONOMICAL EYEPIECE
Having looked at the succession of eyepiece
designs over the past four centuries,
it is easy to recognize the evolution
in thinking that lies behind each
stage of development.
Huyghens’ concept of a field lens to reduce
the focal length of the objective
and enlarge the apparent field of view,
and at the same time correct lateral
colour and minimize spherical aberration,
set the stage and dictated all
thinking upon the subject until 1977
and the first computer auto-optimized
design.
Ramsden’s concept placed a flat focal
plane, corrected for rectilinear distortion,
in an accessible position. For the
first time micrometer threads could be
seen clearly, and not marred by spherical
aberrati on and fa lse colou r.
Ramsden’s concept has set the stage
for all eyepiece designs intended for
micrometers, comparators and sighting
telescopes.
Kellner’s achromatic eyepiece was
essentially an achromatized Ramsden.
Altering the plano-convex field lens to a
bi-convex, and over-correcting the eye
doublet to compensate, resulted in a
wider apparent fiel d th an the
Huyghenian, with greater eye relief.
Plössl’s develo pmen t of the
Symmetrical, Dial-Sight and assymetric
Plössl followed from h is earlier
designs of camera objectives and an
i m p roved achromatic and apla natic
doublet. Zeiss’ Abbé duplet, with the
later addition of an optional field lens,
led naturally to the wide angle Erfles
and their derivatives.
Abbé’s Orthoscopic eyepiece was truly
innovative, there being nothing to base
it on. It is ironic that this design, in its
best forms, remains the most useful
a s t ronomica l eyepiece for genera l
observation.
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König’s design was at first called the
Kellner II, and it is obviously a variation
of the Kellner. König worked with
Bertele, and the König II and Bertele
designs ar e conceptual extensions of
König’s initial work.
It i s not dif ficu lt to recognize the
desi gn de vel opment of the Zeiss
A s t ro p l a n o k u l a r. Post W. W. I I
a s t r onom ical eyep iece desi gn experienced
a ma rked d ow n-tur n du e
to mil itary su rplus satura ting the
ma rket wi th Orthoscopi c, Kellner,
Symm etrical, Dia l-Sight, and Erf l e
types a t prices which undercut all
commercial design and manufacturing
costs. It was only when this surplus
market dried up in the mid to late
1970’s that commercial astronomical
eyepiece design began to increase once
more. Even so auctioned military eyepieces
ar e still being offered, many of
them Galoc wide angle designs, developed
during the 1930’s from the work
of Er flé.
It is unfortunate that all the modern
computer optimized ultra-wide angle
designs, post Fleischman, share the
same 8 to 10 air-glass surfaces, and
adopt the negative-positive in many
instances too, which introduces excessive
rectilinear distortion.
The first true wide angle design was the
Erflé, and to date, the only significant
impr ovement has been the Galoc II and
the Bertele-Galoc hybrid, which combine
the better features of both the
Erflé and the Orthoscopic. Greenwood
has fabricated an excellent 1-2-1 wide
angle design called the Panscopic, in
which the collecting doublet is severely
over-corrected and almost zero power.
This empirical design is superior to the
König, yielding a highly corrected 70˚
field. A similar 1-2-2 ultra-wide angle
called the Platyscopic of fers similar
per formance to the Galoc.