Evolution of the Astronomical Eyepiece - Brayebrook Observatory

EVOLUTION of the ASTRONOMICAL EYEPIECE
DERIVATIVES OF THE PLÖSSL AND
SYMMETRICAL:
The first wide-angle modification of the
Plössl was the aplanatic eyepiece of Dr.
Otto Schröder, described in Gill’s articles
on telescopes in the 9th. edition of
the Encycl opaedia Britta nica . The
glass employed is re f e r red to as
Dauget’s crown (Cb1) and flint (Fb1). It
comprised a pair of achromats spaced
at 83% their ef fective focal length. The
crown elements being plano-convex,
plane surfaces facing the eye, and the
flin ts being co ncave meniscus. Eye
relief was 0.54Fe, and the apparent
field 72˚. Thi s design w as initi ally
employed by Schönfeld in his southe
rn Durchm usterung, and su bsequently
marketed by Browning.
ERFLÉ I - A 1-2-2 design having a 60˚
field but eye relief only 0.3Fe. The
glasses used were SF2, PSK3, FK5 and
SF10.
ERFLÉ II - A 2-1-2 design using the
same glass types having a 70˚ field and
0.6Fe eye clearance. Paten ted after
Erflé’s death in 1923.
ERFLÉ III - A 1-2-2 design using the
same glass types having a 55˚ field and
eye clearance of 0.32Fe. Patented after
Erflé’s death in 1923.
KAPELLA - Patented after Erf l é ’ s
death in 1923 for Zeiss by Kapella, one
of his assistants. A 1-2-2 design having
a 70˚ field and 0.685Fe eye clearance.
The glasses used were BK1, SK15 and
FN11.
Zeiss’ m odifi cation of t he Plö ssl,
and the subse qu en t i nc orporation
of the field lens to w ide n t he f ield,
led Hein ri ch Erf lé to desi gn th e f irst
true w ide-an gle ey epi ece f or m ili tary
use in 1917 .
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KASPEREIT - A modification of Erflé’s
designs by Kaspereit having a 2-2-2
for m, giving fields in excess of 68˚ and
eye r elief 0.3Fe+. The glasses used in
modern variants are SF2, BK7, SK20
and SF10. Some W.W.II (World War II)
variants using Thorium or Uranium
glass have fields wider than 70˚.
All these Erflé types and their derivatives
suf fer from lateral colour, astigmatism
and rectilinear distortion.