Evolution of the Astronomical Eyepiece - Brayebrook Observatory

ORTHOSCOPIC EYEPIECES:
EVOLUTION of the ASTRONOMICAL EYEPIECE
The quest for distortionless imaging
characteristics of photographic objectives
in the 1850’s and 1860’s led C.A.
Steinheil to design the Periscop lens in
1865. Prior to this in 1859 the Petzval
Orthoskop appeared. The fundamental
property of any orthopscopic lens is
that it should have a wide flat field free
from rectilinear distortion and angular
magnification distortion.
to 40˚ at f/5. In the mid-1930’s Barr
and Stroud patented an orthoscopic
eyepiece in which the simple eye lens is
bi-convex, with the steeper convex surface
remote from the eye having a parabolic
figure. This widened the distortion
free field to 64˚ and the eye clearance
was extended to 0.91Fe. Th e
glasses used were boro-silicate crown
and extra-dense flint.
In 1880 Ernst Abbé brought out an
Ortho scopic eyepiece for Zeiss. The
field lens is an overcorrected triplet
combination with a negative component
in the middle, followed by a simple
plano-convex eye lens, convex surface
almost in conta ct. This eyepiece is
remarkable for great eye clearance and
has given rise to a whole family of eyepieces,
some of them of very complex
for m. In Abbé’s design hard crown and
dense flint glasses were used to secure
an apparent field of 30˚, and an eye
cleara nce of 0.8Fe. The Zei ss
Orthoscopic patented in 1930 made
use of less u sual gla sses; barium
c rown, extra -dense flint an d borate
flint. This increased the apparent field
Another aspheric variant was designed
for Zeiss, probably by Robert Richter in
1934.
A c c o rdi ng to th e Briti sh paten t, the
t h i rd surf ace ( a steeply con ve x fa ce
of an over- c o r rect ed eye triplet) wa s
p ara boliz ed and pro vi ded a w idean
gle eyepiece w ith a 53˚ field in
w hi ch distorti on wa s eliminated; the
l ateral colou r removed w itho ut use
bein g ma de of mo re than o ne lens of
flint gla ss; a st igmatism reduced t o
t hat resu lti ng f rom the Pe tzval curv
a t u re, a nd spherical aberration o f
the exi t pu pil el iminat ed or marg i n-
all y reve rsed. Eye c leara nce w as
reduced sligh tly to 0. 66Fe.
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