Evolution of the Astronomical Eyepiece - Brayebrook Observatory

EVOLUTION of the ASTRONOMICAL EYEPIECE
The important basic optical properties of an astronomical eyepiece are its focal length,
apparent field of view and eye relief. When fitted to a particular telescope it will be afflicted
by various aberrations and exhibit the following characteristics to some extent or
another:
a) longitudinal chromatic aberration
b) chromatic inequality of magnification (lateral colour)
c) spherical aberration
d) coma
e) astigmatism
f) field curvature
g) distortion
h) spherical aberration of the exit pupil
i) inter nal reflections (ghost images)
Because optical aberrations ‘a’ thru’ ‘e’ are proportional to the diameter of the exit pupil,
the longer the focal length the more pronounced they become.
Eye relief is given by:
Er = bfl.Fe 2
F
where bfl is the eyepiece back focal length. It increases with the focal ratio of the objective,
i.e. as the focal ratio becomes faster.
a) Longitudinal chromatic aberration is a first order aberration in which the final
image does not lie in a single plane. An undercorrected eyepiece will have a longer effective
focal length in red light, an overcorr ected eyepiece a longer effective focal length in
blue light.
b) Chromatic inequality of magnification is a consequence of ‘a’, where the image is
magnified by slightly dif ferent amounts at different wavelengths. When the image is displaced
towards the field stop, lateral colour manifests itself, red inwards in undercorrected
types and blue inwards in overcorrected types. It is therefore possible to detect the
colour correction of an eyepiece by examining the colour fringing around the field stop
when it is held up to a white light source and placing the eye at the eye point. If the field
stop is fringed with red light the eyepiece is undercorr ected, and if fringed with blue light,
overcorrected.
c) Spherical aberration is suppressed in multi-element designs, but it is present in
single or two element designs to some extent or another. The faster the focal ratio of the
objective the more objectionable spherical aberration, if present, becomes, increasing as
the square of the f/no.
d & e) Coma and astigmatism are off axis aberrations. In aplanatic (ref.p23) eyepiece
designs coma is well suppressed, and in orthoscopic designs both coma and astigmatism.
However in wide angle designs it is not possible to corr ect both distortion and
astigmatism in the outfield. Because, in astronomical applications, distortion is judged
to be less objectionable, astigmatism is suppressed at its expense. Both astigmatism and
coma occur in combination and manifest themselves by an assymetric appearance of the
Airy disc.
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