Evolution of the Astronomical Eyepiece - Brayebrook Observatory

TESTING EYEPIECES (cont.):
EVOLUTION of the ASTRONOMICAL EYEPIECE
Is it in focus or diffuse How objectionable
ghosting is depends to a large
extent on the type of observing you do.
An eyepiece that produces bright on
axis ghosts that either follow or mirror
the image would not be a nuisance if
you are observing M32 say, but would
be when observing M13. Likewise a diffuse
off axis ghost that follows the
image might cause confusion when
comet hunting.
vii) Lateral colour is easy to detect,
and almost all eyepieces exhibit it,
some more so than others. It is also
more obvious with fast objectives. As
the star image is displaced towards the
edge of the field, it becomes drawn out
into a short spectra, red towards the
c e n t re when underc o r rected, blue
when overcorrected. The eye is remarkably
tolerant of this malady, especially
w hen using wide angle eyepieces,
where it is worst, because, when the
eye is fixated on axis, the off axis image
towards the edge of field falls on the
portion of the retina which is capable
of only giving black and white images!
Also faint stars exhibit no sensible
colour in any case, masking the aberration
somewhat.
viii) Astigmatism, due mainly to the
eyepiece, and coma, due mainly to the
objective of a reflector, are never seen
in isolation. Any star, when displaced
towards the edge of field will become
either elongated or triangular or fan
shaped, apex inner most. When the eye
is fixated on axis, astigmatism and
coma need to be very marked for the
eye to resolve it. It is only when the eye
is fixated towards the field boundary
that these aberrations become obvious.
However no eyepiece is completely free
of it. The wider the apparent field and
the faster the objective, the worse it
becomes.
ix) Field curvature can easily be
tested too. Focus the star on axis and
displace it to the field edge. Does the
image defocus If you need to rack the
eyepiece towards the objective then the
focal surface is convex towards the eye
and vice versa. The eye is remarkably
tolerant of field curvature, especially
in the young, because you unconsciously
accommodate as you shi ft
your dir ection of fixation. As the range
in visual accommodation diminishes
with advancing old age field curvature
becomes a problem. And the wider the
apparent field and the faster the objective,
the bigger the problem becomes.
x) Distortion is difficult to detect
visually except when the coefficient
exceeds 20%, and only then on bright
extended objects containing rectilinear
or regularly spaced features.
xi) Contrast and transmission, go
hand in glove. Image contrast may be
tested by comparing the appearance of
the outermost visible diffraction rings
to the Airy disc, or the faintest visible
detail on a planet. Transmission may
be tested by seeing how difficult it is to
detect the faint companions of very
bright stars, e.g. Vega or Rigel or
Regulus. The darkness of the field
background is also an important indicator.
Contrast and transmission are
the most important properties of any
eyepiece, being crucial to good definition.
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