Sky and Telescope - July 2017 - 52
JULY 2017 OBSERVING
Exploring the Solar System by Thomas A. Dobbins
Eyepieces for Planetary Observing
his generation of amateur astronomers can choose from a bewildering
variety of eyepiece types made to suit
all purposes and pocketbooks. Complex
designs that provide apparent ﬁelds of
view of 80° or even 100° can deliver a
dramatic "spacewalk" viewing experience. Many of these optical marvels
command prices surpassing a decent
8-inch Newtonian reﬂector complete
with an equatorial mount.
Just as some telescope designs provide discernibly superior visual views
of solar system targets, so do certain
eyepiece types. The planets all subtend
very small apparent angular sizes, so
devoted planetary observers don't place
a premium on wide apparent ﬁelds of
view (or even 2-inch-format eyepieces)
that require a multitude of lens elements, air-glass surfaces, or complex
The Virtues of Simplicity
Instead, contrast and deﬁnition are the
qualities of paramount importance to
the fastidious planetary observer. Discerning all of the planetary details that
a telescope's optics and the state of the
atmosphere allow demands eyepieces
that have high light transmission and
freedom from ghost images, internal
reﬂections, and scattered light.
Ghost images are caused by double
reﬂections from air-glass surfaces that
come to focus at or near the eye's focal
plane. The more of these interfaces
within an eyepiece, the greater the
chance that ghost images will arise.
Modern anti-reﬂection coatings dramatically reduce such spurious reﬂections, but eliminating scattered light
requires optical elements with wellpolished surfaces that are free of sleeks
and scratches, blackened lens edges, a
A steady hand and steady sky - along with an 18-inch reﬂector and a binocular viewer
with high-quality 4.5-mm eyepieces - all came together for this sketched rendering of
Jupiter on March 27, 2017.
J U L Y 2 0 1 7 * SK Y & TELESCOPE
ﬁnely threaded and effectively blackened interior of the eyepiece barrel, and
a sharp, well-deﬁned ﬁeld stop.
Conventional wisdom has long held
that the best planetary eyepieces are
those with the smallest number of lens
elements and air-glass surfaces that can
still provide a well-deﬁned image in the
center of the ﬁeld of view. Three optical
conﬁgurations that satisfy this "minimum glass" paradigm have emerged.
Monocentric: Introduced by Hugo
Adolf Steinheil in 1883, the monocentric design consists of a cemented
triplet lens with spherical surfaces that
share a common center and radius
of curvature. With only two air-glass
surfaces, monocentrics provide images
of unsurpassed brightness and contrast.
But they have a very narrow apparent
ﬁeld of view, only 25° to 30°, that many
observers have compared to looking
through a drinking straw. In 1911
Charles Hastings patented a reﬁnement
of the design that Carl Zeiss produced
until the 1950s. The ﬁrm TMB brieﬂy
revived this design a decade ago, but
today monocentrics are only available
on the secondhand market.
Orthoscopic: Designed by the
brilliant German mathematician and
physicist Ernst Abbe, the orthoscopic
(from the Greek roots for "straight
seeing") was the ﬁrst eyepiece to offer
virtually complete correction of optical
aberrations and distortion. Introduced
in 1880, it consists of a cemented triplet
ﬁeld lens paired with a single-element
biconvex or plano-convex eye lens.
Widely available even today, orthoscopic
eyepieces offer excellent sharpness,
color correction, and contrast combined
with a 40° to 45° apparent ﬁeld of view.
PE TER V ERCAU TEREN / ASTRONO M Y DR AWINGS.CO M
Jupiter and Saturn are easy to observe now - but what eyepiece type will give the best views?