Sky and Telescope - July 2017 - 53

Plössl: This brainchild of Viennese
optician Georg Simon Plössl originally
consisted of a pair of identical cemented
doublets. It was also known as the symmetrical eyepiece. Twentieth-century
reﬁnements by Rudolf König and Chester
Brandon of the original 1860 design
place the interior surfaces almost in
contact to minimize ghost reﬂections
and employ an eye lens of shorter focal
length than the ﬁeld lens. Providing a
45° to 50° apparent ﬁeld of view, the
best Plössls rival the performance of
orthoscopics. But the quality of today's
commercial offerings varies widely.

Importance of Eye Relief
During a typical planetary observing
session, you'll maintain a prolonged
vigil at the eyepiece, waiting patiently
for those ﬂeeting moments when the
atmosphere steadies momentarily to
provide what Percival Lowell called
"revelation peeps." Comfort is of
paramount importance if you want to
maintain visual acuity for long periods.
To fully exploit the resolving power
of any telescope when viewing a lowcontrast target, you'll need to use a
magniﬁcation of at least 25× per inch
of aperture (or 1× per millimeter) -
and, when the seeing permits, you
can double that value proﬁtably (S&T:
March 2015, p. 54). A classic, long-tube
achromatic refractor with a focal ratio
of f/15 achieves this range of magniﬁcation with eyepiece focal lengths of 15
to 7.5 mm. The popular f/10 SchmidtCassegrain requires focal lengths of
10 to 5 mm, while the increasingly

p Eyepiece designs with fewer lenses tend to deliver the most light to your eye, but those with
complex optical combinations often provide more expansive views and better eye relief.

the short focal lengths required for
planetary observing involve squinting
through tiny eye lenses located a small
fraction of an inch from your eye. Not
only does this present an insurmountable obstacle to eyeglass wearers, but the
surface of the eye lens is also prone to
smearing with eyelash oils.
The time-honored solution to this
ergonomic difﬁculty is to combine an
eyepiece of moderately long focal length
(and thus comfortable eye relief and an
eye lens of reasonable diameter) with
a Barlow lens or other image ampliﬁer
that doubles or triples the magniﬁcation. Most observers ﬁnd that any loss
of image quality resulting from the Barlow's two additional air-glass surfaces is
almost imperceptible - and more than
offset by the combination's ease of use
and comfort.
In fact, many of the best eyepiece
designs offered today integrate a Barlow
lens to provide a remarkably generous

G REGG DINDER M A N / S&T

Contrast and definition are the qualities of paramount
importance to the fastidious planetary observer.
common "fast" f/5 Newtonian reﬂector
needs 5 to 2.5 mm.
The problem is that such short focal
lengths tend to offer poor eye relief, the
distance from the outer surface of the
eyepiece's eye lens within which you can
view the full viewing angle. Although
monocentric, orthoscopic, and Plössl
eyepieces all provide decent eye relief,
about 70% to 80% of their focal length,

eye relief of 20 mm even in eyepiece
focal lengths as short as 3 mm, combined with well-corrected apparent
ﬁelds of 50° to 60°. At one point, TMB
offered six-element eyepieces of this
form that the designer, the late Thomas
M. Back, claimed were the equal of the
ﬁrm's "gold standard" monocentrics in
sharpness, contrast, and lack of scattered light while overcoming their poor

eye relief and narrow ﬁeld of view. If
you use a Dobsonian reﬂector or some
other undriven telescope, the wider ﬁeld
is a welcome bonus because the target
doesn't drift out of the ﬁeld as quickly.
Frankly, much of the conventional
wisdom about what constitutes virtue
in the design of a planetary eyepiece
has ceased to be true. Maximizing light
transmission while minimizing internal reﬂections and contrast-robbing
scattered light remain essential goals,
of course. But achieving them no longer
limits optical designers to a small number of lens elements and uncomfortably
tight eye relief.
Modern high-index glasses and
efﬁcient, broadband, multilayer antireﬂection coatings allow many eyepieces
with as many as 10 air-glass surfaces
to rival the performance of traditional
eyepiece designs in critical side-by-side
comparisons on the most challenging
planetary details.
The "minimum glass" paradigm for
planetary eyepieces still has its vocal
adherents, just as there are ardent
audiophiles who disparage digital components in favor of old analog technology. But there's been a marked shift
toward complex, high-quality eyepieces
- many of which, happily, don't command substantially higher prices than
those of older, simpler designs.
¢ Contributing Editor TOM DOBBINS has
forsaken his old orthoscopic eyepieces in
favor of modern, short-focus offerings by
Pentax and Baader.
s k y a n d t e l e s c o p e .c o m

* J U LY 2 017

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Sky and Telescope - July 2017

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