Sky & Telescope - May 2020 - 67

signiﬁcantly changes the telescope's
magniﬁcation and ﬁeld of view. And
while popular Schmidt- and MaksutovCassegrain telescopes that focus with
moving primary mirrors can extend
their back focus far enough to accommodate a conventional binoviewer,
doing this also increases the telescope's
effective focal length and thus changes
the magniﬁcation and ﬁeld of view of a
given eyepiece.
There are other ways the Orion
BinoViewer is different from conventional models, so let me start with the
day the unit we borrowed for this review
arrived. I was doing routine maintenance in my backyard observatory
when the UPS truck rumbled up to the
driveway. Something as straightforward
as the Orion BinoViewer doesn't need
a mandatory study of its instruction
manual before a ﬁrst look, so I swung
the 4-inch refractor I'd been using the
night before toward a neighbor's ﬂowerbed about 700 feet away and visible
through a scattering of tree branches.
Using the scope "straight through," I
racked the focuser out until a 24-mm
eyepiece was sharply focused on the
ﬂowers. I removed the eyepiece from
the focuser, replaced it with the Orion
BinoViewer, and slipped the eyepiece
into its left eyepiece holder. The image
was perfectly focused and the ﬁeld
looked the same, just as advertised, and
rather impressive at that. I repeated this
test with a handful of other 1¼-inch
eyepieces, including modern wide-ﬁeld
designs, all with the same result. (Careful tests later showed that the Orion
BinoViewer does introduce a small
increase in magniﬁcation with a corresponding reduction in the ﬁeld of view.
It could easily go unnoticed.)
Over the years, telescopes have
come and gone in my life, but eyepieces
seem to ﬂow in only one direction.
Nevertheless, on that ﬁrst day the only
"matched" pair I could ﬁnd suitable for
a binoviewer were two of the ubiquitous
Meade 26mm Super Plössls that ship
with many of the company's telescopes.
They were of different vintages, but the
same mechanically and optically.
My ﬁrst two-eyed look through the

u Rather than pivoting on
a central axis to vary the
spacing of the eyepieces,
the Orion BinoViewer's
eyepieces slide linearly
outward from a center position. The scale shows the
eyepiece separation, which
ranges from 58 to 74 millimeters. Eyepieces are held
in place with solid, locking
collets, and the right eyepiece has a precision helical focuser to compensate
for any variations between
an observer's eyes.

Orion BinoViewer was shocking - the
scene appeared truly three-dimensional,
even being enhanced by a squirrel
scampering around the ﬂowerbed. This,
I thought, had to be an extreme example of the power of suggestion since I
was looking through a telescope with a
single objective. The scene couldn't possibly be real 3D the way it is in binoculars that have separate optical systems
for each eye. But before I had time to
marvel at this lesson in eye-brain gymnastics, things got really weird.

As I moved my head, shifting my gaze
through the Orion BinoViewer's eyepieces, there was clearly visible parallax.
The nearby tree branches and shrubs
behind the ﬂowerbed appeared to be at
different distances. And, to make matters even more confusing, the distances
seemed inverted - the nearby branches
appeared to be behind the ﬂowers. This
was simply nuts! I couldn't think of any
reason why this was happening.
The problem is, I wasn't thinking hard enough, since the answer

p As detailed in the accompanying text, the Orion BinoViewer is unusual because it divides a
telescope's aperture into two halves, putting the light collected by each half into the separate
eyepieces. For this photograph a bright green light was placed into one of the Orion BinoViewer's
eyepiece holders and projected "backward" through a 12-inch f/5 Newtonian onto a wall, showing
a silhouette of half of the secondary mirror and spider vanes.
s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 0

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Sky & Telescope - May 2020

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