Sky & Telescope - May 2020 - 68

S&T Test Report

p The Orion BinoViewer gives a right-side-up and correct-reading view (like conventional binoculars) when used straight through on refractors and Cassegrain telescopes. To have the same field
orientation with a 90° star diagonal requires a pentaprism diagonal such as this one from Orion.
Note that this style of diagonal requires more back focus than conventional mirror or prism models.
The total back focus for the Orion Pentaprism is 4½ inches.

was obvious the instant I turned the
telescope toward the stars after dark.
Out-of-focus star images appeared like
quarter moons rather than circular diffraction disks.
Unlike binoviewers that employ
beamsplitters to deliver 50% of the light
collected by a telescope to each eye, the
Orion BinoViewer optically splits the
aperture in half. Each eye gets all of the
light collected by a physical half of the
aperture. Furthermore, in the straight-

through refractor setup I used earlier in
the day, the light collected by the right
half of the objective comes out of the
left eyepiece and vice versa. With that
nearby scene, even the small separation afforded by each half of a 4-inch
objective was enough to render a 3D
view, and the right-to-left swap caused
the inverted sense of depth. On later
days of terrestrial viewing I could often
coax my brain into perceiving depths
properly, but your mileage may vary.
t Because the Orion BinoViewer works with any
Barlow lens and eyepiece
combination that already
reaches focus with a
telescope, it's easy to vary
magnifications without
having to own multiple
sets of matched eyepieces. As described in
the text, the author used
this pair of 26-mm Plössl
eyepieces with 2×, 3×,
and 5× Barlow lenses to
observe with the effective
magnifications offered by
eyepiece sets having 26-,
13-, 8.6-, and 5.2-mm
focal lengths.

M AY 2 0 2 0 * S K Y & T E L E S C O P E

For astronomical observing none
of this really matters. Regardless of
telescope aperture, the distances are
too great to show real 3D. And the
Orion BinoViewer still delivers half
of the light gathered by a telescope to
each eye. I did note, however, that the
half-moon diffraction pattern seemed
to make me more sensitive to even
slight differences in focus between the
left and right eyepieces. Fortunately,
the Orion BinoViewer has a very precise
and smooth-operating helical focuser
on one eyepiece holder, making it very
easy to keep the focus of both eyepieces
critically in sync.
Orientation is often an issue for people looking through telescopes. Think
how many times you've had to explain
the "upside-down" view in a telescope
eyepiece to ﬁrst-time observers. For the
record, here's what you get with the
Orion BinoViewer. In a straight-through
refractor or Cassegrain the view is rightside up and right-reading just as it is
with a standard binocular. With a regular 90° star diagonal the view is upside
down and mirror-reversed (the included
manual is wrong on this point, saying
the view is "non-reversed"). With an
image-correcting star diagonal the view
through the Orion BinoViewer is still
upside down, but right reading (again,
contrary to the manual). If you want
to use a diagonal that gives a right-side
up and right-reading view, you need a
pentaprism diagonal, such as the one
Orion offers as a $149.99 option. But
be aware, this diagonal consumes about
4½ inches of back focus.
The Orion BinoViewer's internal
relay-lens system that makes it work
without additional back focus is very
good. I didn't notice any internal reﬂections, and star images were sharp across
the ﬁeld. During daytime observing
there was a slender blue fringe circling
the extreme edge of the ﬁeld. It wasn't
objectionable, and it varied somewhat
depending on the eyepieces I tried.
There was one issue that cropped up
during daytime observing of terrestrial
scenes and when viewing the Sun (with
a proper solar ﬁlter) and Venus during
daylight. The Orion BinoViewer's aper-

Sky & Telescope - May 2020

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