Sky & Telescope - August 2022 - 72

ASTRONOMER'S WORKBENCH by Jerry Oltion
Over-Under Binoculars
You don't need prisms to make a great pair of binoculars.
BACK IN APRIL 2019, I wrote about
Frank Szczepanski's unique approach to
making a binocular telescope using only
four mirrors instead of the traditional
six. Frank has since reduced the number
of mirrors required to two.
Of course, that requires a set of
objective lenses up front instead of primary
mirrors in back, but there's plenty
of innovation in between.
One of the biggest problems with
any large binocular telescope is that
the distance between the centers of the
primary light-gathering elements - be
they mirrors or lenses - is wider than
the spacing between the observer's eyes.
That means the light must be bounced
off matching sets of mirrors to bring
the optical axes closer together.
A Clever Solution
Except it doesn't, really. With his Newtonian
telescope design, Frank sends the
light from one scope through the light
path of the other one,
using unequal-diameter
secondary mirrors
to bring the two optical
axes together as
tight as you'd please.
He does a similar
thing with his
80-mm refractor
binos. Because the
light paths are coneshaped,
they don't
actually intersect
the way they do in
a Newtonian of the
same design, but
the principle is the
same: Run the light
path of one telescope
beneath the light
path of the other one and put the eyepieces
as close together as you want.
Using a pair of the same 80-mm
doublet objectives that I wrote about in
our November 2021 issue, Frank built
the telescope you see here. One objective
is slung beneath the other one and
off to the side by the interpupillary
distance required for Frank's eyes and
mounted quite a way behind the other.
A diagonal mirror interrupts the light
path from that objective and sends it
upward to an eyepiece.
The upper scope uses the same
arrangement, but the lens is positioned
well forward of the other, and the diagonal
bends the light path much farther
down the cone than the lower scope
does. That light path reaches the other
eyepiece at just the right spot for the
two eyepieces to be perfectly spaced for
Frank's eyes. This scope doesn't have
the capability to adjust the interpupillary
distance, but with a pair of 2-inch,
wide-field eyepieces it accommodates
a pretty good portion of the average
human eye spacing.
The diagonals are just front-surface
mirrors from a copy machine. Adjusting
screws allow Frank to tip one from side
to side and the other up and down to
merge the images, and once they're set,
they seldom need re-adjustment.
The scope is made from scrap
plywood, and the focusers are simply
friction-fit tubes where adjustments
are performed by sliding the eyepieces
inward and outward. But the view
through it is nothing short of stunning.
Frank Szczepanski's binocular scope uses
offset objective lenses to bring the interpupillary
distance closer together than the objective
lens diameter. It's mounted on an extendable
umbrella stand that can be cranked up and
down to the perfect height for observing any
part of the sky.
72 AUGUST 2022 * SKY & TELESCOPE
ALL IMAGES COURTESY OF AUTHOR UNLESS OTHERWISE NOTED
Sky & Telescope - August 2022

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