Sky and Telescope - October 2016 - 72

Jerry Oltion
Astronomer's Workbench

Weijenberg's Propdob Nano
An 8-inch triumph of miniaturization and packaging.
I can tell

ROEL WEIJENBERG (2)

there's going to be one serious hazard in
writing this monthly column: I want to build everything
I write about. That's especially true this month. The
moment I saw Dutch ATM Roel Weijenberg's 8-inch
travel scope, I knew I'd found my next project.
Seriously! Who can resist the lure of a scope that
breaks down into a package so small you could lose it
under a couch, is light enough (11 pounds!) to carry on
a camping trip, yet provides enough aperture to see the
spiral arms of M51? This is a must-make project.
Roel was a contender in Gary Seronik's travel scope
challenge (S&T: Apr., Aug., and Sep. 2009 issues), but for
his upcoming trip to the U.S. to witness the 2017 eclipse
he wanted something even smaller. He chose an 8″ f/4
mirror made by fellow ATMer Jean Pierre Grootaerd and
designed the scope around that. Why 8″ f/4? Roel says,
"For me personally an aperture of 8 inches is 'the biggest small telescope' or 'the smallest big telescope' - a
size that's very versatile at resolution, light gathering,
and portability." Roel also wanted the scope to be solid,
not just for stability in observing but also to withstand
baggage handlers if he packed it inside a suitcase. And

Roel calls this scope the "Propdob Nano" because it's tightly
compressed for transport. "Prop" roughly translates to "wad" or
"half-pint" in Dutch.

October 2016 sky & telescope

Everything packs neatly inside the box.

he wanted it sized to use without a chair, which would be
more diﬃcult to carry than the telescope.
The rest, as the old saying goes, was just engineering. All the parts would ﬁt into a hard case, so Roel used
the top and bottom of the case as the ground board and
rocker box, Sumerian Optics style. The eight three-piece
aluminum trusses ﬁt inside the box, along with the mirror box, altitude bearings, secondary ring, red-dot ﬁnder,
helical focuser, four eyepieces, Barlow, light shield,
secondary heater, shroud, solar ﬁlter, red ﬂashlight, and
collimator. Yes, all that goes inside a case 10.55″ long,
9.25″ wide, and 3.94″ high.
When the case is opened out, the trusses clamp to
the corners of the mirror box, and the clamps provide
enough wood to bolt the altitude bearings to. The eﬀective diameter of the altitude bearings is a respectable
11.5″, which provides a smooth motion and puts the
center of gravity close enough to the center of rotation to
require just a little tension of a bungee to balance.
The secondary ring is made of 15 mm plywood and
holds a three-vane spider. The 2″ helical focuser is made
out of the inner working of an old camera objective lens
- very lightweight and very low proﬁle. A red-dot ﬁnder
and a foldable 14″ light shield keep the weight of the
upper tube assembly just below 1 pound.
The primary mirror is collimated from behind with
nylon screws in what has to be the simplest system I've
ever seen. When asked if there was a push-pull spring
system securing the mirror, Roel replied, "No springs,
just bolts and gravity."

Sky and Telescope - October 2016

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