Sky and Telescope - March 2016 - 64

Gary Seronik
Telescope Workshop

A 20-inch Dream Scope
This instrument is more than a Dobsonian alternative.
A lot of dobsonian-style

PIERRE LEMAY

telescopes have
appeared in this department over the years, and for good
reason. They're popular because they have the potential
to deliver plenty of aperture in an eﬀective, simple-to-use
package. But another design that shares these attributes
is the so-called ball scope. Indeed, but for one signiﬁcant
detail, they would probably be more popular than Dobs.
And what is that detail? In a word, the ball. More about
that in a moment.
The attributes of the ball-and-socket design are well
demonstrated by this 20-inch, f/3.9 reﬂector built by
Blainville, Quebec ATM Pierre Lemay. He describes it as
his dream scope, and I suspect his dream is one shared
by quite a few readers. For Pierre, the "dream" includes

Building this 20-inch f/3.9 ball-and-socket telescope took Quebec telescope maker Pierre Lemay took more than 20 years. The fully assembled
instrument tips the scales at 130 pounds, including its innovative, dualaxis-drive platform.

March 2016 sky & telescope

large aperture and motorized tracking in an instrument
that is comfortable to use and lightweight enough to be
easily transported and set up. But Pierre's dream took
more than 20 years to come true.
"The project began in 1990 when I saw a classiﬁed
ad in Sky & Telescope for a 20-inch, conical Pyrex mirror
blank advertised for $500," he says. "It was a rough cast
piece of glass that needed a lot of work before grinding could even begin." First stop was a local tombstone
maker's shop, where a curve of about the correct depth
was generated by sandblasting. Next, Pierre attacked
the rear surface of the blank with a grinding wheel and
jig to thin it and take the edge thickness down from 1
to ½ inch. This reduced the mirror's weight from 50 to
32 pounds. Additionally, he perforated the center of the
mirror to facilitate its mounting in the scope later. After
that, it was simply a matter of grinding and polishing a
20-inch f/3.9 paraboloid!
Following the primary mirror, the next big hurdle
was fabricating a "ball." This really is the make-or-break
part of the design. Failing to ﬁnd a suitable ready-made
hemisphere, Pierre resigned himself to making one, or
rather, two. "My ﬁrst attempt recalls the joke about how
the operation was a success, but the patient died," he
says. "But I learned a lot and developed the methods I
used in my second try."
To begin, Pierre made a crude sphere by stacking
2-inch-thick disks of Styrofoam in incrementally varying
diameters - like a layer cake. He included an aluminum plate on the bottom (to support the mirror cell),
and plywood ring on top for attaching the truss poles.
Next, he used a router mounted in a large, pivoting jig to
generate a smooth shape from the stair-step layers. The
resulting Styrofoam hemisphere would serve merely as
a form, over which Pierre applied auto-body ﬁlling putty.
Once the putty hardened, the assembly went back into
the jig for more routing and then sanding. The resulting
hemisphere received eight coats of liquid epoxy resin
(sanded between each coat) for a smooth, strong ﬁnish.
All that remained was to separate the sphere from the
form by breaking up the Styrofoam. "I was then left with
a hollow, 3/16 -inch-thick hemisphere weighing about 40
pounds," he says.

Sky and Telescope - March 2016

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