Sky and Telescope - April 2015 - 70

Gary Seronik
Telescope Workshop

Build the Art Swivel
Here's a great alternative to traditional altazimuth designs.
I'm struck by how frequently diﬀerent telescope
makers arrive at similar solutions when confronted with
similar problems. Remember Chuck Lott's "pseudo-ball"
telescope mount that I featured in the December 2013
issue? By coincidence, just as that issue was going to
press, I received an e-mail from Art Gamble of Wichita
Falls, Texas, describing a remarkably similar design. His
implementation was a little diﬀerent, but the root of the
idea - a ball-and-socket mount utilizing a segmented
"ball" - was the same. Like Chuck, Art was looking for a
way to improve on the conventional altazimuth mount.
The diﬃculty with ball-and-socket mounts is ﬁnding
a rigid ball of the necessary size. It's probably the main
reason the design isn't as popular as it could be, given its
utility. "My goal was to build a mount for my 90mm f/8.8
refractor that operated like a ball-and-socket, but without

ART GAMBLE

As Art
Gamble's wife
Sara demonstrates, a balland-socket
mount can be
an excellent
choice for a
small refractor like their
90mm f/8.8.

April 2015 sky & telescope

the ball," Art says. The solution that he, Chuck Lott, and a
few others arrived at is making a virtual ball from several
plywood arcs.
When you sit down to build one, the ﬁrst questions to
arise are how many arcs, and how big do they need to be?
Experimentation led Art to conclude that ﬁve or six arcs
are optimal to ensure that the "ball" would always have
several points of contact with the socket. The dimensions of the individual arcs depend on the diameter of the
telescope tube and the size of the socket base. The ball
should be big enough to enclose the rear of the telescope
tube and have enough extra material to ensure a rigid
structure. Once you settle on the approximate size of the
ball, you can select a suitable base and then determine
the dimensions of the ball segments. "I came up with a
simple formula: multiply the diameter of the base ring
by 0.572 to calculate the radius of the arc segments," Art
says. The arc sections for his scope have a radius of 6½
inches, which, when assembled, simulate a ball 13 inches
in diameter.
Art cut each arc from ¼-inch plywood, though larger
scopes would obviously beneﬁt from thicker wood. "I
made a template of stiﬀ, thin cardboard and traced the
shape on the plywood, which I cut on a band saw, leaving about 1/8 inch of extra material," he recalls. Next he
stacked the segments and clamped them together for
ﬁnal shaping. "I used a combination belt and disk sander
to reﬁne the pieces and ensure uniform curves."
The mount segments were ﬁnished with several layers of clear polyurethane, including the edges that slide
against the rim of the socket. The socket is simply the top
of a 5-gallon plastic bucket reinforced on the bottom with
a disk of ¾-inch plywood. The base is aﬃ xed to a sturdy
surveyor's tripod. Although the bucket has a bit of ﬂexibility, this actually turns out to be a beneﬁt since it helps
ensure good contact with the ball.
One of the problems of a ball-and-socket mount when
used with refractors like Art's (and small Cassegrains) is
that the eyepiece ideally should be positioned as near the
center of the ball as possible. However, doing so results in
a front-heavy conﬁguration. To cope with the imbalance,
Art cast an 8-pound lead counterweight and aﬃ xed it to
the rear of the ball.

Sky and Telescope - April 2015

Table of Contents for the Digital Edition of Sky and Telescope - April 2015