Sky and Telescope - November 2016 - 66

Jerry Oltion
Astronomer's Workbench

Braking Bad Habits
How to make your lightweight scope behave.
If you've ever used a lightweight Dobsonian

ALL PHOTOS BY GIUSEPPE VIARIZZO

telescope, you've probably experienced the frustration of
having the scope head for the zenith when you remove
the eyepiece. When a scope is so light that the eyepiece
weight is a significant fraction of the total, its removal
will throw the balance wildly askew.
Italian ATM Giulio Tiberini has come up with a
simple yet elegant solution: a disk brake on the altitude
bearing. Normally closed, the brake keeps the scope
locked in position until you squeeze the handle, which
loosens the brake and allows vertical adjustment. Let
go of the handle and your scope won't rise even if you
remove the whole secondary cage.
It's an idea that's been staring us in the face for generations. Altitude bearings are perfect disk-brake rotors,

66

Giulio Tiberini with his 300-mm f/6
scope equipped with a disk brake.

November 2016 sky & telescope

just begging for a caliper to hold them firm when you
want the scope to stay put. Yet most of the brake systems
I've seen involve increasing friction rather than locking
the bearing's motion completely. Increased friction just
makes the scope harder to move, which isn't an ideal
solution to the problem.
The disk brake is a much more elegant answer. As
Giulio says, "The presence of side bearings suggests a
disk-type brake. Their curved shape of large radius provides a good mechanical moment of resistance, and they
seem made to allow a simple and practical installation."
Disk brakes require only two parts: a disk (commonly
called the "rotor") and a caliper to squeeze and hold that
disk. For his caliper, Giulio used a clamp from a car
battery charger and lined the jaws with polyurethane
blocks. For the rotor he didn't use the altitude bearing
surface itself, because that was too thick for the caliper.
Instead he added a steel semicircle, cut with a hand held
jigsaw from 2-mm steel plate, to the side of the bearing.
He mounted the caliper so the rotor would slide through
it freely when the caliper was opened and would be
locked tight when the caliper was closed.
That would have been sufficient, but it would require
reaching down to loosen the caliper any time he wanted
to change the scope's aim in altitude - impossible to
do while hunting down or tracking an object unless
you're using a very small telescope. But Giulio's scope
is taller than he is. He needed to be able to open and
close the caliper while looking through the eyepiece, so
he installed a bicycle brake lever on one of the trusses
where it would be within easy reach. When he wants to
move the scope, he simply squeezes the lever against the
truss (where he would normally hold the scope anyway)
and pushes the scope as he would with any Dobsonian
mount. When he reaches the target, he lets go of the
handle and the brake locks the scope in place.
Of course, if the scope is imbalanced with the weight
of a heavy eyepiece, there's some flexure. As Giulio says,
"The mechanical parts can never be perfectly rigid,
so there is a little backlash due to the structure of the
telescope and the settling of the caliper under load. Thus
you learn to adjust your aim a little above the target, but
it's easy because the brake is so intuitive to use."



Sky and Telescope - November 2016

Table of Contents for the Digital Edition of Sky and Telescope - November 2016

Contents
Sky and Telescope - November 2016 - Cover1
Sky and Telescope - November 2016 - Cover2
Sky and Telescope - November 2016 - 1
Sky and Telescope - November 2016 - Contents
Sky and Telescope - November 2016 - 3
Sky and Telescope - November 2016 - A
Sky and Telescope - November 2016 - B
Sky and Telescope - November 2016 - 4
Sky and Telescope - November 2016 - 5
Sky and Telescope - November 2016 - 6
Sky and Telescope - November 2016 - 7
Sky and Telescope - November 2016 - 8
Sky and Telescope - November 2016 - 9
Sky and Telescope - November 2016 - 10
Sky and Telescope - November 2016 - 11
Sky and Telescope - November 2016 - 12
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Sky and Telescope - November 2016 - 17
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Sky and Telescope - November 2016 - 83
Sky and Telescope - November 2016 - 84
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