Sky and Telescope - August 2015 - 36

Gary Seronik
Telescope Workshop

An f/13 Holcomb Reflector
This sturdy planetary Newtonian utilizes a neglected mount design.
It may be true

VAUGHN MALKIN

that there's nothing new under the
Sun, but when it comes to building telescopes, there are
so many diﬀerent approaches that it can take a while
for an idea to resurface once it has fallen out of favor.
Yet sometimes a particular observing goal leads down
a nearly forgotten road, as the long-focus 8-inch Newtonian reﬂector built by Dunedin, New Zealand, telescope
maker Vaughn Malkin demonstrates.
To begin with, Vaughn's scope bucks the current
trend for fast and superfast primary mirrors with focal
ratios of f/4 or less. To satisfy his planetary observing
needs, he went long - all the way to f/13. Such mirrors
aren't as easy to ﬁgure as is generally believed.

Although unusual in appearance, form follows function with this
long-focus planetary reﬂector built by New Zealand telescopemaker Vaughn Malkin. The scope is ideally suited to provide
detailed views of the Moon and planets.

August 2015 sky & telescope

Apart from a generous image scale, a long-focus primary also allows the use of a small secondary mirror to
minimize diﬀraction eﬀects that can mute low-contrast
planetary details. Vaughn's scope is equipped with a secondary mirror only 32 mm (1.26 inches) across its minor
axis, yet it still maintains a generous fully illuminated
ﬁeld. This size amounts to only a 16% obstruction - well
under the 20% recommended for a planetary reﬂector.
With the optics sorted, the next challenge was to
come up with a tube assembly rigid enough to hold the
mirrors in perfect alignment as the telescope is aimed at
diﬀerent parts of the sky. At 9 feet long, Sonotube wasn't
going to be the answer. After some hunting online,
Vaughn secured a length of 10-inch-diameter unplasticized polyvinyl chloride (uPVC) tubing. Although more
expensive than regular PVC, uPVC is substantially more
rigid. The piece was too short for the mirror's focal
length, so he cut the tube in two and joined the sections
with eight 20-mm-diameter aluminum poles clamped at
each end with pillow blocks aﬃ xed to the uPVC pieces.
The tube housing the primary mirror is 35 inches long;
the upper section is about 62 inches. "The assembly
turned out to have plenty of rigidity," Vaughn reports.
In addition to its long tube, the telescope's other striking feature is its Holcomb mount. This obscure design
saw its heyday in the middle of the 19th century. "I
stumbled across the Holcomb mount only after countless
hours trawling the internet," Vaughn says. The website of
well-known telescope maker Mel Bartels (bbastrodesigns.
com/HolcombeMount.html) provided the solution.
The mount was invented by America's ﬁrst commercial telescope maker, Amasa Holcomb. And while it may
look unusual, it's an elegant solution to the problem of
pointing a telescope with a very long tube. How does it
work? Think of a tripod in which one of the legs is the
telescope itself with its back end resting on the ground.
The scope is crudely aimed by picking up the front legs
and pivoting the scope toward the region of sky you
want to examine. Fine motion is facilitated by adjusting
the lengths of the two front legs by means of two pulley
systems - one for each leg. Adjusting both legs raises
or lowers the front of the scope, while adjusting just one
causes the telescope to move diagonally, allowing objects

Sky and Telescope - August 2015

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