Sky and Telescope - October 2015 - 72

Gary Seronik
Telescope Workshop

Meet Leah
This 16-inch reﬂector is a marvel of craftsmanship and engineering.
Some people like to name their telescopes. As

TOM FAIRBANKS

Wichita, Kansas ATM Tom Fairbanks explains, "I have
two beautiful daughters, but despite my pleas, my wife
wouldn't agree to name either of them Leah. One day,
as I sat looking at my telescope, it occurred to me that
since I was solely responsible for bringing this baby into
the world, I could name it whatever I wanted - Leah
it would be." In this case, the name not only suits the elegance of the telescope, but it also serves as a humorous
(and painfully true) acronym: Large Expensive Astronomical Hobby. But Leah is much more than a clever
name - she's also a pretty amazing telescope.
A professional electrical engineer and hobbyist woodworker, Tom was up for something more challenging

This 16-inch, f/4.2 Newtonian rides on a split-ring mount - a
conﬁguration that originates with Russell Porter, designer of the
famed 200-inch Hale Telescope. The 16-inch features a rotating
upper cage and computer-controlled go-to pointing.

October 2015 sky & telescope

than a plain-Jane Dobsonian for his ﬁrst scope. "After
searching the internet for ideas, I found a few images
of the split-ring design," he recalls. "It had that cool
look I was going for and oﬀers the advantage of being a
true equatorial mount." This was important because he
wanted an instrument that would be suited to computerized pointing and tracking. Of course, such a mount is
considerably more complex and diﬃcult to build than a
Dob; doubly so for a ﬁrst-time builder.
To ensure that he understood the design in detail,
Tom built a ¼-scale model as a ﬁrst step. This exercise
demonstrated a few important consequences of the splitring conﬁguration. As Tom recalls, "while playing with
the model, I realized that the eyepiece could not remain
ﬁ xed and needs to be able to rotate to any side of the
tube." The model also showed that the mount's bearings
would have to cope with both radial and axial forces, and
that a traditional Dobsonian-style sling mount used for
supporting the primary mirror wouldn't suﬃce for all
the scope's various orientations.
To solve the ﬁrst of these issues, Tom decided to
equip Leah with a rotating upper cage so that the eyepiece could easily be orientated to the most comfortable
viewing angle. "For the upper cage assembly, I settled on
thin, interlocking rings of plywood kept in position by
small rollers originally made for a sliding shower door,"
Tom says. One of the cage's best features is that the rotation is friction-controlled - there's no need to clamp or
unclamp anything to change the eyepiece position.
Leah's 2-inch-thick, 16-inch f/4.2 primary mirror
sits on a conventional 18-point ﬂotation cell. Nothing
unusual about that. What is clever, though, is how the
mirror is supported laterally. Instead of a Dobsonian's
single sling, Tom's cell incorporates what he calls a
"quad sling system" - essentially four individual slings
arrayed around the mirror's circumference. Each one
supports part of the load in turn as the orientation of
the scope changes. Turnbuckles allow the tension of all
four slings to be adjusted and ensure the mirror remains
exactly centered in the cell.
Critical to the scope's success was the quality of the
bearings used in the mount. Tom's initial choice of gokart axle bearings proved to be unsuitable. In the end he

Sky and Telescope - October 2015

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