Sky and Telescope - September 2015 - 72

Gary Seronik
Telescope Workshop

Stilburn's Hyperbolic Newt
This coma-corrected telescope gives a sterling performance.
I've always had great admiration for telescope

GARY SERONIK

makers who not only craft ﬁne optics, but also have the
talent to come up with new designs. Many of us can
grind a decent mirror for a Newtonian reﬂector but
couldn't even begin to concoct a new optical system.
Similarly, there are computer jockeys who can crank
out interesting telescope recipes but have never actually pushed glass - and a design isn't a telescope until
it's actually built. ATM Jim Stilburn of Victoria, British
Columbia, is one of the few that can do both very well.
Readers may recall Jim's previous eﬀort, a 6-inch corrected Dall-Kirkham that was featured in the August 2013
issue (page 66). That telescope was the spark of inspiration for his newest creation, a 10-inch f/5 hyperbolic
reﬂector. "My 6-inch provided tantalizing views at the
Costa Rica Southern Sky Party, but it was only a matter of
time before I came down with aperture fever," Jim says.
The ﬁrst step in curing the fever was to sit down at
the computer to start working up a design using Zemax

Victoria, British Columbia, ATM Jim Stilburn has come up with another
gem: a 10-inch f/5 hyperbolic reﬂector of his own design. The instrument is
very portable and breaks down for airline travel.

September 2015 sky & telescope

optical software (zemax.com). Jim chose a 10-inch
aperture for a full magnitude brightness gain over his
6-inch, and a focal ratio of f/5 for the sake of portability. But the coma inherent in a f/5 Newtonian troubled
him. His solution was a design that utilizes a hyperbolic
primary mirror with a matching corrector lens. "Having chosen the aperture and focal length, I was left
with three degrees of freedom: the conic constant of the
primary mirror, and the positions of the two corrector
elements," he notes.
To achieve coma-free performance, Jim elected to
make a primary mirror with a conic constant of −1.35
(which is "overcorrected" relative to the paraboloid used
in a conventional Newtonian) matched with a 2-element,
sub-diameter corrector lens. As he recounts, "After
much brainstorming with Zemax, I realized that excellent ﬁeld corrections could be obtained by including just
two simple lenses ahead of the focal plane."
Jim added one additional constraint to keep the
project manageable - the corrector had to utilize lenses
purchased oﬀ-the-shelf. Experimenting with diﬀerent
lens types and spacings in Zemax, he ultimately settled
on pairing a plano-concave lens with a double-convex
element. Both parts were purchased from Newport
(catalogue numbers KPC070 and KBX166, from newport.com) for a bit more than $100 and have broadband
anti-reﬂection coatings. The lenses are mounted in a
cell attached to the base of the focuser and positioned a
ﬁ xed distance from the focal plane. The only drawback
to this design is the distance to the focal plane requires
a slightly larger diagonal mirror (2.14-inch minor axis),
which gives a central obstruction of 23 percent.
As important as the rest of the telescope is, half the
work went into grinding and ﬁguring the primary mirror.
"The most diﬃcult part was making the mirror," Jim
says. "But 'diﬃcult' isn't quite the right word, because
designing and making any part of a telescope is pure
enjoyment." Fabricating the primary went smoothly and
was accomplished with the same techniques used to produce a regular paraboloid, except, of course, for the extra
work needed for the hyperbolic ﬁgure. Although a standard Foucault test would suﬃce, Jim prefers the simplicity
of the Ross null test.

http://www.zemax.com http://www.newport.com http://www.newport.com

Sky and Telescope - September 2015

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