Sky and Telescope - August 2018 - 70

ASTRONOMER'S WORKBENCH by Jerry Oltion

Wringing the last ounce of performance out of a scope.

t Steve Hosein has fine-tuned
his telescope into a high-contrast, high-quality instrument.

IN HIS YOUTH, Steve Hosein spent

many evenings lying back on the teetertotters in a neighborhood park looking
at the stars. In later years, he saw the
sky from a Moroccan desert, the Peruvian Andes, and at sea near Tasmania.
He was used to dark sky and crisp stars
even before he began using a telescope,
so it's no surprise that high contrast
and quality were paramount concerns
when he acquired a set of mirrors and
began to build a scope of his own.
But other than using excellent optics,
what can a person do to improve the
view through a standard Dobsonian
telescope? Turns out, there's quite a bit.
Cooling the primary mirror is the
ﬁrst and most obvious thing. A mirror
70

AUGUST 2 018 * SK Y & TELESCOPE

that's warmer than the
air around it will warp,
and air currents will
form in the light path,
leading to even more
distortion. Yet in an
enclosed tube, primary
mirrors spend most of
the night cooling down,
seldom achieving ambient temperature until
early morning. People use
fans to blow air against
the back of the mirror,
and sometimes fans
in front to sweep away
the boundary layer, but
there's still no place for
the warm air to go except
up the tube, causing turbulence the entire way.
So Steve drilled a
bunch of 1¾-inch-diameter holes in the tube near
the primary mirror. He
has two rows in front of
the primary, one even with the front
face, and one behind. Those holes allow
air to circulate around the primary mirror and exit the tube without running
all the way forward, cooling the mirror
quickly and effectively.
But holes in the side of the tube
allow light from the sides to shine in,
reducing contrast in the image if any of
that light makes it to the eyepiece. So
Steve built a shroud for the holes. It's a
lightweight shield made from thin plastic that wraps around the holes after the
mirror has cooled, effectively blocking
any unwanted light.
There's still plenty of light entering
the front of the tube, though, and the
only part of that you want is the light

p Ventilation holes allow air to flow around
the primary mirror, cooling it quickly. A shroud
covers the holes to reduce stray light during
observation, and a moveable counterweight
allows precise balancing of the telescope.

G A RTH TAY LOR (3)

An Optimized Dob

from your target object. Anything else
can brighten the tube walls and reﬂect
around until it enters the eyepiece,
reducing contrast. Refractors use bafﬂes
along the length of the tube to minimize this, but reﬂectors can't do that
because any bafﬂes below the secondary
can reﬂect light back up into the eyepiece. However, Steve realized that the
portion of the tube from the secondary
upward could still be bafﬂed, signiﬁcantly reducing light incursion. So he
built a set of closely spaced bafﬂes that
do just that. He used sheet aluminum
cut with a Dremel tool and epoxied
10 rings together into a framework he
could slide into the tube.
He painted the bafﬂes and the inside
of the tube with an ultra-ﬂat black
paint called "scenic black" formulated
for the ﬁlm industry. The result is an
ultra-dark interior.
The traditional four-vaned secondary
spider is another source of stray light,
this time in the form of diffraction.
Reasoning that fewer, thinner vanes
would reduce this diffraction, Steve
made a three-vane wire spider using
0.008-inch-diameter guitar strings. The
wire attachment points are carefully
placed one right over the other so their
cross section from the primary mirror's
perspective is minimal. They're held
with guitar tuning pegs on the outside
Sky and Telescope - August 2018

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