Sky and Telescope - November 2017 - 73

they're an engineering challenge to say
the least. Then add to that the decision
to make the system modular so the
scope could be switched from f/4 to f/8
in the ﬁeld - by completely swapping
secondary cages. This clearly wasn't
going to be a standard Dobsonian.
For the optical systems, friend and
optical designer Roger Ceragioli started
with a 12½-inch f/4 Zambuto mirror.
To ﬁx the ﬁeld aberrations inherent
in an f/4 paraboloid and ﬂatten the
focal plane, he came up with a modiﬁed Wynne corrector. Werner says, "I
kept being a pain in the you-knowwhat to Roger, and he kept coming up
with better and better designs." What
they wound up with is a set of corrector lenses that ﬁts partly in the focuser
drawtube and partly in front of the
secondary mirror. The f/8 conﬁguration
is similar, with an apochromatic Barlow
in front of the secondary and additional
lenses built into the drawtube, which
results in a low 25% central obstruction.
Not surprisingly, the secondary
spiders are non-traditional as well.
Werner says, "I was never a fan of big
fat diffraction spikes. It looks good on a
Christmas card but not in astrophotos."
So he went with a three-vane design
using stainless steel sheet stock that's
only .003 inch thick. That's thinner
than a typical sheet of paper. Why
three, which would generate six spikes,
rather than the traditional four? Roger
explains: "A 3-vane spider has 25% less

p Many of the machined parts for Werner's scope. Some assembly required.

diffracted light than a 4-vane spider,
and since there are six spikes instead of
four, those six end up being a lot dimmer." Werner adds, "The other reason is
that I needed room for the side opening
in the secondary can for the reﬂected
light rays." The result speaks for itself:
Almost no diffraction spikes are visible
in photos taken through the scope.
The tube sections are made of
machined aluminum, which provides
excellent rigidity while keeping the
weight down. The entire scope (in either
conﬁguration) weighs 55 pounds, including 7 cooling fans on the primary mirror.
Perhaps the most impressive feature of
this entire system is the way one second-

* For more information on this innovative telescope design, contact Werner at
dwsfx2@gmail.com.

ary cage can be removed and the other
one attached, and the corrector lenses
in the focuser drawtube can be swapped
out - and when you're done you have
a completely different system without
even having to re-collimate. To accomplish that, the mating surfaces have to be
extremely ﬂat and precisely aligned.
Oh, and the secondary cages rotate
to place the eyepiece in a convenient
position in any part of the sky, also
without affecting collimation.
Then there's the pier the scope
mounts to. Werner didn't want to do
a pier ﬂip when the OTA reached the
meridian, so he designed an angled pier
for his Astro-Physics 1200GTO mount
that stays out of the way. Werner
reports, "With either system on the
mount I can fully rotate while pointing
to Polaris, so no more pier ﬂip, and I
can go nap during imaging."
The dual focal-length scope was a
great learning experience. Werner says
"It was fun to push the boundaries with
Roger, without whose design skills this
project wouldn't have happened."

■ Contributing Editor JERRY OLTION
p The Cone Nebula photographed at f/4. Note the round stars and absence of diffraction spikes
on all but the brightest stars.

welcomes your project submissions.
Contact him at j.oltion@gmail.com.
s k y a n d t e l e s c o p e . c o m * N O V E M B E R 2 0 17

http://www.skyandtelescope.com

Sky and Telescope - November 2017

Table of Contents for the Digital Edition of Sky and Telescope - November 2017