Australian Sky & Telescope — January 01, 2018

http://www.skyandtelescope.com.au 73

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 31.75-cm (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-know-what 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
inthefocuserdrawtubeandpartly
infrontofthesecondarymirror.The
f/8configurationissimilar,withan
apochromatic Barlow in front of the
secondary and additional lenses built
into the drawtube, which results in a low
25% central obstruction.
Notsurprisingly,thesecondary
spidersarenon-traditionalaswell.
Wernersays,“Iwasneverafanofbig
fat diffraction spikes. It looks good on a
Christmascardbutnotinastrophotos.”
Sohewentwithathree-vanedesign
usingstainlesssteelsheetstockthat’s
only .076 mm thick. That’s thinner
thanatypicalsheetofpaper.Why
three, which would generate six spikes,
rather than the traditional four? Roger
explains:“A3-vanespiderhas25%less

diffracted light than a 4-vane spider, and

sincetherearesixspikesinsteadoffour,

thosesixendupbeingalotdimmer.”

Werner adds, “The other reason is that

Ineededroomforthesideopeningin

thesecondarycanforthereflectedlight

rays”.Theresultspeaksforitself:Almost

nodiffractionspikesarevisibleinphotos

takenthroughthescope.

Thetubesectionsaremadeof

machined aluminium, which provides

excellentrigiditywhilekeepingthe

weight down. The entire scope (in either

configuration)weighs25kg,including7

coolingfansontheprimarymirror.

Perhaps the most impressive feature

is the way one secondary 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 [the pole], 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.”

For more information, contact Werner at

[email protected].

■ JERRY OLTION welcomes your

project submissions. Contact him at

[email protected].

SMany of the machined parts for Werner’s scope. Some assembly required.

SThe Cone Nebula photographed at f/4. Note the round stars and absence of diffraction spikes
on all but the brightest stars.