8" Stellascope two-pole
Honorable mention for craftsmanship, Stellafane 2005
Early in the summer of 2005 I found I had the opportunity to attend
the Stellafane convention coming up in July. I wanted to bring a
telescope and just for fun enter it in the scope competition. My initial
idea was to build a "scope on a board" like those popularized by
Russell Porter and the Springfield Telescope Makers back in the
1920s. To make it special I would replace the center section of the
solid board with two aluminum tubes, and make each component
small enough to fit in an airline carry-on bag.
The optical tube was built mainly of softwood core-oak veneer ply,
with some solid oak, finished with a red mahogany stain and
Initially I planned to mount the scope on a short tripod with an
alt-az head fabricated from aluminum angle. This arrangement
proved too rickety, so in a considerable panic (I only had three
days before Stellafane) I turned to an idea that was in my design
notebook from a few years earlier.
This hybrid "dobsonian-fork" mounting used a conventional dob
trunnion on one side and a pivot on the other, carried by what is
essentially half of a fork mounting. Surprising to me at the time,
this worked very well, with much smoother motions and less
vibration than my earlier tripod idea.
The plate glass 8" f/4.5 mirror is in a very shallow mirrorbox, protected
by a hinged lid. In use the lid is held up by a rare earth magnet.
The dobsonian bearings are Teflon running on Ebony Star laminate.
The fork pivot is a stainless steel machine screw run through a bronze
bushing, with Teflon washers. Both the pivot and the azimuth axis are
clamped by plastic knobs with nylon inserts. I can "dial-in" as much
friction as I need and the knobs won't loosen as the scope turns. The
ability to control friction is critical in a lightweight travel telescope.
The top end of the scope is reduced to a focuser board,
secondary holder, helical focuser, and red-dot finder. The
curved secondary holder is fabricated from 1/8" thick aluminum
The Kydex plastic secondary baffle is sized to prevent light
behind the secondary mirror from getting to the eyepiece. To
keep it small, it is placed inside the light path attached to the
secondary holder itself. Since the primary mirror "sees" it edge
on it has little impact on the image.
A diaphragm of Kydex plastic helps baffle the helical focuser. This
also helps keep the size of the secondary baffle down.
My first Stellafane! Here I am with the Stellascope (not a very
original name--probably most newbies like me thought they were
being clever giving their scopes that name).
During the Saturday morning telescope competition on Breezy
Hill I set up on the rock in front of the Porter Turret Telescope,
seen in the background.
The best part of every competition is talking telescopes with
people walking by. I got great feedback (and lots of
encouragement) from the wonderful conventioneers. I was
...and here's the toughest part: the judging. Stellafane
judges come by in two groups, sometimes each visiting twice.
They are always perfectly polite and encouraging, but you
can see them zero in on a telescopes faults as the give each
instrument a thorough going-over.
They immediately noticed a slightly wobbly azimuth bearing
(damn--I knew I should have put a bushing in there!). Also,
the rubber feet on the ground board added their own bit of
shimmy and sway. Lesson learned!
And yet, that evening at the awards ceremony in the natural
amphitheater they awarded my scope an honorable mention
for craftsmanship. I was totally thrilled, and grabbed the
microphone and babbled my thanks. It was a fantastic
experience, and energized me for future competitions.
Ross Sackett's amateur telescope making
My telescopes and ATM Projects