"circus cannon" dob
During the summer and fall of 2000 my design notebook was filling up with lots
of clever telescopes, but nothing was getting built. I had an Orion 10" f/4.5
mirror set, so I decided to build a quick conventional "sidewalk" scope using
John Dobson's plans. I didn't intend it to be pretty, so it would be made of
cardboard concrete form tube and 3/4" shop plywood. At the time I was
fascinated by architect Frank Geary's early "construction site tech" buildings,
so I decided that the telescope would have a similar look.
I had never used a dobsonian before, so I was thoroughly impressed by the
smooth motions and stable views. But the weight! At 75 pounds, the scope
as originally built was way too heavy for casual skywatching, so I immediately
began to cut material away to lighten the load.
I trimmed plywood from the tube box and significantly shortened the rocker. This
shed about 15 pounds of 3/4" plywood, which helped a lot. I also rebuilt the
ground board from its original easy-to-cut square to a triangular shape.
On the belly of the scope I attached a strip of Velcro so that I could adjust the
balance with a shot-filled "bean-bag". This worked great.
I used a very light yellow stain on the plywood to match the unpainted varnished
cardboard tube. Its takes a lot of effort to get that informal construction-site look!
Even with the cut-outs the tube box is massive. I
easily could have built the scope using 1/2" ply
with little sacrifice in stiffness. This was a very
good learning experience for my later telescopes.
I think handles are a must on telescopes.
Whenever I build a scope I think about how I am
going to get it in and out of the house, transport it
in a car, and set it up.
Look at that groundboard! How embarassing--the thing is way
overbuilt, just like the rest of the telescope. I am blushing.
Both the azimuth and altitude bearings are of virgin Teflon pads
running on Ebony Star laminate. The motion is very smooth and
the telescope holds position nicely when I let go.
Dob owners always crow about the "buttery-smooth" motion of
their scopes. I didn't really know what they meant until I waxed and
Armoralled the bearings. Then the scope pretty much OOZED in
the direction I pushed it. Very sexy.
I had a nice crayford focuser but I thought it would be
overkill for the scope (and yeah, it wouldn't fit that
post-apocalyptic aesthetic). I bought an inexpensive
Delrin helical focuser that works fine.
The spraypaint around the focuser looks like hell but
greatly helps to cut the glare in my bright Memphis back
yard. I am sure that this will be the last city in America
to introduce dark-sky legislation. It's much worse even
than my hometown of Los Angeles.
The secondary holder is just a chunk of pine and
three cedar shims. You adjust it by sliding the legs
around the inside of the cardboard tube. It seems
crude, but it works very well. Dobson is a clever guy!
But yes, there are six bright diffraction spikes around
every object. Jupiter looks very festive.
Following the instructions I peeled the waxy lining
away from inside of the tube before spraying it with
Krylon Ultraflat black. This took away a layer of
paper, which weakened the tube significantly. I later
discovered that the "wax" lining (actually a polymer
film) takes paint just fine and this was unnecessary.
Ross Sackett's amateur telescope making
My telescopes and ATM Projects