The following images show a basic platform for a Dobsonian telescope which allows the scope to track an object simply by rotating the platform about its axis. It was easy to build and the parts cost under $40.
This animated gif
shows how it works (~2Mb).
The swinging section is braced with two pieces of ply. This
seems to be the key to the strength of the whole platform. Initially I
thought the base would also need to be braced lengthways, but the
rigidity of the swinging part also strengthens the base because it is
attached rigidly to the base. The platform is surprisingly sturdy - it
easily supports 65kg (me), so the 21kg of my Dob is not an issue.
Ideally, the centre of gravity of the scope and the swinging section combined should lie close to the axis. Most of the weight of the base section of my Dob falls below the axis so it's only really the tube and a small part of the base which is above the axis. The mass of the swinging part of the platform offsets much of the weight of the tube.
and Dob base each need to be roughly balanced. The base of my Dob
needed a couple of kilos on the base opposite the handle because the
upright part of the frame is predominantly on one side of the altitude
bearing. The balancing doesn't need to be exact, the friction in the
platform prevents it from rotating freely.
The motion of the platform is reasonably smooth. Initially, the motion was a bit jerky as there was some friction between the wood and the bolts, but a little oil on the bolts has mostly eliminated the 'sticking'.
there is some wobble when moving the platform. However, without any
wind, I was still able to use a 10mm eyepiece with a 2x Barlow to view
Saturn. I suspect the wobble would be reduced by using something other
than the bolts as the bearings - maybe two short, fat axles sitting on
teflon pads a bit like a Dob's altitude bearing. One of my early ideas
was to use ball joints at each end - this would make construction quite
easy, but they might swing a little too freely. Perhaps a ball joint at
one end and something like a Dob's altitude bearing at the other? Maybe
a piece of threaded pipe screwed into a tee as in the seesaw example
below. [Update: I think a tangent arm attached toward the
the swinging part would eliminate the wobble - it would also allow
finer control of the movement. This might work well with the
The whole platform weighs about 12kg, which is about the same as the base section of my Dob. It isn't exactly portable, so it's more of a backyard fixture.
The total cost of parts was under $40. It uses approximately 5 metres of 90x45mm structural pine ($3/m), one sheet of 12mm ply ($14), two bolts, two nuts, around 25 washers and around 25 screws.
Early sketches I made:
Email 'eq' at the following domain:'brindabella.org'