Equatorial cradle

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 platform swings between the two bolts which attach it to the base. The angle between the bolts is set at 35 degrees (my latitude). The virtual axis formed between the two bolts needs to be aimed at the celestial pole. This animated gif shows the platform in action (file size is ~2Mb). The platform has approximately 15 degrees range of motion - the range is inherently limited by the crossbeam of the swinging section hitting the central beam of the base.

The swinging section is braced with two pieces of ply. This bracing 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.

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'.

Unfortunately, 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 bottom of the swinging part would eliminate the wobble - it would also allow finer control of the movement.  This might work well with the ball joints.]

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:

Other alternative tracking ideas


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