New Project - 18” Ultracompact Dobsonian

Draft Specification:


18” f4.5 (mirror set shared with my main observatory scope)
Starlight Instruments Feathertouch Focuser with integrated Paracor.


Total Weight; about 30kg
Size; will fit in the boot of my car, (except truss poles!)
Max eyepiece height; 2.1 metres
Truss poles; 20mm Aluminium tubes covered in black
heat shrink. Each side
fixed as a pair.


Geared stepper motors driving through toothed belts on both axes.
10000 tick encoders
Nexus DSC to SkySafari for scope position readout & control
Raspberry PI computer driving motors through Phidget modules

Phase 1:I started calling this an Ultralight Dobsonian, but during the build realised that it wouldnt be stiff enough unless I significantly increased the weight, especially of the top cage and truss poles. This is mostly due to the weight of the focuser, intergrated Paracor, and 21mm Ethos eyepiece! Weight isnt so much an issue anyway - what is important is that I can get it to UK Star Parties in my car, along with all my camping stuff, and my wife! For transport the top cage and base will sit on top of the rocker assembly, within the rocker sledges. This produces a total size of 90x56x46 cm (36” x 22” x 18”). The truss poles assemblies can either go on the roof, or will fit between the seats inside the car. Photos of Phase 1 here.

Phase 2: testing with the new servos showed that the PTFE bearings were giving too much friction. I have now replaced them with ball bearings and now it runs very smoothly and easily. I will even have to add some damping for when I use the scope in manual“push tomode. Have added the encoders, wifi link and starting to finish off the wiring and installation of ancillaries. Photos of Phase 2 here.

Phase 3: Having proved the drive system to work very well, the decision was made to go into small scale series production! Working with Craig we designed a compact unit built into a high quality aluminium case, laser etched panels and improved user setup and interface. Photos of the production standard are here. The drive is available from

AstroKeith 2018