This is a remote focus attachment for an 8 inch Newtonian telescope. I suppose this is mostly useful together with a PC-controlled camera, where you can focus without disturbing nor being near the telescope. Automated focus would also be possible with some suitable software, although I have not tried this. The attachment is split into one part that attaches to the focuser base, and one that holds the servo. In addition there is a cup that connects the servo axle to the focuser knob. This makes it possible to revert to manual mode when needed, or to adjust the range of focussing, simply by removing the part that carries the servo. The design is parametric within reason, so you should readily be able to adapt to other size telescopes, other focusers and other servos. The RC servo can be used as is, which will provide a repeatable focus (within the mechanical limitations of the focuser - always approaching from one side will help). This mode will limit travel to about half a revolution of the screw, which is enough for most cases. A simple interface is to use a common RC servo tester (available for $5 or so) as the controller, powering everything from the 5V of a USB port. More sophisticated setups would control the servo by a computer, for instance a tiny Arduino. The alternative way is to strip the internals of the servo, and use only the motor and the gears. In this mode, there is no limitation to travel, but also no feedback on the absolute position. A simple 3 position DPDT switch (spring loaded in both directions ideally) can be used to control the motor. Yet another alternative is to use a continuous rotation servo, like this one, available from Pololu: http://www.pololu.com/product/2149 (I have not tried this nor verified its suitability) You will also need: 1 powerful RC servo, e.g. HK15328A, with mounting hardware as supplied 5 M3x20 screws (3 is really enough) 3 M2.5 countersunk screws with nuts for the knob to servo adapter, or simply use a foam pad with glue All 3 items should build readily from the STL files, without support and with little or no need for cleaning. I used PLA.