Amateur astronomers are sometimes very skilled and creative, but also want to enjoy astronomy without buying expensive devices.
Alternative astronomy hardware
For focusers and telescope mount control, the general idea is to add some stepper motors that are accurate and powerful enough to do the job, mount them somehow with a 3D-printed or metal-bent support, use a microcontroller and motor drivers, and finally develop the interfacing software. In many cases, the motors are better and cheaper than those found on the market devices.
Using belts instead of gears is quite popular and cheap these days too, and helps reducing backlash and periodic error.
Focusers
Arduino focus controller pro, a stepper motor focus controller (DIY) based on Arduino Nano/Uno.
Telescope mount motor control units
To be able to control your telescope mount with a hand controller or a computer, it needs to have two things: motors and a control unit. Solutions on the market are at least $400 for a small automatic slewing system, called GoTo.
Slewing interfaces
For smaller mounts like EQ3, EQ5 or EQ6 and similar mounts, which covers entry and mid-level amateur telescopes, see the AstroEQ and OnStep projects.
The MCMTII french open project targets large telescopes that have custom mounts and drives big motors.