Amateur astronomers are sometimes very skilled and creative, but also want to enjoy astronomy without buying expensive devices. This page tries to list active and open source projects that offer alternate solutions to market products, for lower cost or higher quality.
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. An updated version can be found on github, called the ArduiStepFocuser.
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.
Guiding only interfaces
Weather station
A proper weather station that can detect cloud coverage and rain, which is mandatory for remote or automatic observatories, costs on the market at least $350. Two approaches seem to exist for the cloud coverage: infrared sensor and a Peltier device that measures temperature difference between the ground and the sky.