I have built a DIY off-axis guider (OAG) using a mirror from a DSLR camera, some tube extenders (2 in and 1.25 in diameter), and a webcam. Best guiding performance currently at 0.33″ (arcsecond) RMS error, at 900 mm focal length, using a mount with DIY controller.
In off-axis guiding, the telescope functions both as an imaging scope and a guide scope. In this configuration, a mirror or a prism receives a portion of the light without blocking the main imaging sensor, sending the light to a guide camera. In this build, I used a high-quality mirror I happen to have found in a non-working Canon 1100D. To build the OAG, I removed the lens from a Barlow so I could get a 1.25 inch barrel for the webcam attachment, and then fastened it perpendicular to a 2 inch extender, where an appropriate side hole has been made. I then fabricated a small mirror mount (like a secondary mirror mount in a Newtonian) using some brass material, to send the reflected light on to the side. The placement of the mirror and the proper spacing to achieve focus required trial-and-error. To use the OAG, focus the main camera first, and then slide the guide camera in or out to achieve focus.
Related link: View all home-brewed DIY astronomy equipment
Night Sky in Focus | Astronomy and Amateur Radio © Anthony Urbano | Manila, Philippines