This Apertura 25mm Extension Tube is designed for the GSO 10" Ritchey-Chretien Astrograph, but it will also work with the GSO 12" RC or any product that accepts M117 threads. It is meant to be placed between the telescope and the focuser to extend back focus by 25mm. You can stack different lengths of extension rings to achieve the back focus you require for your imaging set-up. With an extension added at this location, you won't have to extend the draw tube as far, which will minimize downward tilt with heavy loads.