Extension tubes are used when the optical tube is too short and the focuser has not enough travel to reach the focus. This accessory increase the optical path of your telescope of 50mm.
The tube include two thumb screws to secure the camera or the accessory used. The internal compression ring avoid any scratch on your accessory.
The bottom part is threaded (M48x0.75) to allow the connection of standard 2" filters. The safety groove stop the extension to fall inside the focuser.