Tele Vue's 2" ATT-2125 Paracorr Tunable Top permits precise positioning of all eyepieces for optimum coma correction when mounted to the BIG Paracorr Type-2. Simply follow the included guide to find the correct position on the the sliding scale and set the position by tightening the top's thumbscrew. It requires the Visual/Imaging System Adapter to mount to the Paracorr. A 1.25" adapter is included to use smaller-barrel eyepieces.