For Astro T-2 & Other Modular Systems 28mm Insertion Length 3mm T-2 Male Thread on Eyepiece Side 1.25″ Outer Diameter on Telescope Side M28.5 Filter Thread 1.5mm Optical Path Length The Baader 1.25″ T-2 Nosepiece from Alpine Astronomical is a barrel-style adapter compatible with Alpine’s Astro T-2, Baader Planetarium, and Celestron systems. It’s designed with a 28mm insertion length and proprietary Baader safety kerfs. On the eyepiece side, it features a 3mm T-2 male thread designed for modern digital cameras, which generally have a short optical height that works better with the shorter 3mm thread than the older 5mm type. On the telescope side, the nosepiece has a 1.25″ outer diameter. The nosepiece offers an M28.5 filter thread for all 1.25″ filters. The optical path length is 1.5mm.