Engineered to address a persistent problem with off-axis systems or folding mirror attachments where the holder depth is too short to achieve proper focus, the Alpine Astronomical Baader DT-4 Eyepiece Extension adds 31.8mm of optical length to your eyepiece's nosepiece to enable you to get the sharp focus you want. The extension has standard 1.25" filter threads on the front and back, and the inner dimensions are made to accept the 1.25x and 1.7x Glasspath correctors. This extension is part of the T-2 Astro modular accessory system, and is compatible with all components in the system.