Part of the T-2 Astro modular mounting system, the Alpine Astronomical Baader 2" Male to T-2 Female Reducer Ring converts larger 2" female thread mounts to smaller T-2 female threads to make your equipment compatible with their full array of T-2 Astro photo accessories that include rings, extensions, and adapters. Its compact form factor achieves the thread conversion with just a 1mm optical length, so there will only be a minimal effect on focal distance.