Male thread - M42X0.75; Female thread - M42X0.75; eliminates a range of focusing problems The extension tube has a T2 interior thread and a T2 exterior thread for elegantly solving spacing problems in astrophotography Use this extension tube for getting the correct image scale or the exact spacing required for reducers or field flatteners All metal construction and anodizing aluminium process; durable for long time use The required spacing can be achieved by combining several extenders of differing lengths; you can use 1 piece 2 pieces 3 pieces or 4 pieces of the extension rings as you like Sometimes you cannot get to the right focal plane for focusing with a camera or eyepiece. This extension tube kit solves this problem. You can use this extender with any camera or eyepiece or with various adapters. The M42*0.75 threads on both ends make it extremely versatile. Use this extender for obtaining the correct image scale or for the exact spacing required for a reducer or field flattener. You can reach the extension length required by combining several extenders of different lengths.