1.Male thread - M54x0.75. Female thread - M54x0.75, eliminates a range of focusing problems. 2.The extension tube has a M54x0.75 interior thread and a M54x0.75 exterior thread for elegantly solving spacing problems in astrophotography. 3.Use this extension tube for getting the correct image scale or the exact spacing required for reducers or field flatteners. 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 M54 x 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. The Extension Ring has length of 4mm,5mm,6mm,7mm,8mm,9mm. Outer diameter: 60mm male thread length: 3mm 4mm-female thread length is 3mm 5mm-female thread length is 4mm 6mm-female thread length is 4.5mm 7mm-female thread length is 5.5mm 8mm-female thread length is 6mm 9mm-female thread length is 7mm Colour: Black Material: Metal Package Contents: 6 x Telescope Extension(4/5/6/7/8/9mm)