Fully metal construction and anodizing aluminum process, durable for long time use. Use this extension tube for getting the correct image scale or the exact spacing required for reducers or field flatteners. The extension tube has a T2 interior thread and a T2 exterior thread for solving spacing problems in astrophotography and eliminates a range of focusing problems. Please Note: This is M42 (42mm) thread caliber, not M48 thread caliber. Please make sure this is the correct size and thread before purchasing; If you have any problems about your purchase, please don't hesitate to contact us and we'll make it right. Specification: Type: Astronomical T2 Thread Extension Tube Male thread: M42*0.75mm Female thread: M42*0.75mm Length: 3mm 5mm 7mm 10mm 12mm 15mm 20mm 30mm(Optional) Color: Black Size Chart: 42mm/1.65inch 3mm/0.12inch 5mm/0.20inch 7mm/0.28inch 10mm/0.39inch 12mm/0.47inch 15mm/0.59inch 20mm/0.79inch 30mm/1.18inch Package Includes: 1*Extension Tube Note: 1. Please allow slightly error due to manual measurement. 2. Due to the difference between different monitors, the picture may not reflect the actual color of the item. 3. Wish you a wonderful online shopping!