Made of high-quality aluminum material, and long service life. It can be used for a long time. The internal thread design can eliminate stray light and reflected light, making the image clearer and more perfect. The extension tube adopts M48 female thread and M48 external thread to solve the problem of astrophotography spacing and eliminate a series of focusing problems. By combining several extension rings with different lengths to achieve the required spacing, you can use 1, 2, 3 or 8 extension rings as needed. A total of 8 telescope extension rings with of 3/5/7/10/12/15/20/30mm can perfectly meet daily use. Name: Telescope Extension Ring Extended length: 3mm-30mm, choose the corresponding length according to your own needs. Extinction treatment: with extinction thread inside. Interface internal thread specification: M48 x 0.75mm External thread specification of interface: M48 x 0.75mm Colour:As Shown Material:aluminum Package Contents: 8 x Telescope parts Only the above package content, other products are not included. Note: Light reflection and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.