Bahtinov Focus Mask for Telescope - Dobson 10 Inch or any 12.75 Inch External diameter telescope.  External  SLIDE ON fit like cap. Condition is New. Shipped with USPS First Class. Software Designed and tested. Works great with these and similar models. 3d printed and designed by Spencer 3d. As always fast USPS SHIPPING. 2 TO 3 DAY SHIPPING. BLACK,  If need in different color let me know or if need one to fit your telescope let me know I may already have a design, if bot I can design to fit yours. May fit others same size or other brands.