Outer Diameter 176mm
Inner  diameter 170mm
Fits telescope or lens with an outer diameter of 167mm to 170mm

Bahtinov Focus Mask for Telescope - Celestron C6-R... 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 free USPS SHIPPING. 2 TO 3 DAY SHIPPING . Other colors and types available just ask. Need a custom made part or mask let me know I make with no added fees for design.