A Tri-Bahtinov mask for SCT collimation and focusing - Equipment (No astrophotography) - Cloudy Nights
A beginner's guide to Bahtinov masks | BBC Sky at Night Magazine
Amazon.com : Astromania Bahtinov Mask Focusing Mask for Telescopes - FM200mm for a Tube Outer Diameter (OD) from 215mm-260mm - can be Mounted with The Brass Screws Inside or Outside The drawtube :
What is a Bahtinov Mask and How Does it Help With Focusing?