How to achieve a perfect focus (Using a Bahtinov mask) - YouTube
Bahtinov Aid
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 :
Bahtinov Mask - 14 Inch to 17 Inch OTAs – FarpointAstro
Better alternative to Bahtinov Focusing Mask - ATM, Optics and DIY Forum - Cloudy Nights
Amazon.com : Astromania Bahtinov Mask Focusing Mask for Telescopes - FM180mm for a Tube Outer Diameter (OD) from 195mm-240mm - can be Mounted with The Brass Screws Inside or Outside The drawtube :
Farpoint Adjustable Tri-Bahtinov Mask - 15 Inch to 18 Inch OTAs - Camera Concepts & Telescope Solutions
Farpoint Bahtinov Focus Mask for Telescopes with Dew Shield or Front End Diameter from 2.5" to 4.5" # FP400
Using APT 3.81 - Focusing using a Bahtinov Mask - YouTube