Although I get satisfactory collimation from my 9.25 SCT using defocused star collimation technique, it's not always easy to get perfectly tight concentric Airy discs. I have always wondered if collimation could be done better. After searching the internet, I came across the Duncan mask, which was created by Duncan Evenden of the Orpington Astronomical Society, UK.
http://alpha-lyrae.co.uk/2013/12/31/sch ... ncan-mask/
http://www.cloudynights.com/topic/38397 ... ask/page-3
After procrastinating for several weeks (busy with work), I decided to create a Duncan mask for myself as a weekend project. For materials, I used corrugated cardboard recycled from a used but clean cardboard box. After marking a central point, 60-degree arcs and sectors were created using ruler and compass construction. Holes were then cut using #11 blade scalpel. I modified the original design by adding side notches for easy removal. The mask fits fairly snuggly around the secondary mirror housing. Unfortunately, it was a cloudy weekend, so no chance of actually testing it out.