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Hi Philip,
You are right about the duncan mask it only roughly collimates your telescope.
A duncan mask is only needed if you don't know how far to defocus the star or have trouble seeing the amount of defocus.
Step 1:
For me roughly collimating a scope is defocusing a star so I can see 3 rings - after the collimation is done at this amount of defocus I then adjust the defocus to 2 rings and continue to adjust the collimation screws until it's as close as I can get it.
At this stage the collimation is close but for me it's not good enough for high resolution work such as planetary imaging.
Step 2:
Fine collimation - I focus on a star using a Bahtinov mask then I take a short video of the focus star. After this I process the video using autostakkert (stacking program) to determine what screw needs to be adjusted and I repeat this process until I get an image that looks like the one attached.
Only after the second step I will then start to use the scope. Hope this helps or have more questions just let me know
ps. the image below is an actual image not simulated!
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