The mask worked very well last night. Sirius is bright in the south and easy to focus on from our porch. This is with the 200mm Takumar lens f/4 ISO 6400 1 second.
I taped the lens to keep focus after this shot but I see that I moved the focus ring way too much when I adjusted it according to this. I’ll untape it and move the ring so it’s closer to the infinity mark. It will probably mess up the focus but that’s easy enough to fix. It was surprising how much i would move the focus ring to move the diffraction spike.
I’ve loosened and turned the focus ring per the video and hopefully the infinity focus is just to the right of the infinity mark with enough play for fine adjustment. I note that to bring the central diffraction spike UP on the Y mask i have to move the ring to the right – i.e. closer to the infinity mark.