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.
I made a new Y mask sized for my 50mm f/1.8 lens(which has a very small objective by the way) and tried it last night. In the moment I did not find the mask helpful at all. I couldn’t see the spikes on my LCD at all but when I uploaded them after focusing by eye I see the spikes perfectly. So, my by-eye focusing worked as well as the mask would have in this instance but the mask is not hopeless. The shot was taken with the lens at f/4 4 seconds ISO 1600. The star showing the spikes is Sirius by the way. Also by the way, when the focus was really off you could see the Y mask in the bloated star. The thumbnail below was taken at f/1.8 1/2 second ISO 6400.
I took a couple of further shots. The best one captures most of Orion. It’s below both original and after stretching it with the Paintshop Pro histogram functions. They were shot at f/4 2 seconds ISO 1600.
I’ve heard of Bahtinov focusing masks masks but now the cool kids are using Y shaped masks. From what I can see the dimensions and even the angles are not critical. Somehow the shape causes a diffraction pattern that has a specific shape when the camera is well focused(see below where the three spikes meet). I’m hoping to try it tonight if the sky is clear enough.