Is this Physics or Quantum mechanics?

My answer is neither! The Bahtinov mask is just pure magic.

Okay, seriously- the light waves travel past the sharp edge of the mask and causes a diffraction spike in a wave form. As each peak of the wave aligns it creates a bright spot. This is the first order in the spectrum. As each third peak aligns, a third order spectrum spot is illuminated. Then a 5th, 7th and so on until the aptitude of the wave has dissipated. So this is why the math formula uses focal length to calculate the optimum distance between the cutouts. The nice thing is that you can align the spectrum on the rising, falling, or peak of the sin wave and still get a nice diffraction spike pattern. It is only a minor difference in the third order spectrum from a mask that is dead on optimal and one that is way off. This is why I always recommend that you spend more time on getting nice straight lines with a sharp edge, and not get too caught up with the focal length.

See my post above for a photo I made with a mask that was way off of optimal cutout as calculated by the formula. You see the first order is a mess, but the third order still is quite useful in getting good focus.

I hope I explained this okay.

And by the way- I’m not a big time banker, just a guy that works on engines by day and takes time out on clear nights to enjoy the cosmos. Once and a while I’ll climb on my 1952 8N tractor and ride for the fun of it.