drag the knob to slide the ellipse
δ 90° · state CIRCLE
x = A·sin(1·t + δ)
y = A·sin(2·t)
BEAM X+0.00
BEAM Y+0.00
TANGENCY1 : 2
integer frequencies close the figure · phase δ rotates the closed loop from line to circle · persistence is phosphor decay — raise it to burn the whole lattice in