Try:
Manipulate[ With[{ax = 2 ay, ty = 2 tx, co = 2}, ContourPlot[ co - 2 tx Cos[kx ax] - 2 ty Cos[ky ay], {kx, 0, 2 Pi}, {ky, 0, 2 Pi}]], {ay, 0, 10}, {tx, 0, 10}]
