d1=f0[[20;;147,90;;,1]]-f0[[20;;147,90;;,3]];

Dimensions[d1]

m=Dimensions[d1][[1]]

n=Dimensions[d1][[2]]

A1=Im[Fourier[d1]];

hx=2 π/n; hy=2 π/m;

uF[x_,y_]:=Sum[N[A1[[i+1,j+1]] Sin[ i hx x] Sin[j hy y]],{i,12,16},{j,12,16}] N[4/(Sqrt[m n])];

ContourPlot[uF[x,y],{x,0,n/2},{y,0,m/2}]

Fd0=Table[uF[x,y],{y,0,m/2-1},{x,0,n/2-1}];

Dimensions[Fd0]

Dimensions[d1]

ListContourPlot[Fd0]

?Norm

RelError=Norm[Fd0-d1[[1;;64,1;;102]]]/(m*n/4)