In the Abbe$-$Porter's experiment, either a horizontal slit or a vertical slit is adopted for performing filtering of the object with amplitude transmittance function

$t_A(x,y)=$circ$(\frac{r}{R})*{\displaystyle \underset{m=-1}{\overset{1}{}}}{a}_m{e}^{j2\frac{m}{D}x}*{\displaystyle \underset{n=-1}{\overset{1}{}}}{b}_n{e}^{j2\frac{n}{D}y},$ which has a spatial period $D$ in both the $x-$and the $y-$direction. If we now orient the slit at $45$ degrees to both $x$ and $y$ axes, what will the spatial period of the filtered image be$?(12\%)$