 Largest contiguous Fresnel aperture

The largest radius of the normalized Cornu spiral occurs on its first outer turn at $w_{\ast }\simeq 1.20938$, where $\mathrm{Re}[F(w{)}^{\ast }{F}^{\prime }(w)]=0$. Oddness makes the chord from $F(-w_{\ast })$ to $F(w_{\ast })$ attain twice that radius, while the triangle inequality bounds every other chord by the same length. The interval is therefore the strongest freely selectable connected aperture at that observation point, with normalized point power about1.80142. Signed physical limits follow from $x=w\sqrt{\lambda z/2}$. Existing openings can constrain this optimization.