= Solution
For a spatially coherent, on-axis source, the coherent spatial-frequency cutoff is
$$
f_c=\frac{\mathrm{NA}}\lambda.
$$
A line-space grating must place at least its first diffracted orders within the pupil. Its limiting pitch is therefore $\lambda/\mathrm{NA}$, and its equal line/space half-pitch is
$$
\boxed{a_{\min}=\frac{\lambda}{2\,\mathrm{NA}}
=\frac{193}{2(1.35)}\ \mathrm{nm}\simeq71.5\ \mathrm{nm}.}
$$
The corresponding full pitch is \b[143 nm]. Explicitly distinguishing pitch from half-pitch avoids a factor-of-two ambiguity in the phrase “feature size.” This is an ideal optical cutoff estimate; finite resist contrast and process requirements can make the usable feature size larger.
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