  Gratings enable dispersive spectroscopy from the X-ray to the optical, and
feature prominently in proposed flagships and SmallSats alike. The exacting
performance requirements of these future missions necessitate assessing whether
the present state-of-the-art in grating manufacture will limit spectrometer
performance. In this work, we manufacture a 1.5 mm thick, 1000 nm period at
grating using electron-beam lithography (EBL), a promising lithographic
technique for patterning gratings for future astronomical observatories. We
assess the limiting spectral resolution of this grating by interferometrically
measuring the diffracted wavefronts produced in +/-1st order. Our measurements
show this grating has a performance of at least R ~ 14,600, and that our
assessment is bounded by the error of our interferometric measurement. The
impact of EBL stitching error on grating performance is quantifed, and a path
to measuring the period error of customized, curved gratings is presented.
