The diffraction grating is fabricated out of stainless steel so that
its radius of curvature will expand and contract at the same rate as
the spectrometer housing and thus maintain proper focus. The grating
characteristics are given in Table 1-1, and it is shown in cross
section in Figure 2-4. It was ruled holographically on an optical
master and then replicated onto the stainless steel blank. For
Astro-2, the grating has been coated with silicon carbide and is about
80
more efficient than the osmium-coated grating used on Astro-1.