  Silver (Ag) mirrors for astronomical telescopes consist of multiple metallic
and dielectric thin films. Furthermore, the topmost surface of such Ag mirrors
needs to be covered by a protection coating. While the protection coating is
often deposited at room temperature and the entire mirrors are also handled at
room temperature, various thin film deposition techniques offer protection
coatings with improved characteristics when carried out at elevated
temperatures. Thus, in this work, high-performance Ag mirrors were designed and
fabricated with a new benchmark. The resulting Ag mirrors were annealed (i.e.,
post-fabrication annealing) at various temperatures to investigate the
viability of introducing thermal processes during and/or after fabrication in
improving overall optical performance and durability of protected silver
mirrors. In our experiments, Ag mirror samples were deposited by electron-beam
evaporation and subsequently annealed at various temperatures in the range from
60 {\deg}C to 300 {\deg}C, and then the mirror samples underwent an
environmental stress test at 80 {\deg}C and 80% humidity for 10 days. While all
the mirror samples annealed below 200 {\deg}C showed negligible corrosion after
undergoing the stress testing, those annealed below 160 {\deg}C presented
spectral reflectivity comparable to or higher than that of as-deposited
reference samples. In contrast, the mirror samples annealed above 200 {\deg}C
exhibited significant degradation after the stress testing. The comprehensive
analysis indicated that delamination and voids caused by the growth of Ag
grains during the annealing are the primary mechanisms of the degradation.
