  Although silver-based telescope mirrors excel over other materials such as
gold and aluminum in the visible-infrared spectral range, they require robust
protective coatings to overcome their inherent low durability. Our research
shows that a single-layer of aluminum oxide (AlOx) deposited through thermal
atomic layer deposition (ALD) using trimethylaluminum (TMA) and water (H2O) at
low temperatures (~60{\deg}C) serves as an acceptable protective coating
without adversely impacting the optical performance of the mirrors. While
silver-based mirrors protected with a single-layer of AlOx perform decently in
the field, in environmental tests under high-humidity at high-temperature
conditions that accelerate underlying failure mechanisms, they degrade quickly,
suggesting that there is room for improvement. This paper describes a study
that compares the performance and endurance of two sets of silver-based mirrors
protected by a single-layer of AlOx prepared by thermal ALD with two types of
oxygen precursors: H2O and pure ozone (PO). The study shows that while the two
types of samples, regardless of their oxygen precursors, initially have
comparable spectral reflectance, the reflectance of the samples with AlOx
protective coatings prepared with PO remain nearly constant 1.6 times longer
than those with AlOx protective coatings prepared with H2O in the environmental
test, suggesting promising characteristics of AlOx protective coatings prepared
with PO.
