In order to increase throughput and maximize sensitivity the next-generation of astronomical instrumentation is moving toward cryogenic, all-reflective, off-axis optical design solutions. These off-axis systems require mirrors which are produced with complex conic sections, demand a thermal optical performance at cryogenic temperatures, and must support lifetimes on the order of 5-10 years. SSG specializes in the design, development, fabrication and testing of off-axis, all-reflective optical systems, having produced > 40 such systems over the last 20 years. The majority of these system have been produced using nickel plated aluminum mirror substrates and aluminum metering structures in order to obtain a passively systems has long been a point of debate. In this paper we demonstrate the long term stability of nickel plated aluminum optics by presenting interferometric test data obtained on > 10 optical elements over a period of 10 years. Cryogenic stability is demonstrated by presenting system level wavefront data obtained over a wide thermal range down to 115K. In addition, we will present thermal test data obtained from a number of alternate metal optical materials: beryllium, bare aluminum, and aluminum/beryllium alloys.