 We demonstrate a cryogenic optomechanical system with a flexible psi underscore 3n underscore for membrane in a 400-backslash-zero mk environment, observing a mechanical quality factor q-backslash-zero, backslash-zero for backslash-zero times backslash-zero 106 for the 261-backslash-zero khz fundamental drumhead mode and a cavity resonance half width of 60-backslash-zero khz. We monitor thermal motion using a heterodyne optical circuit, find that observed optical spring and damping agree with theory, and develop a phano line shape explanation for mechanical sidebands. We discuss future requirements for laser cooling the large and low frequency mechanical element to its quantum mechanical ground state. This article was authored by A. M. J. H., J. C. Sanky, K. Berkia, and others.