A consequence of a slow thermal time constant is the formation of a thermal boundry layer above the optical surface. As the image at the left illustrates, there is a significant amount of turbulance associated with convective heat transfer from stored heat in the mirror to ambient air. A Gas-Fusion mirror with thin face plates drastically reduces this effect. Observatories with large mirror substrates manage thermal issues with primary mirrors using active cooling. This method has also been employed with great success in Hextek substrates for little cost. Such a system can be as simple as a low flow ambient air exchange via a tube inserted into a back plate hole of each cell. Where passive thermal management is the only option, Hextek substrates provide the best performance.