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Thermal Time Constant

The quality of an image not only depends on the accuracy and smoothness of the optic but also on temperature differences between the ambient air and mirror.  Hextek substrates have the ability to achieve thermal equilibrium quickly (short thermal time constant) due to the thinness of the glass cross-sections that form the monolithic blank structure.  This ability, minimizes thermal turbulance over the mirror's surface yeilding a better quality image.  All mirror materials suffer from this problem not because of the thermal expansion coefficient of the material but due to  mass.  Gas-Fusion thermal time constants are driven by the thickness of the faceplate.  The table to the right shows the comparision between three different configurations.  

Theoretical Thermal Chart

Boundary Detection

Boundary Layer

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.