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Mirror seeing is caused by natural or weakly mixed convection over a mirror warmer than ambient air. The seeing effect is generated in a thin region just above the viscous-conductive layer where the temperature fluctuations are largest and most intermittent. If its cause could be visualized, seeing would appear to come from a thin but very turbulent layer "floating" a few millimeters above the surface.

On the base of this physical description we have expressed the hypothesis that the average amplitude of the seeing would be essentially a function of the surface heat flux. The profile of can then be described by a similarity equation (gif), valid down to the interface with the tiny viscous-conductive layer. Computations of mirror seeing with this similarity model show a good agreement with experimental results.

The results of different experiments, done by various researchers, have been processed to get a homogeneous database. In absence of forced air flow, mirror seeing appears not to depend on the mirror size. The effect of inclination is more controversial: it is reported to be large for a small 25-cm mirror but does not appear in our analysis of the 3.6-m CFHT telescope. For the purpose of engineering parametric studies the following relationship is proposed:

with a possible spread of 25%.

In the case of a ventilated mirror, the seeing decreases showing a dependency on the Froude number. The relationship derived from the experiments is:

The dependency on the Froude number suggests the influence of the mirror size. Thus larger mirrors should produce more seeing in the same conditions of ventilation.

Lorenzo Zago,, Sun Feb 26 22:57:31 GMT+0100 1995