The data from the 62-cm experiment suggest an approximately linear relationship of seeing FWHM with the temperature delta for any given airflow speed. This reflects the fact that also in these conditions the seeing is caused by buoyant intermittent fluctuations like in the free convection case. Forced ventilation reduces this seeing by creating a mixed convection regime, which should be here characterized by the non-dimensional Froude number:

where **D** is the mirror diameter. We would therefore
expect that the Froude number has the role of a stability parameter
with respect to the FWHM normalized with
the mirror-air temperature temperature difference.
If this is the case the rate of mirror seeing with respect
to the mirror-air temperature temperature difference is a function of
the Froude number:

The results of both the 25-cm and 62-cm experiments are plotted against the Froude number in fig. . They do follow a function of the Froude number as assumed by equation () which has the form:

Equations () and () allow us to derive an empirical similarity law for the scale of mirror seeing from a ventilated mirror:

It follows that

The dependency on the Froude number and the similarity relationship () suggests a relationship with the mirror size which would mean that larger mirrors have more seeing in the same condition of ventilation, although this is still to be verified on large mirrors.

Lorenzo Zago, zago@elgc.epfl.ch, Sun Feb 26 22:57:31 GMT+0100 1995