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Other laboratory measurements

[Barr] studied the mirror seeing of a 1.8-m horizontal mirror in the range of mirror-air temperature differences -1.5 K 1.9 K by analyzing interferograms of the front surface. Unfortunately the large aberrations of the mirror and the change of tilt and focus between and during the test runs limit exceedingly the accuracy of the results and the comparison of different mirror conditions. The researchers also intended to verify the utility of small blowers placed around the mirror to create an artificial airflow . However their power was too weak to create a sustaining airflow capable to predominate the free convection flow pattern.

Possibly the most significant result of this work is the analysis performed on the phase structure functions of the interferograms which concluded that they follow well the form predicted for Kolmogorov turbulence for separation distances up to 80 cm.

Since the largest temperature fluctuations are found at a much shorter distance from the surface and are therefore associated to small turbulence length scales, this result of [Barr] makes admissible to use a parameterisation also in the case of mirror seeing.

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