On the 8-m primary mirror of the VLT, wind will have both a negative effect as it will deform dynamically the surface shape and a positive effect as it will decrease the mirror seeing when the mirror is warmer than ambient air. To analyze this combination we need to parameterize both effects in the same terms.

* Dynamic deformations due to wind buffeting*

Combining equations () and () the rms slope error of the optical aberration is given by the expression

where **U** is the mean wind flow speed about 60 cm above the mirror.

Inputing the data of the VLT 8.2-m mirror
(cf. page )
at the pressure height of 2500 m ()
equation (6.1) becomes

* Mirror seeing*

The mirror seeing FWHM angle is parameterized from equations () and () as

obtaining for the particular case of the VLT mirror and an average temperature

For small speed values of ( m/s), approaching the free convection condition, is parameterized by equation ():

The quadratic sum of and are plotted in fig. 6.1 for different positive mirror-air temperature differences.

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