A first test with pressure measurements on the primary mirror was performed at the LASEN boundary layer tunnel as part of the program for wind loading on the VLT telescope in a retractable enclosure (cf. section 4.1.4). The 1/80 model of the telescope was surrounded by a low platform as it was at the time alternatively envisaged for an open air arrangement of the VLT (fig.).

**Figure:** Distribution of pressure taps
and sectors on the 1/80 models of the primary mirror

Because of its sharp edges the mirror model was considered to be Reynolds independent. Two types of measurements were done:

- Average, peaks and r.m.s. of the pressure field.

This was done with 32 sensors distributed over the surface (see figure ). The pressure signals were sampled at 250 Hz and the results were taken from 16 runs of 10 sec each. The frequency response of the sensors was flat up to 100 Hz. The following table shows the values of the mean pressure coefficient [= and its rms for various zenith angles and zenithal angle towards the wind : - Mean pressures over eight sectors of the mirror.

Since the measuring equipment did not allow the simultaneous measurement of the instantaneous pressure at tens of pressure taps, an original method was devised to obtain directly the main modal components of pressure. For this test a different model with the same dimensions but altogether 152 pressure taps was used. The pressure taps were grouped into eight sectors, which are shown in figure , the pressure taps were connected, thereby giving the average pressures over these sectors. By forming one could measure properties of a mode similar to defocus and by forming properties of a mode similar to the y-component of astigmatism. Both the average values and the r.m.s. values were measured for various zenith angles. The results for are shown in figure . To get the corresponding coefficients of defocus and the y-component of astigmatism these data have to be multiplied by a factor of two. If one assumes reasonably that the x- and y-component of astigmatism are of the same order of magnitude, a multiplication of the second coefficient with a factor would give the coefficient of the total astigmatism.

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