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 (see section 4.1.4).
The 1/80 model of the telescope was surrounded by a low platform,
which was at the time one of the alternative designs 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 can be considered to be Reynolds independent. Two types of measurements were done:
).
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 azimuth
and
zenithal angles 
measured with respect to the wind
direction:
, 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.
Figure: Mean and rms
values of modes defocus (circles)
and y-component of astigmatism (crosses)