The surface figure of the relatively thin primary mirrors of
the new 8-m generation are quite sensitive to the turbulent pressure
fluctuations caused by wind buffeting. For instance in the case of
the VLT, the relationship
(
) leads to specify, for a wavefront error of
200 nanometers, a threshold for
the admissible pressure fluctuation at 1.3 Pa. This is a very low value
that will generally be reached with a wind speed close to the mirror
of about 3 m/s.
From the practical engineering standpoint there are in principle three possible technical solutions to this problem:
) and () will
allow the designers to relate the airflow speed near the
mirror with the
pressure fluctuations, thereby providing the quantitative
data for evaluating the
optical performance of the mirror under wind loads and then for
determining the best operating strategy for
the venting devices (windscreen, louvers) of the enclosure.
This topic will be further discussed in section 6.2 below.
The other alternatives require the upgrading of "active optics" mirror support systems in order to provide a reaction capability of the mirror to fluctuating wind loads.
), where it is planned to provide the
mirror with six fix-points instead of three. This will reduce the
mirror deflections, hence the optical aberrations by a factor of about
four.
.