The VLT Active Optics System Due to the low ratio between their
thickness and their diameter, the VLT primary mirrors will be
rather flexible and sensitive to various disturbances, requiring
permanent control of their optical shape. Active optics consists
in applying controlled forces to the primary mirror and in
moving the secondary mirror in order to cancel out the errors.
The scheme was developped by ESO for the 3.5-m New Technology
Telescope (NTT) and is now applied to the VLT. The system must
essentially compensate for static or slowly varying deformations
such as manufacturing errors, thermal effects, low frequency
components of wind buffeting, telescope inclination, ... It is
also used when changing between Cassegrain and Nasmyth foci. A
schematic view of the system is shown below. (Drawing by Ed
Janssen, ESO) DescriptionThe different elements of the active
optics system of the VLT are the primary mirror, with its active
support system located within the M1 Cell structure, the M2
unit, the CCD Shack-Ha