Mechatronic distributed control system for telescope primary mirror active optics

The last generation optical telescopes use the active optics technology to actively compensate the errors of the optical system due to mirrors imperfections, thermal and gravity reasons; e.g. when the altitude angle changes the mirror shape slightly modifies depending on its aspect ratio (thinner mirrors have lower deformations) and on the mirror supporting system. The primary mirror shape is controlled by 84 force actuators disposed along concentric rings. The number of actuators along both radial and azimuthal directions depends on the maximum symmetry and order of the optical aberrations to be corrected. A distributed control system well suites to the problem of the primary mirror actuators control. The present paper is mainly focused on the control electronics and is referred to the experience related to the VLT Survey Telescope (VST), a telescope with a 2.6 m. primary mirror designed and developed by I.N.A.F., Osservatorio Astronomico di Capodimonte (OAC) in cooperation with the European Southern Observatory (ESO).