A mirror control mechanism for space telescope (2003)

The high resolution optical instruments require more and more stability on the relative position between their different mirrors. The use of a mirror control mechanism (MCM) allows to correct in flight the position of the mirror (in particular the focusing and the 2 tilts). The mechanism described hereafter is designed for a Cassegrain telescope secondary mirror. The selected concept is based on 3 vertical actuators which produce the focusing and tilts movements, and three horizontal actuators which produce the transverse movements. This architecture offers 5 degrees of freedom which guarantee the absence of rejection for any kind of correction. After the design phase, a demonstrator was manufactured and characterised by functional and mechanical tests. This mechanism is able to control any type of axisymmetric mirror within 5 degrees of freedom. The mass of the model presented is 3.5kg with overall dimensions ∅280mm/H77mm (except electronics). This concept can be adapted to smaller versions of mirror requiring an active control, and in a more general way to equipments for which the pointing precision is a key requirement. 1. CONTEXT The optical instruments for observation at high resolution must have a very important dimensional stability of the structural elements which connect optics together. Two ways of work allows to progress in this direction. The first one consists in using materials having thermoelastic and hygroelastic sensitivity almost null and setting up integration procedures with fine metrological adjustments. The second one consists, in the opposite, to relax the constraints on materials and integration by adding in the instrument an in-flight adjusting device able to correct the optical defects during the mission. Such a device can be placed behind the secondary mirror of a Cassegrain telescope, allowing to minimize the inertia to be moved and to work on the relative stability with regard to a passive primary mirror. The knowledge in mechanisms of precise positioning resulted in drawing aside all the solutions bringing of the tribological contacts (ball screw, kneecaps, ball bearings) because of the nonlinear behaviour to the small movements, of problems of performance stability versus time and the potential problem of optics pollution. It was thus decided to work with piezoelectric actuators which are already universally used in ground systems of precise positioning, and to use techniques of guidance by elastic strain to avoid non-linearities The mechanism described hereafter is designed for a Cassegrain telescope secondary mirror to correct in flight the focusing, the 2 tilts, and the decenters (see fig1). Figure 1 : Definition of the geometrical instabilities 2. OBJECTIVES The need for the opticians is first to carry out corrections according to 3DOF (refocusing and 2 tilts). But the fact that this mechanism is intended for a secondary mirror of standard Cassegrain telescope is constraining. That imposes the obstruction of the mechanism does not occult the field of view of the telescope, or in other words it must be included in a cylinder of diameter equal to the mirror’s one and placed behind this one. So kinematics will be completely off-set compared to the optical centre around whose the movements must be done. Thus, a M1 Mirror Decenter