Design and Development of Refocusing Mechanism for Secondary Mirror of a Space Telescope Using Thermal Actuator

Abstract

Satellite telescope is consisting of various components such as primary mirror, sec- ondary mirror, lenses, detector unit, spider bladesand metering structure etc. When light rays enter in satellite telescope, it first travels to the primary mirror than reflects back towards the secondary mirror and hence forth setting a focal point after passing through the centrally provided hole on primary mirror in case of Cassegrain focus. Generally, space borne telescopes are subjected to orbital loads like temperature ex- cursion, absences of gravity, moisture release of metering structure and due to this, there is the change in focal length & misalignment of optical system which ultimately reduces the quality of images. The actual location of focal point is very important for qualitative images. So to get the perfect focal point, it is necessary to reset the distance between primary and secondary mirror by means of some actuating mechanism. This distance can be corrected with the help of re-focusing mechanism. This re-focusing mechanism is placed at the back side of secondary mirror for correcting the distance between primary and secondary mirror. Very precise mechanism is required for resetting the distance be- tween primary and secondary mirror. Active refocusing mechanism uses either thermal or piezoelectric actuator. This research work has been carried out on thermally driven refocusing mechanism, which is used for changing position within ±10 micron by means of only heating. In this dissertation, thermally driven refocusing mechanism is modelled using Cro Parametric 3.0 and thermal structural analysis has been carried out to achieve ±10 micron decenter and ±15 arc second tilt. Amount of temperature variation required for specified movement mechanism ability to perform on both side have been checked. Thermal expansion has also been checked experimentally and compared with simulation results.