This work presents a new technological concept for large aperture, lightweight, telescopes using thin deployable active mirrors, currently under a feasibility study for spaceborne Lidars.
The study is mainly addressed to a DIAL (Differential Absorption Lidar) at 935.5 nm for the measurement of water vapour profile in atmosphere, to be part of a typical small ESA Earth Observation satellite to be launched with ROCKOT vehicle. A detailed telescope optical design will be presented, including the results of angular and spatial resolution, effective optical aperture and radiometric transmission, optical alignment tolerances, stray-light and baffling. Also the results of a complete thermo-mechanical model will be shown, discussing temporal and thermal stability, deployment technology and performances, overall mass budget, technological and operational risk and system complexity.
This work presents the latest results of new technological concepts for large aperture, lightweight telescopes using thin deployable active mirrors. The study is originally addressed to a spaceborne DIAL (Differential Absorption Lidar) at 935.5 nm for the measurement of water vapour profile in atmosphere, as an output of an ESA contract (whose preliminary results were presented at ICSO 2006).
The high versatility of these concepts allows to exploit the presented technology for any project willing to consider large aperture, segmented lightweight telescopes. A possible scientific application is for Ultra High Energy Cosmic Rays detection through the fluorescence traces in atmosphere and diffused Cerenkov signals observation via a Schmidt-like spaceborne LEO telescope with large aperture, wide Field of View (FOV) and low f/#.
A technology demonstrator has been manufactured and tested in order to investigate two project critical areas identified during the preliminary design: the performances of the long-stroke actuators used to implement the mirror active control and the mirror survivability to launch. In particular, this breadboard demonstrates at first that the mirror actuators are able to control with the adequate accuracy the surface shape and to recover a deployment error with their long stroke; secondly, the mirror survivability has been demonstrated using an electrostatic locking between mirror and backplane able to withstand without failure a vibration test representative of the launch environment.
This paper describes an innovative approach for a new generation of large aperture, deployable telescopes for advanced
space LIDAR applications, using the thin active mirror technology. The overall telescope design is presented with a
special attention to the optical performances analysis. The mechanical layout with details of the deployment and baffling
technique is shown; the complete satellite thermo-elastic analysis mapping the primary mirror deformation due to the
thermal loads is presented; the control system architecture is explained and the optical design including the angular and
spatial resolution, effective optical aperture and radiometric transmission, optical alignment tolerances, straylight and
baffling is deeply discussed. Finally an overview of different mission profiles that this technology can satisfy is
presented; the imaging performances can be achieved using the shown technology tuning the surface control to higher
performances.
This paper describes the design, manufacturing and test of a ground demonstrator of an innovative technology able to
realize lightweight active controlled space-borne telescope mirror. This analysis is particularly devoted to applications
for a large aperture space telescope for advanced LIDAR, but it can be used for any lightweight mirror. For a space-borne
telescope the mirror weight is a fundamental parameter to be minimized (less than 15 Kg/m2), while maximizing
the optical performances (optical quality better than &lgr;/3). In order to guarantee these results, the best selected solution is
a thin glass primary mirror coupled to a stiff CFRP (Carbon Fiber Reinforced Plastic) panel with a surface active control
system. A preliminary design of this lightweight structure highlighted the critical areas that were deeply analyzed by the
ground demonstrator: the 1 mm thick mirror survivability on launch and the actuator functional performances with low
power consumption. To preserve the mirror glass the Electrostatic Locking technique was developed and is here
described. The active optics technique, already widely used for ground based telescopes, consists of a metrology system
(wave front sensor, WFS), a control algorithm and a system of actuators to slightly deform the primary mirror and/or
displace the secondary, in a closed-loop control system that applies the computed corrections to the mirror's optical
errors via actuators. These actuators types are properly designed and tested in order to guarantee satisfactory
performances in terms of stroke, force and power consumption. The realized and tested ground demonstrator is a square
CFRP structure with a flat mirror on the upper face and an active actuator beneath it. The test campaign demonstrated the
technology feasibility and robustness, supporting the next step toward the large and flat surface with several actuators.
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