We report on the use of Fiber Bragg Grating (FBG) sensors integrated onto an aircraft landing gear for remote and realtime load monitoring. Several FBGs strain sensors, both in a linear and tri-axial configuration, have been integrated on different locations of true landing gears (both Main and Nose gears) based on their load condition derived from FEM numerical analysis and exposed to numerous qualification lab tests where the load applied to the gears was varied in the range 0-20kN. To this aim, the gears were mounted on a 25kN hydraulic press, that changed the shock absorber route from 0 mm up to 200 mm (corresponding to the maximum take-off weight,~4600 kg). Obtained results are in good agreement with those provided by reference electrical strain gauges located very close to their optical counterparts, and demonstrate the great potentialities of FBG sensors technology to be employed for remote and real time load measurements on aircraft landing gears.
MAD5 is a Multi-Conjugate Adaptive Optics (MCAO) system conceived to demonstrate the feasibility of MCAO on the sky. The wave front sensor part is divided in two channels: a Shack-Hartmann sensor and a Layer Oriented sensor. We will describe the construction of the latter one. Assembly, integration and test of the instrument are the first steps for ESO acceptance, before integrating the Layer Oriented sensor with the other components of MAD. We will show qualitative and quantitative results of optical and mechanical tests: in particular we will describe the alignment of the references selection unit, constituted by sixteen motorized linear positioners and eight star enlargers, of the beam compressor and of the two re-imaging objectives, each one conjugated to a different altitude. Being the pyramid the core of this kind of wave front sensor, we will focus our attention on its construction difficulties and we will discuss all the optical tests made to choose the best ones to be installed on the wave front sensor. Finally we will present the sensor performance showing the first open loop results.
Layer Oriented represented in the last few years a new and promising aproach to solve the problems related to the limited field of view achieved by classical Adaptive Optics systems. It is basically a different approach to multi conjugate adaptive optics, in which pupil plane wavefront sensors (like the pyramid one) are conjugated to the same altitudes as the deformable mirrors. Each wavefront sensor is independently driving its conjugated deformable mirror thus simplifying strongly the complexity of the wavefront computers used to reconstruct the deformations and drive the mirror themselves, fact that can become very important in the case of extremely large telescopes where the complexity is a serious issue. The fact of using pupil plane wavefront sensors allow for optical co-addition of the light at the level of the detector thus increasing the SNR of the system and permitting the usage of faint stars, improving the efficiency of the wavefront sensor. Furthermore if coupled to the Pyramid wavefront sensor (because of its high sensitivity), this technique is actually peforming a very efficient usage of the light leading to the expectation that, even by using only natural guide stars, a good sky coverage can be achieved, above all in the case of giant telescopes. These are the main reasons for which in the last two years several projects decided to make MCAO systems based on the Layer Oriented technique. This is the case of MAD (an MCAO demonstrator that ESO is building with one wavefront sensing channel based on the Layer Oriented concept) and NIRVANA (an instrument for LBT). Few months ago we built and successfully tested a first prototype of a layer oriented wavefront sensor and experiments and demonstrations on the sky are foreseen even before the effective first light of the above mentioned instruments. The current situation of all these projects is presented, including the extensive laboratory testing and the on-going experiments on the sky.
We are currently working on four projects employing Multi Conjugate Adaptive Optics in a Layer-Oriented fashion. These ranges from experimental validations, to demonstration facility or full instrument to be offered to an astronomical community and involves telescopes in the range of 4m to 24m equivalent telescope aperture. The current status of these projects along with their brief description is here given.
The sky coverage and the Strehl Ratio (SR) uniformity over Field of Views (FoV) of some arcmin are two key points for the development of Natural Guide Star (NGS) based Multi-Conjugate Adaptive Optics (MCAO) systems. We developed a numerical code able to simulate the behavior and to evaluate the performance of MCAO Layer-Oriented systems. In this paper we study the two issues and present the results of the simulations. In particular we consider the Multiple Field of View (MFoV) version of the Layer-Oriented concept. This technique allows looking for the reference stars in sky-areas bigger than the FoV corrected by the adaptive system, with a considerable gain in term of sky coverage. In the MFoV approach of the Layer-Oriented the NGS are selected in two or more concentric annular FoVs. In the configuration we take into account the guide stars are chosen in two different FoVs. The references for the ground layer loop are chosen in an annular FoV which inner diameter has the dimension of the scientific one. This annulus has only technical purposes and only a ground layer correction is applied in those sky directions. The reference stars for the highest loop are selected, as usual, in the corrected FoV. First we take into account the sky coverage considering different galactic latitude cases and studying the results distribution of the simulated cases. In order to enable a statistical approach to the problem we considered a big number of NGS configurations for each galactic latitude case considered. Then we take into account the level of SR uniformity in the cases where a useful correction is achieved. We define a function to quantify the SR uniformity over the FoV and we study its distribution using the results computed by the numerical simulations.