The 40m WaveFront Sensing Experiment

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Figure 1:

 In the framework of the FP6 funding for the design and study on an European Extremely Large Telescope an experiment has been conceived to sense the atmosphere turbulence on a scale of the order of the size of such a telescope. Nominally, a 40m sized patch in the atmosphere can still be measured by using an extremely large Field of View Wavefront Sensor. With this concept in mind the so called "40m WFS Experiment" takes shape. The basic idea is to observe a rich and bright star cluster with a panoramic wavefront sensor having on the focal plane a sinusoidal transmission grating. The movements of the stars because of the atmosphere turbulence modulates the light transmitted through the grating. The stars could moves individually or coherently depending upon where the turbulence occurs. Turbulence occurring very close to the primary mirror, or in other words in the very low altitude layers, happens to influence the stars coherently over a large field of view, while the opposite happens for turbulence occurring far away from the telescope, up in the atmosphere. Moreover, thanks to the very large field of view of the sensor, the beam collected, being of the size of the telescope at its height, that is about 8m, expand with height and, properly choosing the right parameters, reach a diameter of about 40m at a significantly interesting altitude of a few km. In this way one can determine the power spectrum of the turbulence by a direct measurements and not through interpolations that are somehow model depending. Furthermore the frozen layer hypothesis, often recalled as Taylor's hypothesis can be tested thoroughly. Such hypothesis assumes that the time evolution of a layer in its own is much longer than the effect on the starlight image because of the drifting wind. Both these measurements are of large impact in the design of an ELT. The first measurements gives directly the maximum stroke of a Deformable Mirror needed for such a telescope, while the second open the opportunity to achieve wavefront sensing with integrations times much larger than the usual, allowing for using very faint stars to perform very wavefront sensing and hence augmenting the sky coverage. The design actually consists of four independent wide field sensors to be attached to the foci of a VLT telescope.

People: C. Arcidiacono, A. Baruffolo, M. Dima, J. Farinato, R. Ragazzoni

Collaboration: INAF OA Bologna, Nice Univ., ESO, Instituto de Astrofisica de Canarias

Publications: Metti et al. (2008), SPIE 7012,143

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