Correction of non-common path aberrations in pyramid wavefront sensors to recover the optimal magnitude gain using a deformable lens.

Adaptive optics (AO) correction based on pyramid wavefront sensors (P-WFSs) has been successfully implemented in several instruments for astronomical observation due to the P-WFS advantages in terms of sensitivity with respect to other WFSs, such as the Shack-Hartmann. The correction of non-common path aberrations (NCPAs) between the sensing and the scientific arm, commonly performed introducing offsets to the Zernike coefficients of the measured wavefront in the AO closed loop, reduces the sensitivity of P-WFSs causing a loss in sky coverage and scientific throughput. We propose a technique to exploit the full capabilities of P-WFSs compensating the NCPAs up to the fourth order on the WFS channel by means of a multi-actuator adaptive lens (MAL). We show the preliminary results obtained in a dedicated laboratory test bench.

The tracking control system of the VLT Survey Telescope.

The VLT survey telescope is the latest telescope installed at European Southern Observatory's Paranal observatory that is considered one of the best sites for optical astronomy for the excellent seeing conditions. The exceptional quality of the site imposes tight requirements for the telescope tracking system that shall perform very well to fully exploit the extreme sharpness of the Chilean sky. We describe the specific solutions adopted for pointing, servo and guiding systems and the results obtained during the commissioning of the telescope. The hardware implementation relies on industry components and the control solutions privilege both the performance and the future maintainability of the system.

Adaptive-optics corrections available for the whole sky

Adaptive-optics systems can in principle allow a telescope to achieve performance at its theoretical maximum (limited only by diffraction), by correcting in real time for the distortion of starlight by atmospheric turbulence. For such a system installed on an 8-m-class telescope, the spatial resolution and sensitivity could be up to 100 times better than conventional imaging. Adaptive-optics corrections have hitherto been achieved only for regions of the sky within a few arcseconds of a bright reference source. But it has been proposed theoretically that by using multiple guide stars, the tomography of atmospheric turbulence could be probed and used to extend adaptive-optics corrections to the whole sky. Here we report the experimental verification of such tomographic corrections, using three off-axis reference stars approximately 15 arcsec from the central star. We used the observations of the off-axis stars to calculate the deformations of the wavefront of the central star, and then compare them with the real measured values. This tomographic approach is found to reduce variations in the wavefront by approximately 92%. Our result demonstrates that a serious barrier to achieving diffraction-limited seeing over the whole sky has been removed.

Minimum diameter of a laser projector for some perspective-based laser guide-star tilt retrieval schemes.

Because of the finite speed of light, a laser guide Star that is seen from the side is not exactly a straight line. When such a laser guide star is used to sense tip-tilt with some of the perspective-based techniques that are used to retrieve an absolute tip-tilt laser, such nonstraightness introduces an error. We estimate this effect for various diameters of a laser projector, assuming a Kolmogorov turbulence spectrum and figuring the maximum achievable Strehl ratio. We found that under poor seeing conditions the effect is not negligible and that laser projectors larger than those currently used are required.