A low cost seeing monitor to measure the isokinetic patch on the edge of the moon

In most of the techniques currently proposed to overcome the well known limitation of the laser Guide Star of not providing a useful tip-tilt signal, the size of the isokinetic patch plays a major role. Isokinetic patch size, defined as the distance at which the tilt correlation falls below a given threshold, can be measured, in principle, using a star cluster, as the few available measurements in the literature report. We have used the edge of the moon as a reference for the measurement of the tilt correlation over angles spanning from a few arcsec to several arcminutes. We constructed a seeing monitor using a 20-cm portable telescope coupled, through a pair of commercial objectives, with an anamorphic optical relay and a small commercially available 12-bit CCD. The image of the edge of the moon (relayed using the anamorphic relay) is frame-shifted in groups of six or eight in a fast manner, and it is read out later. In this way there is no need for a fast analog-to-digital converter, and only a minor modification to the original commercially available devices is needed. A three-day run at the Asiago Observatory during January 1997 has been performed. The collected data have been extensively analyzed in order to assure their reliability. A comparison with seeing measurements done simultaneously at the focus of the main observatory is presented, showing good agreement. Dispersion of the edge tilts consistent with Poisson statistics is pointed out. Scaling to the planned run at the Canary Island in order to pursue the Adaptive Optics module for the TNG telescope is briefly discussed together with the implication of the results for the various LGS tilt-recovery techniques.