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

Details Low cost seeing monitor to measure the isokinetic patch on the edge of the moon Low cost seeing monitor to measure the isokinetic patch on the edge of the moon

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998spie.3219...73g&link_type=abstract

Proc. SPIE Vol. 3219, p. 73-82, Optics in Atmospheric Propagation and Adaptive Systems II, Anton Kohnle; Adam D. Devir; Eds.

Physics

Optics

Scientific paper

In most of the techniques currently proposed to overcome the well known limitation of the laser guide star to do not provide an 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. The edge of the moon or of the sun have been used to sense the tilt and/or to establish a reference system for speckle transfer function. 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. The edge of the moon is bright, its curvature is negligible for the purposes of this measurement, and, in most places of astronomical interest, it reaches significant elevations over the horizon. 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 and so allowing to have good tilt resolution even with a large covered field of view along the isokinetic patch measurement direction) is frame-shifted in groups of 6 or 8 in a fast manner and later it is read-out. In this way there is no need of fast analog to digital converter and only a minor modification to the original, commercially available, devices is needed. A 3-days 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 shown giving good agreement. Dispersion of the edge tilts consistent with Poisson statistics (due to the low number of covered realization) 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 with the various LGS tilt-recovery techniques.

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