<title>High-speed seeing measurements at the Keck Telescope</title>

Abstract

We have installed a high-speed camera system at the Keck Telescope, to be used for studying and monitoring atmospheric seeing as well as for telescope diagnostic purposes. This instrument, which consists of a Dalsa camera with a 64 X 64 pixel CCD, a 4 Megabyte Epix frame grabber, and a 486 computer, records sequences of 1248 frames at 181 Hz and 0.2 arcsecond resolution. We note that the Keck Telescope, by virtue of its 10 meter baseline as well as its ability to separate images formed by any or all of its 36 primary mirror segments, is ideally suited to seeing studies, in particular to those involving relatively long baselines and aperture-aperture correlations of wavefront aberrations. We present power spectra for atmospheric wavefront tilts for the primary mirror segments. In general they show the power law frequency dependance expected on theoretical grounds. However the measured segment-to-segment correlations are systematically smaller than theory predicts by a significant factor. It is possible that this effect is a manifestation of a finite outer scale of turbulence.