CFHT Image Quality and the Observing Environment

We analyze stellar images on 36,520 exposures made in the u, g, r, i, and z bands with MegaCam at the focus of the 3.6 m Canada-France-Hawai'i Telescope between 2005 August and 2008 August. The effect on image quality (IQ) of temperature differences (ΔTs) in the telescope environment and of wind speed and direction are first examined and discussed. The contributions of the optics to image spread are then estimated and the frequency distribution of the observatory-free site seeing is obtained. The main findings are: (1) In the convective mode, the thermally-induced image full width at half-maximum intensity (FWHM) grows with the temperature gradient and path length L at the rate of ~0.2''(ΔT/L)6/5L3/5. (2) For a given |ΔT|, thermal convection is ~3 times more detrimental to image quality than thermal inversions. (3) The orientation of the dome slit with respect to the wind direction has important effects on IQ. (4) The median observatory induced seeing is 0.43'' FWHM. (5) The FWHM caused by the optics and slight optomechanical imperfections ranges from 0.46'' in u to 0.28'' in i. (6) The median DIMM-scale zenith atmospheric seeing at a wavelength of 500 nm and an elevation of 17 m above ground at the CFHT site is 0.55''. (7) The characteristics value of the outer scale of turbulence is 30 m. The paper addresses various issues bearing on the management of facility seeing.