MIRROR, DOME, AND NATURAL SEEING AT CFHT

A large and homogeneous set of image quality data, obtained with HRCam at the prime focus of the CFHT, was correlated with thermal sensor data to identify and quantify "local seeing" effects. The main findings are as follows. 1. Mirror seeing is an important source of image spread and amounts to FWHM = 0".40/-degrees-C6/5 when the primary mirror is warmer than the air inside the dome. 2. Dome seeing is marginally significant, at the rate of approximately 0".1/-degrees-C6/5 of temperature difference between the air inside and outside the dome. 3. Optical aberrations from the CFHT primary mirror and from HRCam currently impose a limit of FWHM = 0".38 to the image quality. 4. The median natural atmospheric seeing at the CFHT site on Mauna Kea is FWHM = 0".43 +/- 0".05, the 10th and 90th percentiles being approximately 0".25 and approximately 0".7, respectively.