ABSTRACT
A large and homogeneous set of images-quality data, obtained with HRCam at the prime focus of the Canada-France-Hawaii Telescope (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 arcsec per deg C exp 6/5 when the primary mirror is warmer than the air inside the dome; (2) dome seeing is marginally significant, at the rate of about 0.1 arcsec per deg C exp 6/5 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 arcsec to the imagae quality; (4) the median natural atmospheric seeing at the CFHT site on Mauna Kea is FWHM = 0.43 arcsec ± 0.05 arcsec, the 10th and 90th percentiles being about 0.25 arcsec and 0.7,arcsec, respectively.