On the contrary, free convection at the surface of the primary mirror, the cause of so-called mirror seeing, remains an important issue for telescope designers. Mirror seeing was first suspected by [Woolf 82] to be a main seeing contribution in the telescopes of the latest generation which was earlier hidden in the background of larger dome seeing effects.
More recently, experimental measurements by [Racine 92] showed that at the CFHT, mirror seeing is indeed the main cause of facility-made seeing. [Racine 92] analyzed image frames from the HRCam instrument (for high resolution imaging) during 25 nights over an 8-month period for correlations with temperature differences. He found that the main effect came from the primary mirror and evaluated it at:
where 
is the seeing FWHM contribution and
is the temperature difference between the mirror
surface and the surrounding air.
A lower dependency was found with the temperature difference
between the dome inside and outside air:
arcsec 
.
In the next section we report the results of a subsequent analysis
by the author of a much larger set of image quality data of the CFHT.
A more specific analysis of the mirror seeing phenomenon is then found
in section 
.