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Mirror seeing

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 gif.

Lorenzo Zago,, Sun Feb 26 22:57:31 GMT+0100 1995