A Japanese team from the National Astronomical Observatory, Tokio, performed a mirror seeing experiment ([Iye]) with a 62-cm mirror setup which had been originally conceived to verify the feasibility of a new active optics system.
The test setup is shown in fig. : the wavefront from a 65-cm horizontal mirror was measured by means of a Shack-Hartmann analyzer and expanded by Zernike polynomials. The image quality under the presence of seeing degradation was expressed by evaluating the Strehl ratio by the expression
where is the wavefront rms error.
The mirror was not actively heated or cooled but its temperature as well as that of ambient air followed slightly different diurnal variation cycles so that most of the time there was a mirror-air temperature difference. In order to measure the effect of forced ventilation, the upper surface of the mirror was flushed by the air flow created by an electric fan with a flow-guide nozzle as illustrated in fig. . The measurements were carried automatically over 90 full days.
Fig. shows the average Strehl ratio measurements reported by [Iye] as a function of the mirror-air for different flushing speeds. Some remarks are worth noting:
Figure: Measured Strehl ratios versus mirror-air temperature difference for the 62-cm mirror (as reported by [Iye]).
For the purpose of comparison we have converted the Strehl ratio values to FWHM seeing values by means of the SuperIMAQ computer program for the analysis of mirror image quality [ESO]. The seeing values in term of seeing FWHM are then plotted in fig. .
Figure: Relationship between seeing FWHM and Strehl ratio for a 62-cm mirror (computed with the SuperIMAQ program).
Figure: Seeing FWHM versus mirror-air temperature difference for the 62-cm mirror.