C. M. Lowne of the Royal Greenwich Observatory performed an experiment to investigate the degradation of image quality in a heated mirror [Lowne].
The beam from a HeNe laser was diverged by a 20x microscope objective and than directed to a 254-mm spherical mirror. The return image was accepted by the microscope objective and relayed over a further 2 meters, giving an image magnification of 250x with image scale 2.5 mm to 1 arcsec. A calibrated iris was placed at this focus and the beam was then led to a CdS photo-conductive cell, which measured the light intensity.
The mirror was tested at three zenith angles (0
, 20
and 50
) and was heated and cooled artificially over a
range of temperatures from -2K to +8K with respect to ambient.
The measurements consisted in recording in each test case the iris diameter through which passed 75% of the light intensity. These data, converted in arcsec, are listed in the tables below.
We have converted the 75% intensity diameters to FWHM seeing values
by means of the SuperIMAQ computer program (fig. 
).
Noting that the 75% intensity diameter for a diffraction limited
(perfect) mirror is 0.73 arcsec, an aberration of 70 nm astigmatism
was assumed in order to fit the reference measured value of 1
arcsec. The converted measurements are then plotted in
fig. .
Figure: Relationship between seeing FWHM and 75% intensity diameter
for a 254-mm mirror with 70 nm astigmatism.
Figure: Seeing FWHM for the 254-mm mirror.