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A mirror seeing experiment with a 4-cm mirror


The experiment described here was performed by the author in the ESO optical laboratory.

The experiment is schematically illustrated in fig. gif: a parallel laser beam with a diameter of 3 cm is reflected on a flat horizontal mirror of a diameter of 4 cm and focused on a position sensing detector (PSD). The mirror was heated by self-adhesive film resistances in order to produce seeing which was evaluated from the rms of the image motion on the PSD and converted in terms of equivalent FWHM.

Figure: Optical schematic of the mirror seeing experiment.

We use here the expression, reported by [Sarazin 92], relating the variance of short-exposure angular image motion to Fried's parameter for the long exposure:


where is the wavelength, in the presently case 680 nm, and D the beam aperture of 3 cm. Noting that only the variance along one direction was measured on the PSD:

one obtains:

where f is the focal distance on the PSD, which was 30 cm in this experiment.
was computed from the image motion data recorded by the PSD over an integration time of about 30 seconds.
The value for the full width half maximum is then evaluated by equation (gif):

Figure: The test mirror on steel plate, itself laid on the floor of the laboratory.

Figure: The cardboard dome.

Different power settings of the heating resistances fixed around the mirror were set and the corresponding temperature of the mirror surface was measured by means of a contact thermometer. Because of the heating method, the temperature of the mirror is not completely homogeneous so the values given in the table of results below represent averages with an accuracy of a few percents of the mirror-air .

The resolution of the PSD used is about 3m, which corresponds to a computed seeing (FWHM) of about 1.3 arcsec. Therefore, in order to get seeing measurements with good resolution, the mirror was heated up to a of 100 K.

The tests were performed in two configurations:

  1. In the first configuration the mirror was simply laid on the floor of the laboratory, in "open air" - fig. gif.
  2. The second configuration consisted in having the mirror enclosed by an octagonal cardboard dome of diameter 30 cm and a 6-cm wide slit as shown in fig. gif.

Figure: Power spectrum of the image motion.


For all tested configurations (which included presence/absence of dome and different mirror-air ), seeing measurements were taken continuously over a period of typically half-hour, sometimes longer. From these sequences the mean and rms values were then obtained.

A summary of the main test results for both open-air and dome configurations is found in the table below.

Table 1: Summary of test results.
1) External disturbances minimized.

The use of equation (gif) to evaluate the results in terms of FWHM or image size assumes that the turbulence follows Kolmogorov's law. In order to verify this point, power spectra of the image motion have been computed on the 30-sec sequences. As the example of fig. gif shows, the spectrum have clear Kolmogorov's characteristics with an inertial range with slope .

A fact apparent from these results is that the dome configuration starts showing seeing at lower s than the open-air one: however for larger s the results are quite the same. It looks like there is an anticipated stable-to-unstable transition in the dome due to interaction of the mirror convective flow with the internal dome surface, most likely linked to the tiny geometrical scale of the model. Therefore no undue extrapolation of these curious results should be done to full scale.

Another important feature shown by the measurements is the large scattering of seeing values recorded during a same measurement sequence. This variability depended very much on the "room turbulence" caused by the presence of the experimenters and by other external disturbances, such as opening and closing a door, happening during a measurement sequence. One should remark here that the air motions in the generally very quiet optical lab, which could be caused by people occasionally moving some meters from the experiment, are very small in absolute terms and certainly not of turbulent nature. Nevertheless when such tiny air motions interact with the convective flow from the mirror, they apparently cause strong increases of seeing.

One should underline that this increase of seeing values appears to be clearly due to some interaction of the "room turbulence" (which, as said, is not really one) with the convection flow immediately above the mirror and is not a purely added "room seeing" effect. If the mirror is not heated, the "room turbulence" produces no measurable seeing effect. This indicates the extreme sensitivity of seeing caused by natural convection to even minimal air motions and turbulence that have an external cause. Typically at full scale, this would correspond to the often reported case of seeing allegedly caused by leaving a door open on the observing floor in a telescope dome.


One can also remark that "room turbulence" affects to a proportionally larger extent the seeing of a mirror with a small , which creates weak convective flows. Stronger convective flows from larger s tend to predominate over external disturbances. Some subsequent measurements were taken outside of working hours in order to minimize disturbances that may affect "room turbulence", which resulted in sequences - noted with 1) - with substantially less mean seeing and variability. Fig. gif shows two comparisons between couples of sequences taken in the same test configurations.

next up previous contents
Next: Seeing tests on Up: Experimental studies Previous: Experimental studies

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