Measurements of outer scale length
<P>
M.Andersen (reported by H.Pedersen)
<P>
The outer scale length of turbulence defines an upper cut off in the size of
turbulent  structures.  It is difficult to  measure the  outer scale length,
since it requires simultaneous  wavefront sensing  and  measurements  of the
wind speed  up through the  atmosphere.  The fact, that most telescopes have
very strong local turbulence (dome seeing),  which may have a behavior  that
is completely  dislike that of the free atmosphere,  further complicates the
situation. As a consequence, highly discordant values have been published.
<P>
We have used the  2.56m  Nordic Optical Telescope on  La Palma to obtain 
indirect estimates of the outer scale length.  The Nordic Optical Telescope is
particular well suited for this,  because the local turbulence  is at a very
low level.  Assuming Kolmogorov turbulence with infinite outer scale length,
the statistical weight of  Zernike aberration terms can be derived.  We have
measured  the image motion,  and find it to be only 40-50%  of that predicted.
Our interpretation of this is that the outer scale length is finite.
<P>
Voitsekhovich and Cuevas (JOSA, November 1995) give the  statistical  weight
of low order abberation terms as function of (L0/D), D being the diameter of
the aperture. Using this result, we find an outer scale length in the  range
5-8m, measured on several nights.  The implication is,  that the seeing, 
understood as the image quality obtainable by a perfect telescope, is not only
dependent  on r0,  but also the telescope diameter.  A 10m  telescope on  La
Palma  will essentially be tip/tilt corrected by the aperture itself.
<P>
We plan to  improve the measurement of LO,  by comparing the  RMS  amplitude
of 1.st and 2.nd aberration terms.